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A&A
Volume 602, June 2017
Article Number A124
Number of page(s) 25
Section Catalogs and data
DOI https://doi.org/10.1051/0004-6361/201629463
Published online 27 June 2017

© ESO, 2017

1. Introduction

X-ray and γ-ray satellites have detected a large number of high energy sources, allowing us to explore and study in detail the hard X-ray band, above 15 keV. Specifically, the INTEGRAL (Winkler et al. 2003) and Swift (Gehrels et al. 2004) satellites are able to detect hard X-ray sources with positional accuracy better than 5 arcmin with their instruments IBIS (Ubertini et al. 2003) and BAT (Barthelmy 2004), respectively. There are more than 1200 hard X-ray sources detected by these satellites, mostly active galactic nuclei (AGN, ~60%) followed by X-ray binaries (XRBs, ~15%), cataclismic variables (CVs, ~5%), and finally active stars (e.g. Baumgartner et al. 2013); but more than 20% of the whole set of detections has no precise classification or has no obvious counterpart at other wavelengths and therefore they cannot be associated with any known class of high-energy emitting objects.

Although the cross-correlation with catalogues of surveys at other wavelengths (soft X-ray, radio, or optical bands) is important for improving the positional accuracy of detected objects and pinpointing longer-wavelength candidate counterparts of these sources, only with the detection of emission spectral features especially in the optical can we confirm these associations and reveal the actual nature of these unclassified objects.

The identification and classification of these newly detected objects is important as it is useful, for instance, for binary system studies (Coleiro & Chaty 2013; Scaringi et al. 2010; Landi et al. 2009) and for population analysis of extragalactic objects (e.g. Ricci et al. 2015; Ajello et al. 2008). Moreover, one can find interesting classes of X-ray emitting objects, for example types of unusual AGN obscured by local absorption in their galaxies and Compton-thick AGNs with a line-of-sight hydrogen column NH>1024cm-2 due to the torus of dust surrounding a supermassive black hole (Comastri et al. 2002; Malizia et al. 2009; Lutovinov et al. 2012; Baloković et al. 2014; Koss et al. 2016a; Ricci et al. 2015). Also, new types of hard X-ray binaries have been reported; namely, the INTEGRAL discovered supergiant fast X-ray transients (SFXTs, Sguera et al. 2006) and the γ-ray binary in de Martino et al. (2013). Due to local or Galactic absorption, many of these objects are optically faint, so one needs relatively large telescopes to identify their nature and get information about them, such as the study of stellar populations in their central regions (Morelli et al. 2013).

Many efforts have been made to characterise and analyse the properties of the Swift-BAT sample using optical spectroscopy (e.g. Landi et al. 2007; Winter et al. 2010; Berney et al. 2015; Halpern & Thorstensen 2015; Ueda et al. 2015), but the identification and precise classification of unassociated or poorly studied objects remains incomplete. The work presented here extends to fainter objects the work started in Parisi et al. (2009; 2012; 2014; hereafter PI, PII and PIII), and focuses on the identification and classification of unassociated hard X-ray sources detected by the BAT instrument onboard Swift, that are listed in the Second Palermo Swift-BAT Catalogue (2PBC, Cusumano et al. 2010).

The paper is structured as follows: in Sect. 2, we provide information about the sample selection; in Sect. 3, we describe the optical observations, telescopes employed, and reduction method used; in Sect. 4, the results of the optical classification of different classes (AGNs, XRBs, CVs and active stars) are presented and a statistical analysis of the sample is given; finally, in Sect. 5 we summarise the main conclusions of our work.

Table 1

Observational information of finding charts of the soft X-ray counterpart identifications to the present sample of 2PBC sources.

2. Sample selection

In this work we focus on the optical follow-up of objects with unknown classification and/or redshift that are reported in the Second Palermo Swift-BAT hard X-ray catalogue obtained by analysing data acquired in the first 54 months of the Swift mission (Cusumano et al. 2010).

This survey covers 90% of the sky down to a flux limit of 1.1 ×10-11 erg cm-2 s-1 and 50% of the sky down to a flux limit of 0.9 ×10-11 erg cm-2 s-1 in the 15−150 keV band. It lists 1256 sources, of which 57% are extragalactic, 19% are galactic, and 24% are of unknown type.

From this BAT survey, we selected a sample of unknown type objects and used the available soft X-ray data (<10 keV) to reduce the source positional uncertainty from arcmin to arcsec sized radii, using the ROSAT (Voges et al. 1999), Swift/XRT (0.3−10 keV, Evans et al. 2014), XMM-Newton (0.2−12 keV, Watson et al. 2009) catalogues or the Chandra1 (0.3−10 keV, Aldcroft et al. 2000) and XRT2 databases. This approach was proven by Stephen et al. (2006) to be very effective in associating hard X-ray sources with a strong, softer X-ray counterpart within the high-energy error circle with a high degree of probability, which in turn drastically reduces the positional error circles to a few arcsec in radius, thus shrinking the search area by a factor of ~104.

More specifically, for the present sample, we analysed the soft X-ray (<10 keV) images with longest available exposure of the field of each BAT object in Table 1 to search for sources detected (at confidence level >3σ) within the 90% Swift-BAT error circles. This choice also allowed us to investigate the source hardness in case of multiple objects within the hard X-ray error box, which permitted us to pinpoint the hardest sources, hence the most likely counterparts to the BAT objects.

Nearly all soft X-ray images were extracted from the ASDC XRT database (see footnote 2); that of 2PBC J1419.2+6804 was instead obtained from an XMM-Newton observation downloaded from the HEASARC database3. The information about these images can be found in Table 1 and the coordinates of the most likely counterparts are reported in Table 2 (second and third columns) together with their relative uncertainties (fourth column).

In most of the cases, that is for 33 objects, we found a single bright soft X-ray object within the BAT 90% confidence level error box with emission above 3 keV, and the soft X-ray counterpart positions of these BAT objects are provided in Fig. A.1, where for each image the BAT error circle is shown in blue and the black arrow indicates the soft X-ray counterpart for which the optical spectra were acquired. On the other hand, we found that 11 of the proposed soft X-ray counterparts to BAT objects in this paper are coincident with the proposed ROSAT or XRT counterpart reported in the Cusumano et al. (2010) and/or Baumgartner et al. (2013) catalogues. We list them in Table 2.

For the remaining six objects we needed to consider additional criteria as we detail below.

When no soft X-ray counterpart was found inside the BAT error circle at 90% position confidence level, we expanded the search area by considering the 99% BAT error circle assuming a gaussian uncertainty distribution for the error radius. In our sample, the putative counterparts of three objects were selected considering the 99% BAT error circle: those of 2PBC J0550.7−2304, 2PBC J0620.8−2932, and 2PBC J1832.8+3124. Since no other object is detected inside the 99% positional uncertainty, we assumed these associations to be correct (see Fig. A.2). We remark here that the soft X-ray counterpart indicated with the black arrow for the object 2PBC J0550.7−2304 corresponds to the ROSAT object proposed as a soft X-ray counterpart in the Cusumano et al. (2010) catalogue and lies at the edge of the 99% BAT error circle.

In one single case, we included in our sample an object reported by PIII and for which no soft X-ray counterpart was found within its 99% BAT error circle. Actually, PIII found two nearby soft X-ray sources as possible associations of the object PBC J1020.5-0235, which are the same sources labelled as Swift J1020.5−0237A and Swift J1020.5−0237B in the 70-month BAT Catalogue of Baumgartner et al. (2013). In PIII the object with higher 3−10 keV flux was considered for optical spectroscopy (object B) and identified as Sy 1 galaxy at z = 0.060. We thus decided to complete the information about this field by analysing the optical spectrum of the object Swift J1020.5−0237A (see Fig. A.3).

Finally for two cases, 2PBC J0819.2−2508 and 2PBC J1520.2−0433, the soft X-ray counterparts proposed in this work do not coincide with those reported in the Cusumano et al. (2010) and/or Baumgartner et al. (2013) catalogues (see Fig. A.4).

Table 2

Log of the spectroscopic observations presented in this paper (see text for details).

Then, within the soft X-ray error boxes of each source of our sample, we identified the putative optical counterpart(s) to the BAT object and performed optical spectroscopic follow-up work with long-slit spectroscopy. Following the method applied by Masetti et al. (2004, 2006a,b,c,d, 2008, 2009, 2010, 2012, 2013, hereafter MI to MX respectively) and by PI, PII, and PIII, we determined the nature of 50 selected objects, estimating also redshifts, distances, Galactic and local extinction, and central black hole masses for type 1 AGNs. The observational information of the 50 objects are listed in Table 2.

3. Optical observations

The data presented in this work involved the use of the following telescopes:

  • the 1.5 m at the Cerro Tololo InteramericanObservatory (CTIO), Chile;

  • the 1.52 m Cassini telescope of the Astronomical Observatory of Bologna, in Loiano, Italy;

  • the 2.1 m telescope of the Observatorio Astronómico Nacional in San Pedro Mártir (SPM), Mexico;

  • the 3.58 m Telescopio Nazionale Galileo (TNG) at the Roque de Los Muchachos Observatory in La Palma, Spain;

  • the 1.9 m Radcliffe telescope of the South African Astronomical Observatory (Sutherland, South Africa);

  • the 2.5 m SDSS at the Apache Point Observatory (APO), Sunspot, New Mexico;

  • the 3.5 m New Technology Telescope (NTT) of La Silla Observatory, Chile.

In Table 2, we show a detailed log of all the spectroscopic observations presented in this work. Column 1 indicates the names of the observed 2PBC sources. In Cols. 2 and 3, we list the soft X-ray coordinates of the proposed counterpart (with their respective positional uncertainties in Col. 4) extracted from the XMM-Newton (with an uncertainty better than 2′′, Watson et al. 2009) and the XRT catalogues (with an uncertainty of ~6′′ for the PC interactive mode according to Evans et al. 2014). In Cols. 5 and 6, we list the equatorial coordinates of the proposed counterpart, mostly extracted from the 2MASS (with an uncertainty of 0\hbox{$\farcs$}1: Skrutskie et al. 2006) or USNO-A2.0 catalogues (with an uncertainty of 0\hbox{$\farcs$}2, Deutsch 1999; Assafin et al. 2001).

The telescope and instrument used for the observations are reported in Col. 7, while the main characteristics of each spectroscopic setup are presented in Cols. 8–10. Column 11 reports the observation date and the UT time at mid-exposure, and Col. 12 provides the exposure times and the number of observations for each source.

The instrumental resolutions (in Å) for the different setups of the optical spectra presented here are listed in Tables 2 and 3. The spectroscopic data acquired have been extracted (Horne 1986) and reduced following standard procedures using IRAF4. Calibration frames (flat fields and bias) were taken on the day preceding or following the observing night. The wavelength calibration was performed using lamp data acquired soon after each on-target spectroscopic acquisition; the uncertainty in this calibration was ~0.5 Å in all cases according to our checks that used the positions of background night sky lines. Flux calibration was obtained using catalogued spectrophotometric standards. Finally, the data of a given object were stacked together to increase the signal-to-noise ratio when multiple spectra were acquired. The Sloan Digital Sky Survey (SDSS) spectrum of 2PBC J0838.7+2612 was acquired from the corresponding online archive5, already reduced and calibrated.

Table 3

Instrumental setups of the telescopes used to gather the observations presented in this paper.

4. Results

The spectra of 50 objects were analysed according to the above steps. Of these, 45 were classified as extragalactic objects and the remaining five objects presented features in their spectra consistent with those of galactic sources.

The main criteria used for the classification of the extragalactic sources spectra are reported hereafter. We assume a cosmology with H0 = 70 km s-1 Mpc-1, ΩΛ = 0.7, and Ωm = 0.3; the luminosity distances of the extragalactic objects presented in this paper were determined for these parametres using the cosmology calculator of Wright (2006).

In order to distinguish between type 1 and type 2 emission-line AGN, we used the criteria of Veilleux & Osterbrock (1987) and we used the line ratio diagnostics of Ho et al. (1993, 1997) and Kauffmann et al. (2003) to distinguish among the Seyfert 2, low-ionization nuclear emission-line regions (LINERs; Heckman 1980), HII regions, and transition objects (LINERs whose integrated spectra are diluted or contaminated by neighbouring HII regions, Ho et al. 1997). For the subclass assignation to Seyfert 1 galaxies, we used the Hβ/[OIII]λ5007 line flux ratio criterion of Winkler (1992).

To estimate the E(B−V) local optical absorption in our type 2 AGN sample, when possible, we first dereddened the spectrum by applying a correction for the Galactic absorption along the line of sight to the source. This was done using the galactic colour excess E(B−V)Gal given by Schlegel et al. (1998) and the Galactic extinction law obtained by Cardelli et al. (1989). We then estimated the colour excess E(B−V)AGN local to the AGN host galaxy by using the resulting FHαFHβ\hbox{$\frac{F_{\rm H_{\alpha}}}{F_{\rm H_{\beta}}}$} line ratio and corrected that assuming a Milky Way reddening using the Balmer decrement flux relation of Osterbrock (1989) that for normal density and temperature conditions implies (FHαFHβ)∗\hbox{$ \left(\frac{F_{H_{\rm \alpha}}}{F_{\rm H_{\beta}}}\right) _{*}$} = 2.86.

For most of our type 1 AGN sample we also estimated the mass of the central black hole. We generally used the Hβ emission line flux, corrected for the Galactic colour excess (Schlegel et al. 1998) and its full width half maximmum (FWHM) to measure the broad-line region (BLR) gas velocity (vFWHM). Using Eq. (2) of Wu et al. (2004), we estimated the BLR size, which is used with vFWHM in Eq. (5) of Kaspi et al. (2000) to calculate the AGNs black hole mass. For the single quasi-stellar object (QSO) of our sample, we apply the formulae from McLure & Jarvis (2002), which use the information afforded by the Mg ii broad emission. As already remarked in our previous papers, the main sources of error in these mass estimates generally come from the determination of the flux of the employed emission lines, which is ~15% in the sample, and from the scatter in the scaling relation between the size of the BLR and the diagnostic line luminosity (Vestergaard 2004). Therefore, we expect the typical error to be around 50% of the black hole mass value.

The results of the black hole (BH) masses determination are reported in Sect. 4.4 where we also list the Eddington luminosities which were computed using the observed X-ray fluxes or upper limits in the 15−150 keV band and the median bolometric correction according to Ho (2009). For that, we first converted the observed X-ray luminosity measured in the Swift-BAT survey in the energy range 15−150 keV to the intrinsic 2−10 keV luminosity (L2−10 keV) by dividing by 2.67 following Rigby et al. (2009) and Marconi et al. (2004), and then we applied the bolometric correction factor of CX = 15.8 between the bolometric luminosity (LBOL) and the X-ray luminosity in the 2−10 keV band according to Ho (2009). We would like to stress that our estimation of the bolometric luminosity implies an approximation because Rigby et al. (2009) and Marconi et al. (2004) used a conversion from 14−195 keV to 2−10 keV energy band whereas our reference hard X-ray band is 15−150 keV.

To evaluate the distances of the compact Galactic X-ray sources of our sample, we assumed an absolute magnitude MV~ +9 and an intrinsic colour index (V−R)0~ 0 mag (Warner 1995) for CVs, whereas we assumed MV~ 0 and (V−R)0~ 0 for the X-ray binary (van Paradijs & McClintock 1995). Although these approaches basically provide an approximate value for the distance of Galactic sources, our past experience (MI-X and PI-III) tells us that these estimates can be considered correct to within 50% of the refined value subsequently determined with more precise approaches. The values for the absorption in the optical V band along the line of sight of the galactic objects for the determination of distances were again estimated from the Hα/Hβ flux ratio following Osterbrock (1989).

The X-ray luminosities reported in Tables 4 to 7, D.1 and D.2 are associated with a letter indicating the satellite and/or the instrument, namely the Swift-BAT (C) from Cusumano et al. (2010), Swift-BAT (B) from Baumgartner et al. (2013), and INTEGRAL/IBIS (I) from Baumgartner et al. (2016) catalogues, and from the Swift/XRT (X) and XMM-Newton (N) databases, with which the measurement of the corresponding X-ray flux was obtained. For the cases in which the soft X-ray information was obtained from the ASDC6 archive, we derived the 0.3−10 keV flux by assuming a power law model with photon index Γ = 1.8 and NH value formulated by Predehl & Schmitt (1995) according to Schlegel et al. (1998) using the webpimms tool7 in order to obtain their soft X-ray luminosities.

For galactic objects with a soft X-ray counterpart from the ASDC archive, we assumed different spectral models according to their nature in order to obtain their soft X-ray fluxes. Specifically, for the probable X-ray binary 2PBC J1911.4+1412 we considered a power law model with Γ = 2.1, that is, a Crab-like spectrum, and for the active star 2PBC J0620.8−2932 we assumed a black body model with a typical value for these objects of kT = 1 keV (Pandey et al. 2005).

In the following, we present the object identifications by dividing them into four broad classes (X-ray binaries, CVs, active stars, and AGNs).

thumbnail Fig. 1

Spectrum (not corrected for the intervening Galactic absorption) of the optical counterpart of the probably X-ray binary belonging to the sample of BAT sources presented in this paper, 2PBC J1911.4+1412. For this spectrum, the main spectral features are labeled and ⊕ indicates telluric absorptions.

Table 4

Main results for the probably X-ray binary identified in the present sample of 2PBC sources.

4.1. 2PBC J1911.4+1412: an X-ray binary?

Among the objects in our sample, 2PBC J1911.4+1412 displays optical features of an X-ray binary (see Fig. 1).

In the optical spectrum of this source, we detected emissions of Balmer lines up to Hδ and emission of He I at a redshift consistent with 0 superimposed on a reddened continuum. Moreover, the spectrum displays an emission of He IIλ4686, typical of accretion disks. Table 4 lists the relevant optical spectral information along with the main parameters determined from available X-ray and optical data. The value for the absorption in the optical along the line of sight reported in that Table for the object is lower than the Galactic one along their direction according to Schlegel et al. (1998) (AV~ 5.4 mag), indicating that the object is indeed within the Galaxy.

A detailed study of the photometric optical/near-infrared (NIR) information is needed to constrain the nature of the secondary star in this binary system but, due to the faintness of the optical counterpart along with the optical spectral shape and the NIR non-detection in the 2MASS catalogue (Skrutskie et al. 2006), the presence of an early type companion can be ruled out and, consequently, a classification as a high mass X-ray binary (HMXB) is excluded.

thumbnail Fig. 2

Spectra (not corrected for the intervening Galactic absorption) of the optical counterparts of the three CVs belonging to the sample of BAT sources presented in this paper. For each spectrum, the main spectral features are labelled and ⊕ indicates telluric absorptions.

A CV nature can also be excluded on the following basis: considering an MV = +9, we obtain a distance of d~ 340 pc, which is too close to justify the optical absorption displayed on the source spectrum. We thus suggest that this object is probably a low mass X-ray binary (LMXB) with a secondary star of late spectral type at a distance of ~21 kpc (see Table 4). This distance is quite large but is not unprecedented (see GS 1354-64, Casares et al. 2004, 2009; Cyg X-2 and XTE J1710-281, Jonker & Nelemans 2004). However we note that the AV value determined from the spectrum, and compared with the galactic AV, is too low for that distance of 21 kpc because this distance would imply that the object is located on the far side of the Galaxy and thus the AV should be close to the value in Schlegel et al. (1998) and not just a minor fraction. One possibility is that the USNO magnitude is underestimated: if the object were actually brighter than its tabulated optical magnitude, it would lie closer. Indeed, the USNO photometry can sometimes be offset by up to 0.3−0.4 mag (e.g. Masetti et al. 2003). Alternatively, the absolute magnitudes that we assumed for the LMXB and CV cases are not correct: the object may either be a low optical luminosity LMXB or a bright CV. This, again, would alter the real distance of the object. In conclusion, we cannot give a precise identification for this object, although we deem more probable that it may be a distant LMXB.

On the other hand, in order to get clues on the measured distance to 2PBC J1911.4+1412, we noted that the object is tabulated in the proper motion catalogue of Roeser et al. (2010) (PPMXL). Unfortunately, the error in the measured proper motion is larger than the proper motion value (pmRA = 2.0 ± 6.3 mas/yr, pmDec = −5.0 ± 6.3 mas/yr), therefore only an upper limit to the velocity (<130 km s-1 in case of a distance of 21 kpc) can be given. We note that this value is typical of LMXBs.

4.2. CVs

Three sources in our 2PBC sample display emission lines of the Balmer complex, as well as He I and He II, consistent with z = 0, indicating that these objects lie within our Galaxy (see Fig. 2). The analysis of their optical features indicates that all of them are CVs (Table 5). The AV of these objects are consistent with ~0 according to their HαHβ\hbox{$\frac{\rm H_{\alpha}}{\rm H_{\beta}}$} flux ratios and, when compared with the Galactic one along their direction according to Schlegel et al. (1998) (AV~ 1.9, ~4.1 and ~1.3 mag for 2PBC J0658.0-1746, 2PBC J0717.8-2156 and 2PBC J0800.5-4306, respectively), this confirms their galactic nature and their proximity to Earth.

Through the equivalent widths (EW) of the Hβ and He IIλ4686 lines we investigated their magnetic nature, that is, the possibility that they are harbouring a magnetised white dwarf (WD) and thus that they may be polar or intermediate polar (IP) CVs. The three objects classified as CVs have EWs of both He IIλ4686 and Hβ larger than 10 Å, and for 2PBC J0658.0−1746 and 2PBC J0717.8−2156 the He IIλ4686/Hβ EW ratio is higher than 0.5, implying that both objects are magnetic CVs likely belonging to the IP subclass, whereas 2PBC J0800.5−4306 is likely a polar CV because its He IIλ4686/Hβ EW ratio is higher than one (see Warner 1995). Optical-NIR time-resolved spectrophotometric followup is thus mandatory to confirm the magnetic nature of all of these CVs.

We note that the object 2PBC J0717.8−2156 was analysed by Halpern & Thorstensen (2015) who indicated a CV nature for it; however, the optical spectrum of this source published in that paper was not flux calibrated. Here we complete the information on this object by presenting a flux-calibrated spectrum and by confirming a (possibly magnetic) CV nature for it.

The main spectroscopic results and the main astrophysical parameters, which can be inferred from the available observational data, are reported in Table 5.

Table 5

Synoptic table containing the main results concerning the three CVs identified in the present sample of 2PBC sources.

4.3. Active stars

The object 2PBC J0620.8−2932 shows the optical features of an active star in its spectrum (Fig. 3). We recall that this object is within the BAT error box at 99% confidence level and does not display X-ray emission at above 3 keV in the XRT observations; nevertheless, we suggest that this source can be classified as a chromospherically active star, with a star-like continuum typical of late-G/early-K type stars, a faint emission of Hα, and absorption bands such as G, NaD, and Mg b at z = 0, and we think that the flux above 3 keV from this source is possibly transient and could be due to flares (Sguera et al. 2016; Krivonos et al. 2007; MVI; MIX; MX). This conclusion is supported by the comparison between hard and soft fluxes of the source in Table 6, which is F15−150 keV/F0.3−10 keV≈ 16, indicating that the star could have been undetected by BAT during a flare.

Table 6

Main observational results for the active star as identified in the present sample of 2PBC sources.

Table 7

Synoptic table containing the main results for the QSO identified in the present sample of 2PBC sources.

An estimate for the distance to this object can be evaluated, as suggested in Rodriguez et al. (2010), assuming similarity with the active star II Peg (which has magnitude R~ 6.9 and distance 40 pc; Monet et al. 2003; van Leeuwen 2007). We considered that 2PBC J0620.8−2932 has a USNO-A2.0 magnitude R~ 14.7 and we consequently obtained the distance reported in Table 6, which is actually an upper limit since we considered AV~ 0 mag because we cannot determine the reddening along the source line of sight with the presently available information.

4.4. AGNs

The results for the extragalactic sources are reported in Tables 7, D.1, and D.2, where for each source we list fluxes, the classification, the estimated redshift, the luminosity distance (in Mpc), the Galactic colour excess, and the colour excess local to the AGN host.

thumbnail Fig. 3

Spectra (not corrected for the intervening Galactic absorption) of the optical spectra acquired for 2PBC J0620.8−2932, the optical counterpart of the active star belonging to the sample of BAT sources presented in this paper. For the spectrum, the main spectral features are labelled and ⊕ indicates telluric absorptions.

We found that among 45 extragalactic objects, 27 have strong redshifted broad and narrow emission lines that are typical of type 1 AGNs (Figs. 4 and 5) and 18 have only strong and redshifted narrow emission lines that are typical of a type 2 AGN nature (see Figs. 6–9).

In our sample, we report the redshift value for most AGNs for the first time and most of them are in the range 0.004–0.294, that is, the AGNs of our sample are located in the local Universe (z< 0.3) except 2PBC J2010.3−2522, the QSO reported in Table 5 which is at redshift 0.828.

Among our AGN sample, 20 objects are intermediate type Sy 1.2–1.5 and 5 are Sy 1.9, whereas of the 18 type 2 AGNs, 14 are Seyfert 2 galaxies, one is a starburst galaxy (2PBC J1555.0−6225, Fig. 7), one is a LINER (2PBC J2238.9+4050, Fig. 8), and one is an X-ray bright optically normal galaxy (XBONG, Comastri et al. 2002) which displays absorption features only (2PBC J1332.1−7751, Fig. 9).

For type 1 AGNs, BH masses and Eddington luminosities are reported in Table D.3, where we corrected the measurement of the observed FWHM of Hβ for the instrumental resolution corresponding to the relevant telescope setup (see Tables 2 and 3) in order to determine the BH masses. We only report the Hα emission for narrow line sources (Table D.2) as the Hα emission required more complex fits and sometimes had issues with the blending of [NII] emission lines and atmospheric lines.

For three Sy 2 objects, 2PBC J0356.6−6252, 2PBC J0608.0+5749, and 2PBC J2029.4−6146, (FHαFHβ)∗\hbox{$ \left(\frac{F_{\rm H_{\alpha}}}{F_{\rm H_{\beta}}}\right) _{*}$} is less than 2.86, which implies that they could be type 2 AGN without local absorption (these are called “naked” Sy 2; Hawkins 2004). But, in particular, the object 2PBC J0356.6−6252 has strong IR emission due to dust according to Yamada et al. (2013), and the analysis of its X-ray spectra according to Ricci et al. (2015) indicates a probably Compton thick nature, thus a more detailed study is needed to confirm the nature of this source.

As can be seen in Fig. 5, the spectrum of 2PBC J1548.5− 3208 displays a composition of an AGN type Sy 1.9 at redshift 0.048 and a foreground star due to the inability of our setup to disentangle the pair of sources.

thumbnail Fig. 4

Spectrum (not corrected for the intervening Galactic absorption) of the optical counterpart of the QSO belonging to the sample of BAT sources presented in this paper, 2PBC J2010.3-2522. For this spectrum, the main spectral features are labelled and ⊕ indicates telluric absorptions.

thumbnail Fig. 5

Optical spectra of type 1 AGNs presented in this work (not corrected for the intervening galactic absorption). For each spectrum, the main spectral features are labelled and ⊕ indicates telluric absorptions.

thumbnail Fig. 5

continued.

thumbnail Fig. 6

Optical spectra of type 2 AGNs presented in this work (not corrected for the intervening galactic absorption).

We note that for the object 2PBC J0819.2−2508, Cusumano et al. (2010) proposed a ROSAT soft X-ray counterpart in the catalogue (1RXS J081915.0−251106) and thus we acquired and analysed the spectrum of the optical object associated with the ROSAT source. However, we subsequently found in the XRT pointings of this field that the source analysed here has no emission above 3 keV; rather, there is another source inside the corresponding BAT error box of 2PBC J0819.2−2508 with emission above 3 keV (labelled as “XRT object” in Fig. A.4, left panel). This second source has coordinates (J2000) RA = 08:19:16.2, Dec = −25:07:06.9 with error radius of 6′′ and is positionally associated with a R≈ 19 optical source. We think that the proposed ROSAT soft X-ray counterpart in Cusumano et al. (2010) could actually not be the real soft X-ray counterpart of 2PBC J0819.2−2508 and instead this other source could be; at the very least, both could be contributing to the detected hard X-ray emission. The nature of this object and, in turn, an optical spectrum of this other source is required in order to confirm if this is a possible soft X-ray counterpart to 2PBC J0819.2−2508.

thumbnail Fig. 7

Spectrum (not corrected for the intervening Galactic absorption) of the optical counterpart of the starburst galaxy belonging to the sample of BAT sources presented in this paper, 2PBC J1555.0-6225. For this spectrum, the main spectral features are labelled and ⊕ indicates telluric absorptions.

Similarly, the object 2PBC J1520.2−0433 has a proposed ROSAT soft X-ray counterpart in the Cusumano et al. (2010) catalogue, 1RXS J152017.4-043621 (see Fig. A.4 right panel, in magenta). We, however, found in the XRT observation of this field that this ROSAT source has no X-ray emission inside its error circle. Rather, there is another source inside the 90% BAT error circle of 2PBC J1520.2−0433, indicated with a black arrow in that figure, with emission above 3 keV. Thus, we considered this latter source as the actual soft X-ray counterpart for the optical follow-up.

thumbnail Fig. 8

Spectrum (not corrected for the intervening Galactic absorption) of the optical counterpart of the LINER belonging to the sample of BAT sources presented in this paper, 2PBC J2238.9+4050. For this spectrum, the main spectral features are labelled and ⊕ indicates telluric absorptions.

thumbnail Fig. 9

Spectrum (not corrected for the intervening Galactic absorption) of the optical counterpart of the XBONG belonging to the sample of BAT sources presented in this paper, 2PBC J1332.1-7751. For this spectrum, the main spectral features are labelled and ⊕ indicates telluric absorptions.

As mentioned in Sect. 2, the object 2PBC J1020.5−0235 has two hard X-ray emitting sources just outside the 99% BAT error circle (see Fig. A.3), listed as Swift J1020.5−0237A and Swift J1020.5−0237B in the 70-month Swift-BAT catalogue of Baumgartner et al. (2013); in this work, we analysed the object A and we identified it as a Sy 2 at z = 0.294 (see Table D.2 and Fig. 6).

For the object 2PBC J2010.3−2522, that we classified as type 1 QSO at z~ 0.8, we found in the literature that this object could be associated with a Fermi transient J2007−2518 that is also spatially coincident with a radio source (Kocevski et al. 2014). Strader et al. (2014) acquired an optical spectrum of this same source and obtained results equivalent to ours. We thus confirm the QSO nature of this object and its possible association to the Fermi transient J2007−2518.

We found that the object classified as a starburst galaxy in our sample, 2PBC J1555.0−6225, has WISE magnitudes and colours that indicate an AGN nature according to the criteria of Secrest et al. (2015): (W1−W2) = 0.56, (W2−W3) = 2.34 > 2. This could be an elusive AGN as in Smith et al. (2014) and we are probably only seeing the starburst contribution in its optical spectrum, with the X-ray emission detected from the hidden nuclear activity.

For the object 2PBC J1332.1−7751, classified as XBONG, Davies et al. (2015) reported a column density in X-ray of Log(NH) = 23.8 cm-2, that is almost at the limit to be a Compton Thick object (with Log(NH) ≳ 24 cm-2) and they classified it as a Seyfert galaxy, but no optical classification is considered for this object in that work. Thus, 2PBC J1332.1−7751 is a peculiar object that is absorbed in X-ray and optically obscured. Our classification as XBONG is then supported by the information from Davies et al. (2015): the AGN optical appearence is likely hidden due to dust obscuration revealed in X-ray spectrum.

thumbnail Fig. 10

Histogram, subdivided into source types, showing how the initial percentage of objects in the 54 months Palermo Swift-BAT catalogue (in green) has changed by means of the identifications performed by other authors (in grey) and by our group in PI-PIII and in this work (R16) (in yellow).

We note that in the field of the object 2PBC J0838.7+2612, there is another object with emission above 3 keV inside the 99% BAT error circle, indicated with tick marks (see Fig. A.1). For the sake of completeness, in Appendix B we thus report the main optical spectroscopic information we gathered about this source (a galaxy pair). Furthermore, the soft X-ray source that we associated with the object 2PBC J2030.7−7530 is the hardest source with emission above 3 keV within the BAT error circle (its count rate is at least 20 times larger than the other sources within the BAT error box). In the same way, the object 2PBC J1419.2+6804 has a count rate two orders of magnitude stronger than the other objects inside the 90% BAT error circle (see Appendix A).

We then looked for Compton-thick AGNs. In order to explore this nature, we applied the diagnostic T of Bassani et al. (1999), that is, the ratio of the measured 2−10 keV X-ray flux to the unabsorbed flux of the [OIII]λ5007 forbidden emission line, to the objects which are classified as type 2 AGNs in our sample.

It was found that all objects show a T parameter >1 (from 2.5 to 430), so all of them apparently lie in the Compton thin regime, with NH<1024 cm-2. We, however, stress that the T values should be considered as upper limits because the soft X-ray fluxes that we considered refer to bands which are wider (0.3−10 keV) than the one for which this method should be applied (2−10 keV).

Moreover, we support this result by applying the criterion of Malizia et al. (2007): we found that all our type 2 AGNs lie in the Compton thin regime with softness ratio Fxsoft/Fxhard from 0.02 to 0.41. Although we use a different soft and hard X-ray ranges (0.3−10 keV and 15−150 keV instead of 2−10 keV and 20−100 keV), the approach can be considered equivalent and the results comparable.

4.5. Statistics

We here give an update of the statistics concerning the identifications of new hard X-ray Swift-BAT sources including the results of our group in this work and those in PI, PII, and PIII.

Considering all works, we identified 171 Swift-BAT objects through optical spectroscopy and their separation into the main classes is the following: 157 (92%) are AGNs, two are starburst galaxies (1%), one (0.6%) is probably an X-ray binary, ten (5.8%) are CVs, and one case (0.6%) is likely identified as an active star. We were able to drastically decrease the percentage of unidentified objects in the BAT survey, as shown in Fig. 10, where the unknown source type percentage decreased from more than 20% to ~10% thanks to our systematic work (yellow bars).

The green bars in Fig. 10 represent the initial 54-month Palermo Swift-BAT catalogue and the grey bars represent the classification of 20 AGNs and 18 galactic objects performed by other authors and available in the literature, as well as by our group using information already existing in the Simbad database (see Appendix C for the relevant information concerning these sources). We would like to stress that those other works have been able to reduce by ~3% the percentage of unclassified sources in the catalogue of Cusumano et al. (2010), whereas our enduring classification work has allowed us to decrease by ~10% the percentage of these objects.

All AGNs (Seyfert 1, Seyfert 2, starburst galaxy, and LINERs) and XBONG in our sample and PI-PIII samples are at low redshifts (z< 0.5) while only two QSOs are located at z> 0.5.

We can see that the high-redshift QSOs found in our identification programme are of type 1. This is reasonably explained by the evidence that soft X-rays, needed for a precise localisation of the source, are not easily detected from type 2 QSOs due to local absorption in these sources, which adds to the faintness tied to their distance.

Figure 11 shows the redshift distribution of AGNs analysed in this work and the previous works of PI, PII, and PIII, represented by a histogram and binned according to the rules of Knuth (2006), which are based on the optimisation of a Bayesian fitness function across fixed-width bins.

The redshift distributions for the present sample and that of the PI-III merged sample were studied through a two sample Kolmogorov-Smirnov (KS) test. When we compare our redshift sample of Sy 1 with that of PI-III, we find that the KS test gives a p-value ~ 0.001, whereas the comparison between the Sy 2 samples gives a p-value ~ 0.178. This suggests that our Sy 1 sample is drawn from a different redshift distribution compared with the Sy 1 sample of PI-III. There is no univocal explanation for this: the difference could be due, for instance, to the fact that we on average used larger telescopes than those used in the PI-III works, thus we were able to observe fainter (and likely more distant) objects. On the other hand, when we compare the distributions of Sy 2 galaxies in the two samples, we see that they are consistent with belonging to the same parent population; therefore the above interpretation cannot be applied to this class of AGNs.

thumbnail Fig. 11

Histograms showing the redshift distribution of Swift-BAT objects analysed and classified as AGNs in this sample (above) and those in the previous works of PI-III (below).

thumbnail Fig. 12

Distribution of the [OIII]λ5007 to 0.3−10 keV ratio for the AGNs of our sample and corresponding best-fit correlations (see text for details). Red continuous line represents the locus for which the parameter T is equal to 1 and the dotted pink line indicates the Berney et al. (2015) relation for the ratio between the soft X-ray flux and the [OIII]λ5007 unabsorbed flux.

Moreover, we noted that the Sy 1 sample has on average a higher z (~0.130) than the Sy 2 sample (at z~ 0.066). Interestingly the objects classified as naked Sy 2 are those few at higher z. This could explain why we see a difference with PI-III samples: indeed, in our selection of sources we are observing more distant Sy 1 galaxies, while this does not occur in the case of Sy 2 AGNs (except the naked ones) given that they are expected to be more absorbed and thus even fainter with distance (which means they are difficult to observe with medium-sized telescopes).

Considering this work and PI-III, we found 12 Galactic objects: ten of these are CVs (i.e. 84% of the galactic identifications) and most of them are of magnetic type, whereas one object is an X-ray Binary, most probably a LXRB (8%) and the remaining one is an active star (8%). We can confirm that the Swift satellite is efficient in detecting magnetic CVs because these objects are hard X-ray emitting sources due to their magnetic nature, and because the BAT surveys allow the study of objects outside the Galactic plane and, among them, hard X-ray emitting CVs.

We additionally tested the distribution of our AGNs by comparing the [OIII]λ5007 unabsorbed flux with the soft X-ray flux following Berney et al. (2015). Specifically, we constructed a plot of the relative strength of the unabsorbed [OIII]λ5007 emission line and the soft X-ray flux. It is important to stress that the soft X-ray fluxes considered refer to a wider band (0.3−10 keV) than that in Berney et al. (2015; 2−10 keV). So, these fluxes should conservatively be considered as upper limits. We found that when we compare the soft X-ray flux with that of the [OIII]λ5007 emission line in our type 2 AGN sample, the relation between them shows a Pearson correlation coefficient of RPear = 0.39 and p-value = 0.13, with a standard deviation of ~0.50 dex. The plot of the relation is shown in Fig. 12 left panel, with the best fit model (slope 0.49 ± 0.09, orange dashed line) and the locus for which the parameter T is equal to 1 (red continuous line). In this panel, we also show the best fit model for our type 1 AGNs, as a dash-dot line (with slope of 0.60 ± 0.09, in green). We found that for this sample the Pearson correlation coefficient is RPear = 0.37 with a p-value = 0.06 and a standard deviation of ~0.55 dex. The dotted line (in pink) in both panels indicates the Berney et al. (2015) relation for the ratio between the soft X-ray flux and the [OIII]λ5007 unabsorbed flux. Finally, in the right panel we compare the soft X-ray flux with that of the [OIII]λ5007 emission line in our entire AGN sample (type 1 and type 2), with the best fit as a dashed line (with slope 0.52 ± 0.03, in grey), finding a Pearson correlation coefficient of RPear = 0.41 with a p-value = 0.007 and a standard deviation of ~0.53 dex. We can see that the Berney et al. (2015) relation has a steeper slope than our fits. This is possibly due to the range of the soft X-ray band we considered. Nevertheless, the results agree with those of Berney et al. (2015), suggesting to us that any correlation between these quantities, if present, is marginal at best.

5. Conclusions

In the present work, we identified and characterised 50 X-ray emitting objects with unknown or poorly explored natures, and which are listed in the 2PBC Swift survey. This was accomplished with the use of seven telescopes of different apertures, from 1.5 to 3.58 m, and of archival data from one spectroscopic survey.

Our main results are as follows.

  • The majority of identifications are made up of extragalactic objects. Most of them are AGNs (26 are Seyferts of type 1, 15 cases are Seyferts of type 2, one is a LINER, one is a XBONG and one is a QSO of type 1) and one is a starburst galaxy. This confirms the trend of our previous findings that optical spectroscopy eminently enables the identification of hard X-ray emitting AGNs.

  • In this work, we reported 30 new redshifts, 13 confirmations and two more accurate redshift values. All AGNs lie at redshift z≲ 0.3 except one of them, 2PBC J2010.3−2522, which is a QSO at z~ 0.8. For most type 1 AGNs we estimated the BLR size, velocity, and the central BH mass as well as their Eddington ratio, and for type 2 AGNs we studied the local absorption.

  • Our sample is thus dominated by extragalactic objects; among them we highlighted some peculiar objects, such as one galaxy displaying LINER features, one starburst galaxy, one XBONG, and two “naked” Seyfert 2 AGNs in the Compton thin regime.

  • Five objects in our sample are galactic sources: one is probably a distant LXRB, three are CVs of magnetic nature, and one is an active star.

This confirms the importance of this identification work on catalogued but unidentified high-energy sources, also because peculiar objects can be found within the considered samples (see MI-X and PI-III).

Surveys at optical and NIR wavelengths, such as the Vista Variables in the Vía Láctea (VVV: Minniti et al. 2010; Saito et al. 2012) public NIR survey, will permit the identification of variable sources in the fields of the objects detected in published and forthcoming hard X-ray catalogues. This will facilitate the detection of putative NIR counterparts for these high-energy sources by means of accurate positional information and/or variability studies. In turn, this facilitates the detection of transient and persistent hard X-ray sources, especially in crowded fields such as those along the Galactic Plane and Bulge (Rojas et al. 2012a,b, 2013; Masetti et al. 2016; Saito et al. 2016).

Furthermore, the NuSTAR satellite (Harrison et al. 2013) with its high spatial resolution can allow the identification of blended sources such as the case of the object 2PBC J1548.5-3208 in our sample. Furthermore, thanks to its high energy resolution, NuSTAR can allow us to identify the different hard X-ray contributions of two sources in the same field, for example in the case of 2PBC J0819.2−2508, as demostrated by Koss et al. (2016b) with Swift J2028.5+2543.

2

Agenzia spaziale italiana Science Data Center (ASDC): http://asdc.asi.it

4

IRAF is the Image Reduction and Analysis Facility made available to the astronomical community by the National Optical Astronomy Observatories, which are operated by AURA, Inc., under contract with the US National Science Foundation. It is available at http://iraf.noao.edu/

Acknowledgments

We thank Silvia Galleti for Service Mode observations at the Loiano telescope, and Roberto Gualandi, Ivan Bruni, and Antonio De Blasi for night assistance; Aldo Fiorenzano, Vania Lorenzi, and Walter Boschin for Service Mode observations at the TNG; Manuel Hernández, Rodrigo Hernández, and Jacqueline Seron for Service Mode observations at the CTIO telescope and Fred Walter for coordinating them. We thank the anonymous referee for useful comments and suggestions. We also acknowledge the use of public data from the Swift data archive. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester, the ASI Science Data Center Multimission Archive, of the NASA Astrophysics Data System Abstract Service, the NASA/IPAC Extragalactic Database (NED), of the NASA/IPAC Infrared Science Archive, which are operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration, and of data obtained from the High Energy Astrophysics Science Archive Research Center (HEASARC), provided by NASA’s GSFC. This publication made use of data products from the Two Micron All Sky Survey (2MASS), which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. This research has also made use of data extracted from the Sloan Digitized Sky Survey archive; the SIMBAD database operated at CDS, Strasbourg, France, and of the HyperLeda catalogue operated at the Observatoire de Lyon, France. Alejandra Rojas acknowledges financial support from Universidad Andrés Bello. Lorenzo Morelli acknowledges financial support from Padova University grant CPS0204. The IASF and IAPS co-authors acknowledge support from ASI/INAF agreement 2013.025.R0. Gaspar Galaz acknowledges the support of Basal center for Astrophysics and Technologies (CATA), PFB-06.

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Appendix A: Finding charts of the new soft X-ray identifications

thumbnail Fig. A.1

Soft X-ray images (0.3–10 keV) of the fields of 2PBC hard X-ray sources selected in this paper for optical spectroscopic follow-up. The object name is indicated in each panel. The BAT error circle is shown in blue and the new soft X-ray counterpart identifications for which the optical spectra were acquired are indicated with black arrows. In all cases, the objects selected as soft X-ray counterparts are the only ones with emission above 3 keV or with emission at 3σ considering the entire band of 0.3−10 keV. Field sizes are 20′× 20′. Nearly all images were extracted from the ASDC XRT database. In all cases, north is up and east to the left.

thumbnail Fig. A.1

continued. The soft X-ray image of the field of 2PBC J1419.2+6804 was obtained from an XMM-Newton observation downloaded from the HEASARC database. In the field of the object 2PBC J0838.7+2612, there is another X-ray emitting object inside the 99% BAT error circle labelled with red tick marks.

thumbnail Fig. A.1

continued.

thumbnail Fig. A.2

XRT soft X-ray images (0.3–10 keV) of the fields of 2PBC sources for which the corresponding soft X-ray counterpart is outside the 90% error BAT circle (continuous circle, in blue) and inside or at the edge of the 99% BAT error circle, indicated with dashed blue line. The counterpart identifications for which the optical spectra were acquired are indicated with arrows. Field sizes are 20′× 20′. In all cases, north is up and east to the left.

thumbnail Fig. A.3

XRT soft X-ray observation of the field of 2PBC J1020.5−0235. The 90% BAT error circle is shown as a continuous blue line while the 99% error circle is indicated with a dashed blue line. The object for which optical spectroscopy was acquired is indicated with an arrow. Information on object B is presented in PIII. Field size is 20′× 20′. North is up and east to the left.

thumbnail Fig. A.4

Soft X-ray observations (0.3–10 keV) of the field of two 2PBC sources for which the positions of the soft X-ray counterparts proposed in this work do not coincide with those reported in the Cusumano et al. (2010) and/or Baumgartner et al. (2013) catalogues (see text). The 90% BAT error circle is shown in blue and our proposed soft X-ray counterpart identifications, for which the optical spectra were acquired, are indicated with arrows. Field sizes are 20′× 20′. In all cases, north is up and east to the left.

Appendix B: Extra X-ray emitting object in the field of 2PBC J0838.7+2612

As reported in the main text, we note that in the field of 2PBC J0838.7+2612 there is another object inside the 99% BAT error circle (labelled with red tick marks in Fig. A.3). This object has emission above 3 keV and is actually positionally coincident with two optical sources.

The spectra of these two objects were acquired on March 27, 2014 (UT start times at 22:29 and 22:49; exposure time of2 × 1200 s) with the TNG Telescope in La Palma, Spain and are presented in Fig. B.1; the information about the main optical properties of the LINER is reported in Table B.1.

We found that the pair of objects is composed of two galaxies at the same redshift z = 0.048 ± 0.001 (DL = 229.7 Mpc). One of them is a LINER and the other one is a normal galaxy; thus, we conclude that the LINER could be mainly responsible for the soft X-ray emission. We thus cannot exclude that the hard X-ray emission detected by BAT and labelled as 2PBC J0838.7+2612 in Cusumano et al. (2010) could partly be due to the LINER in this galaxy pair, albeit it nominally lies outside the 90% error circle of the BAT source.

Table B.1

Main results for the two additional X-ray emission objects observed in the field of 2PBC J0838.7+2612.

thumbnail Fig. B.1

Optical spectra of the putative X-ray emitting objects in the field of 2PBC J0838.7+2612, both at the same redshift z = 0.048.

Appendix C: Information of the 54-month Palermo Swift-BAT catalogue from the literature and the Simbad database

Table C.1

Additional classification of 38 objects listed in the Cusumano et al. (2010).

Appendix D: Additional tables

Table D.1

Synoptic table containing the main results for the 26 low-z broad emission-line AGNs identified or observed in the present sample of 2PBC sources.

Table D.2

Synoptic table containing the main results for the 18 narrow emission-line AGNs observed in the present sample of 2PBC sources.

Table D.3

BLR gas velocities (in km s-1), central black hole masses (in units of 108M⊙), Eddington luminosity, and apparent Eddington ratios for broad line AGNs discussed in this paper.

All Tables

Table 1

Observational information of finding charts of the soft X-ray counterpart identifications to the present sample of 2PBC sources.

In the text
Table 2

Log of the spectroscopic observations presented in this paper (see text for details).

In the text
Table 3

Instrumental setups of the telescopes used to gather the observations presented in this paper.

In the text
Table 4

Main results for the probably X-ray binary identified in the present sample of 2PBC sources.

In the text
Table 5

Synoptic table containing the main results concerning the three CVs identified in the present sample of 2PBC sources.

In the text
Table 6

Main observational results for the active star as identified in the present sample of 2PBC sources.

In the text
Table 7

Synoptic table containing the main results for the QSO identified in the present sample of 2PBC sources.

In the text
Table B.1

Main results for the two additional X-ray emission objects observed in the field of 2PBC J0838.7+2612.

In the text
Table C.1

Additional classification of 38 objects listed in the Cusumano et al. (2010).

In the text
Table D.1

Synoptic table containing the main results for the 26 low-z broad emission-line AGNs identified or observed in the present sample of 2PBC sources.

In the text
Table D.2

Synoptic table containing the main results for the 18 narrow emission-line AGNs observed in the present sample of 2PBC sources.

In the text
Table D.3

BLR gas velocities (in km s-1), central black hole masses (in units of 108M⊙), Eddington luminosity, and apparent Eddington ratios for broad line AGNs discussed in this paper.

In the text

All Figures

thumbnail Fig. 1

Spectrum (not corrected for the intervening Galactic absorption) of the optical counterpart of the probably X-ray binary belonging to the sample of BAT sources presented in this paper, 2PBC J1911.4+1412. For this spectrum, the main spectral features are labeled and ⊕ indicates telluric absorptions.
In the text
thumbnail Fig. 2

Spectra (not corrected for the intervening Galactic absorption) of the optical counterparts of the three CVs belonging to the sample of BAT sources presented in this paper. For each spectrum, the main spectral features are labelled and ⊕ indicates telluric absorptions.
In the text
thumbnail Fig. 3

Spectra (not corrected for the intervening Galactic absorption) of the optical spectra acquired for 2PBC J0620.8−2932, the optical counterpart of the active star belonging to the sample of BAT sources presented in this paper. For the spectrum, the main spectral features are labelled and ⊕ indicates telluric absorptions.
In the text
thumbnail Fig. 4

Spectrum (not corrected for the intervening Galactic absorption) of the optical counterpart of the QSO belonging to the sample of BAT sources presented in this paper, 2PBC J2010.3-2522. For this spectrum, the main spectral features are labelled and ⊕ indicates telluric absorptions.
In the text
thumbnail Fig. 5

Optical spectra of type 1 AGNs presented in this work (not corrected for the intervening galactic absorption). For each spectrum, the main spectral features are labelled and ⊕ indicates telluric absorptions.
In the text
thumbnail Fig. 5

continued.
In the text
thumbnail Fig. 6

Optical spectra of type 2 AGNs presented in this work (not corrected for the intervening galactic absorption).
In the text
thumbnail Fig. 7

Spectrum (not corrected for the intervening Galactic absorption) of the optical counterpart of the starburst galaxy belonging to the sample of BAT sources presented in this paper, 2PBC J1555.0-6225. For this spectrum, the main spectral features are labelled and ⊕ indicates telluric absorptions.
In the text
thumbnail Fig. 8

Spectrum (not corrected for the intervening Galactic absorption) of the optical counterpart of the LINER belonging to the sample of BAT sources presented in this paper, 2PBC J2238.9+4050. For this spectrum, the main spectral features are labelled and ⊕ indicates telluric absorptions.
In the text
thumbnail Fig. 9

Spectrum (not corrected for the intervening Galactic absorption) of the optical counterpart of the XBONG belonging to the sample of BAT sources presented in this paper, 2PBC J1332.1-7751. For this spectrum, the main spectral features are labelled and ⊕ indicates telluric absorptions.
In the text
thumbnail Fig. 10

Histogram, subdivided into source types, showing how the initial percentage of objects in the 54 months Palermo Swift-BAT catalogue (in green) has changed by means of the identifications performed by other authors (in grey) and by our group in PI-PIII and in this work (R16) (in yellow).
In the text
thumbnail Fig. 11

Histograms showing the redshift distribution of Swift-BAT objects analysed and classified as AGNs in this sample (above) and those in the previous works of PI-III (below).
In the text
thumbnail Fig. 12

Distribution of the [OIII]λ5007 to 0.3−10 keV ratio for the AGNs of our sample and corresponding best-fit correlations (see text for details). Red continuous line represents the locus for which the parameter T is equal to 1 and the dotted pink line indicates the Berney et al. (2015) relation for the ratio between the soft X-ray flux and the [OIII]λ5007 unabsorbed flux.
In the text
thumbnail Fig. A.1

Soft X-ray images (0.3–10 keV) of the fields of 2PBC hard X-ray sources selected in this paper for optical spectroscopic follow-up. The object name is indicated in each panel. The BAT error circle is shown in blue and the new soft X-ray counterpart identifications for which the optical spectra were acquired are indicated with black arrows. In all cases, the objects selected as soft X-ray counterparts are the only ones with emission above 3 keV or with emission at 3σ considering the entire band of 0.3−10 keV. Field sizes are 20′× 20′. Nearly all images were extracted from the ASDC XRT database. In all cases, north is up and east to the left.
In the text
thumbnail Fig. A.1

continued. The soft X-ray image of the field of 2PBC J1419.2+6804 was obtained from an XMM-Newton observation downloaded from the HEASARC database. In the field of the object 2PBC J0838.7+2612, there is another X-ray emitting object inside the 99% BAT error circle labelled with red tick marks.
In the text
thumbnail Fig. A.1

continued.
In the text
thumbnail Fig. A.2

XRT soft X-ray images (0.3–10 keV) of the fields of 2PBC sources for which the corresponding soft X-ray counterpart is outside the 90% error BAT circle (continuous circle, in blue) and inside or at the edge of the 99% BAT error circle, indicated with dashed blue line. The counterpart identifications for which the optical spectra were acquired are indicated with arrows. Field sizes are 20′× 20′. In all cases, north is up and east to the left.
In the text
thumbnail Fig. A.3

XRT soft X-ray observation of the field of 2PBC J1020.5−0235. The 90% BAT error circle is shown as a continuous blue line while the 99% error circle is indicated with a dashed blue line. The object for which optical spectroscopy was acquired is indicated with an arrow. Information on object B is presented in PIII. Field size is 20′× 20′. North is up and east to the left.
In the text
thumbnail Fig. A.4

Soft X-ray observations (0.3–10 keV) of the field of two 2PBC sources for which the positions of the soft X-ray counterparts proposed in this work do not coincide with those reported in the Cusumano et al. (2010) and/or Baumgartner et al. (2013) catalogues (see text). The 90% BAT error circle is shown in blue and our proposed soft X-ray counterpart identifications, for which the optical spectra were acquired, are indicated with arrows. Field sizes are 20′× 20′. In all cases, north is up and east to the left.
In the text
thumbnail Fig. B.1

Optical spectra of the putative X-ray emitting objects in the field of 2PBC J0838.7+2612, both at the same redshift z = 0.048.
In the text

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