EDP Sciences
Free Access
Issue
A&A
Volume 561, January 2014
Article Number A67
Number of page(s) 17
Section Catalogs and data
DOI https://doi.org/10.1051/0004-6361/201322409
Published online 03 January 2014

© ESO, 2014

1. Introduction

A critically important region of the astrophysical spectrum is the hard X-ray band, from 15 keV to 200 keV; this band is being explored in detail by two satellites, INTEGRAL (Winkler et al. 2003) and Swift (Gehrels et al. 2004), which carry on board the high-energy instruments IBIS (Ubertini et al. 2003) and BAT (Barthelmy 2004), respectively. These spacecrafts permit the study of a variety of processes that take place in this observational window, thus providing a deeper look into the physics of hard X-ray sources.

Both instruments have detected a large number of known and new objects, discovered new classes of sources, and allowed us find and study highly absorbed objects. In particular, the nature of many sources detected above 20 keV by the two satellites is often unknown, because the sources are optically unclassified and their types are only inferred based on a few available X-ray or radio observations.

Optical follow-up observations of these objects is therefore mandatory. In particular, the optical spectra can provide not only an accurate source classification, but also fundamental parameters that together with multiwaveband studies, for example in the soft X-ray band, can determine the stellar population properties (Morelli et al. 2013), giving important information on these newly detected objects.

In this paper we continue the identification work on Swift/BAT sources that was started seven years ago, which has allowed the identification of about 60 objects through optical spectroscopy up to now (Landi et al. 2007; Parisi et al. 2009, 2012). In this work we focus on the optical follow-up of a number of objects with unknown classifications and/or redshifts that are reported in the 54-month Swift/BAT survey catalogue (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 73 objects (one BAT source has three possible optical counterparts) that either had no optical identification, had not been deeply studied before, or were without published optical spectra. For all these sources, we first analysed the available X-ray data to reduce the source positional uncertainty from arcmin- to arcsec-sized radii. Within the reduced X-ray error boxes, we then identified the putative optical counterpart/s to the BAT object and then performed optical spectroscopic follow-up work. Following the method applied by Masetti et al. (2004, 2006a–d, 2008, 2009, 2010, 2012, 2013) and Parisi et al. (2009, 2012), we determined the nature of all selected objects, estimating also the central black hole mass for type 1 AGNs.

The paper is structured as follows: in Sect. 2, we provide information on the X-ray data reduction to obtain the X-ray coordinates of the likely counterparts; in Sect. 3, we describe the optical observations, the telescope employed, and provide information on the optical data reduction method used. Section 4 reports and discusses the main optical results (line fluxes, distances, Galactic and local extinction, central black hole masses, etc.). In Sect. 5, we discuss some peculiar sources, and finally in Sect. 6 we summarize the main conclusions of our work.

Table 1

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

2. X-ray astrometry

In this section, we provide information on the search for the soft X-ray counterparts of the BAT objects. To obtain the soft X-ray coordinates, we cross-correlated the BAT positions with those in the catalogues of soft (<10 keV) X-ray sources and/or analysed archival observations. More specifically, for the present sample, we selected BAT objects that have, within their BAT error box, source detections by either ROSAT (Voges et al. 1999), Swift/XRT, or Chandra1. This approach was proven by Stephen et al. (2006) to be very effective in associating hard X-ray sources with a softer X-ray counterpart with a high degree of probability, which in turn drastically reduces their positional error circles to a few arcsec in radius, thus shrinking the search area by a factor of ~104.

For 73 of the 75 objects studied in this work, we used X-ray data acquired with the X-ray Telescope (XRT, 0.3–10 keV, Burrows et al. 2004) on board the Swift satellite. The XRT data were reduced using the XRTDAS standard data pipeline package (xrtpipeline v. 0.12.6), to produce screened event files. All data were extracted in photon-counting (PC) mode (Hill et al. 2004), only adopting the standard grade filtering (0–12 for PC) according to the XRT nomenclature. Depending on the source nature (bright or faint), we either used the longest exposure or coadded multiple observations to enhance the signal-to-noise ratio (S/N). For each BAT detection, we then analysed the 3–10 keV image of interest with XIMAGE v. 4.5.1 (single or added over more XRT pointings) to search for sources detected (at a confidence level >3σ) within the 90% Swift/BAT error circles; this 3–10 keV image choice ensured that we selected the hardest sources, hence the most likely counterparts to the BAT objects. We estimated the X-ray positions and relative uncertainties using the task xrtcentroid v.0.2.9 (coordinates of the most likely counterparts are reported in Table 1). For the remaining two sources we used the positional information from the ROSAT Bright all-sky survey (Voges et al. 1999) and Chandra archival data, respectively (see Table 1). The Chandra data were reduced using the CIAO v4.5 software with the calibration database CALDB v4.5.6, provided by the Chandra X-ray Center and following the science threads listed on the CIAO website2. The resulting X-ray coordinates are all reported in Table 1 (second and third columns) together with their relative uncertainties (fourth column).

For six sources the association process has been more complicated than in the other cases: indeed, the putative counterparts of PBC J0030.5−5902, PBC J0243.9+5323, PBC J0818.5−1420, and PBC J0855.8−2855 are all outside the 90%, but inside the 99% BAT error circle; since no other object is detected inside the 90% positional uncertainty we assume these associations to be correct. For PBC J1020.5−0235 no soft X-ray source is detected in the 90% and 99% BAT error circles. We searched for possible associations in the recent 70-month BAT Catalogue (Baumgartner et al. 2013), finding two possible sources (SWIFT J1020.5−0237A and SWIFT J1020.5−0237B) with similar 14–195 keV X-ray fluxes. Taking into account their confidence levels in the energy range 3–10 keV and choosing the hardest one, it is more reasonable to associate the 54-month BAT source PBC J1020.5−0235 with SWIFT J1020.5−0237B, even though we cannot exclude a possible contribution from SWIFT J1020.5−0237A; we have therefore performed optical spectroscopy of the suggested counterpart, but have highlighted this source in Table 1 to recall that this is the only case where no straightforward X-ray association has been found. For PBC J1540.3+1415, two soft X-ray sources lie within the BAT error circle, one of which has two optical counterparts (see Table 1, where the three associations are reported as PBC J1540.3+1415-1, PBC J1540.3+1415-2, and PBC J1540.3+1415-3). In this case, the association of the real optical counterpart is not trivial (see Sect. 4.2.2).

Because we used different satellites to pinpoint the soft X-ray objects associated with the chosen BAT sources, we estimated the probability that a soft X-ray source detected by XRT, Chandra, or ROSAT is associated by chance with a BAT hard X-ray object. For associations of BAT sources with XRT and Chandra objects, we used the method of Tomsick et al. (2012); assuming an average 2–10 keV flux of 10-12 erg s-1 cm-2 and a mean BAT error radius of 4.5 arcmin, we estimated a probability of 2% of spurious association. For the single BAT source association with a ROSAT object we followed the method of Stephen et al. (2006), finding a probability of 1%, similar to the value found for XRT and Chandra. This value is also similar to that found by Stephen et al. (2006) for the associations of INTEGRAL sources and ROSAT bright objects. This means that our associations can be considered reliable, independently of which of the three soft X-ray satellites we used.

Although the error box of the ROSAT object associated with PBC J0325.6−0820 is substantially larger than that of XRT or Chandra, it does not affect our source association since this source is outside the Galactic plane and is associated with only one optical object in its error box.

3. Optical spectroscopy

In this section we describe the optical follow-up studies that we performed for all 75 objects. In Table 1 we list the optical coordinates of all likely counterparts as obtained from the 2MASS catalogue3 (Skrutskie et al. 2006), except for one object whose position was extracted from the USNO-A2.0 catalogue (Monet et al. 2003).

The detailed log of all optical measurements is also reported in Table 1: we list in Col. 7 the telescope and instrument used for the observation, while the characteristics of each spectrograph are given in Cols. 8 and 9. Column 10 provides the observation date and the UT time at mid-exposure, while Col. 11 reports the exposure times and the number of spectral pointings.

The following telescopes were used for the optical spectroscopic study presented here:

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

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

  • the 1.82 m Copernicus Telescope of the Astronomical Observatory of Asiago, Italy;

  • the 2.1 m telescope of the Observatorio Astrónomico Nacional in San Pedro Martir, Mexico.

The data were reduced with the standard procedure (optimal extraction, Horne 1986) using IRAF4. Calibration frames (flat fields and bias) were taken on the day preceding or following the observing night. The wavelength calibration was obtained using lamp spectra acquired soon after each on-target spectroscopic acquisition. The uncertainty in the calibration was ~0.5 Å in all cases; this was checked using the positions of background night-sky lines. Flux calibration was performed using catalogued spectrophotometric standards. Objects with more than one observation had their spectra stacked to increase the S/N.

thumbnail Fig. 1

Spectra (not corrected for the intervening Galactic absorption) of the optical counterparts of the six CVs belonging to the sample of BAT sources. For each spectrum the main spectral features are labelled.

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The identification and classification approach we adopted in analysing of the optical spectra is the following: for the emission-line AGN classification, we used the criteria of Veilleux & Osterbrock (1987) and 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), H ii regions and transition objects (LINERs whose integrated spectra are diluted or contaminated by neighbouring H ii regions, Ho et al. 1997). In the LINER class, some lines ([Oii]λ3723, [Oi]λ6300, and [Nii]λ6584) are stronger than in typical Seyfert 2 galaxies, the permitted emission-line luminosities are weak, and the emission-line widths are similar to those of type 2 AGNs. In particular, as mentioned in Ho et al. (1993), all sources with [Oii] > [Oiii], [Nii]/Hα > 0.6 and [Oi] > 1/3 [Oiii] can be considered as LINERs. For the subclass assignation to Seyfert 1 galaxies, we used the Hβ/[O iii]λ5007 line flux ratio criterion presented in Winkler et al. (1992). Moreover, the criteria of Osterbrock & Pogge (1985) allowed us to distinguish between “normal” Seyfert 1 and narrow-line Seyfert 1 (NLS1): the latter are galaxies with a full width at half-maximum (FWHM) of the Hβ line lower than 2000 km s-1, with permitted lines that are only slightly broader than their forbidden lines, with a [Oiii]λ5007/Hβ ratio <3, and finally with evident Feii and other high-ionization emission-line complexes.

We note that the spectra of all extragalactic objects are not corrected for starlight contamination (see, e.g., Ho et al. 1993, 1997), because of their limited S/N and spectral resolution. However, this does not affect our results and conclusions.

To estimate the E(B − V) local optical absorption in our AGNs sample, when possible, we first dereddened the Hα and Hβ line fluxes 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 comparing the intrinsic line ratio and corrected that for Galactic reddening using the relation for type 2 AGNs derived from Osterbrock (1989) In the above relation, Hα/Hβ is the observed Balmer decrement, (Hα/Hβ)0 is the intrinsic one (2.86), and a is a constant with a value of 2.21. For type 1 objects, where the Hα is strongly blended with the forbidden narrow [Nii] lines, it is difficult to obtain a reliable Hα/Hβ estimate. In these cases, we used the Hγ/Hβ ratio, albeit Hγ may also be blended with the [O iii]λ4363 line; the AGN reddening was evaluated using the same relation as described above but with the intrinsic (Hγ/H ratio of 0.474 and an a value of − 5.17.

To provide extra information, we also estimated the mass of the central black hole for 29 type 1 AGNs found in the sample5. The method used here follows the prescription of Wu et al. (2004) and Kaspi et al. (2000), where we used the Hβ emission line flux, corrected for the Galactic colour excess (Schlegel et al. 1998), and 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. The results are reported in Table 6 where we also list the observed BAT X-ray luminosities in the 15–150 keV band and the Eddington ratios for each AGN considered. To calculate the luminosity distances, we considered a cosmology with H0 = 70 km s-1 Mpc-1, ΩΛ = 0.7, and Ωm = 0.3 and used the cosmology calculator of Wright (2006).

The errors on black hole masses reported in Table 6 generally come from the emission line flux estimate that correspond to about 15%, and also from the scatter in the RBLR − LHβ scaling relation (Vestergaard 2004). This implies a typical error of about 50% of the value of the black hole masses.

To derive the distance of the six cataclysmic variables (CVs) in our sample, we used the distance modulus assuming an absolute magnitude MV ~  +9 and an intrinsic colour index (V − R)0 ~ 0 mag (Warner 1995). Although this method basically provides an order-of-magnitude value for the distance of these Galactic sources, our past experience (Masetti et al. 2004, 2006a–d, 2008, 2009, 2010, 2012, 2013) tells us that these estimates are in general correct to within 50% of the refined value subsequently determined with more precise approaches.

4. Optical classification

In this section we discuss the optical classifications found and highlight the most interesting or peculiar objects discovered. The R magnitudes were all extracted from the USNO-A2.0 catalogue when not otherwise stated. Of the 75 objects studied, the majority are of extragalactic nature (69 AGNs) and only a few are of Galactic origin (6 CVs).

In the optical class, six objects in the sample had the optical type previously reported in the Véron-Cetty & Véron 13th catalogue edition (hereafter V&V13, Véron-Cetty & Véron 2010, and references therein), in the SIMBAD Astronomical Database and in Halpern (2013). Despite this, we chose to report our own data of these six sources to confirm/disclaim their classification and also to provide line flux information.

Table 4

Main results obtained from the analysis of the optical spectrum of the QSO PBC J0030.5−5902.

Table 5

Main optical results concerning sources identified as cataclysmic variables (see Fig. 1).

4.1. Cataclysmic variables

Six sources in our BAT sample display emission lines of the Balmer complex (up to at least Hϵ), as well as He i and He ii, consistent with z = 0, indicating that these objects lie within our Galaxy (see Fig. 1). The analysis of their optical features indicates that all are CVs (see Table 5). Through the equivalent widths (EW) of the Hβ and He iiλ4686 lines we investigated their magnetic or non-magnetic nature. For PBC J0746.2−1610 and PBC J0927.8−6945 the He iiλ4686/Hβ EW ratio is higher than 0.5, and the EW of both emission lines is larger than 10 Å, implying that both objects are magnetic CVs belonging to the intermediate polar (IP) subclass (see Warner 1995, and references therein). A tentative IP classification can also be made for PBC J0325.6−0820, PBC J0706.7+0327 and PBC J0820.4−2801 since here the He iiλ4686 and the Hβ EW are also larger than 10 Å, although their ratio is lower than 0.5. For PBC J2124.5+0503 (classified as LMXB in Halpern 2013) the weakness of its emission lines (<10 Å) and the He iiλ4686/Hβ ratio lower than 0.5 imply a non magnetic nature. The Hα to Hβ flux ratio is lower than 2 for all the CVs found in this work, therefore we assume that the absorption along the line of sight is negligible in all cases. We also estimated their distances (see Sect. 3), assuming no Galactic extinction along the line of sight (see Table 5). All CVs that have a probably magnetic nature are located at a relatively far distance (>150 pc), while the only non-magnetic CV is located quite close to Earth.

thumbnail Fig. 3

Spectrum (not corrected for the intervening Galactic absorption) of the optical counterpart of the starburst galaxy belonging to the sample of BAT sources. For this spectrum the main spectral features are labelled.

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thumbnail Fig. 4

Spectra (not corrected for the intervening Galactic absorption) of the optical counterparts of the three transition objects belonging to the sample of BAT sources. For each spectrum the main spectral features are labelled.

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thumbnail Fig. 5

Spectra (not corrected for the intervening Galactic absorption) of the optical counterparts of the two LINERs belonging to the sample of BAT sources. For each spectrum the main spectral features are labelled.

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4.2. Extragalactic objects

The results for the extragalactic sources are reported in Tables 2 and 3, where for each source we list the Hα, Hβ, and [Oiii] fluxes, the classification, the estimated redshift, the luminosity distance given in Mpc, the Galactic colour excess and the colour excess local to the AGN host. In Table 4, we report information of the only quasi-stellar object (QSO) found in this sample. All the extragalactic optical spectra are displayed in Figs. 25.

4.2.1. Redshifts

We confirm the redshift estimates reported in SIMBAD and V&V13 for 29 AGNs. For the remaining 40 sources, the redshifts derived from our low-resolution optical spectra are published here for the first time (see Figs. 25). Redshift values are in the range 0.006–1.137, that is, all AGNs are located in the local Universe (z < 0.3) except for one source, which is at redshift 1.137.

We note that all redshifts were estimated using the [Oiii] narrow emission line, and when this line was unavailable, from other forbidden narrow emission lines or absorption features.

4.2.2. Optical class

For the first time, we provide the classification of 64 sources in the sample. For the remaining five AGNs, our results agree with the classifications listed in the literature except for one object: PBC J1002.3+0304, also known as IC 0588, is classified as a Seyfert 1 in the V&V13 catalogue, but according to our analysis it is a Seyfert 1.8/1.9. PBC J2157.4−0611 was classified by Halpern (2013) in his preliminary work as a QSO, we refine this classification, stating that it is a Seyfert 1.5.

Summarizing our extragalactic results, we found that of 69 AGNs, 36 have strong redshifted broad and narrow emission-lines that are typical of Seyfert 1 galaxies, while the remaining 33 display only the strong and redshifted narrow emission-lines that are indicative of a type 2 AGN nature (for the subclass assignment see Tables 2 and 3).

Note that among broad-line AGNs, only three are pure type 1 objects, including the QSO at z > 1. One is an NLS1, while the remaining objects are all of intermediate type, nine belong to Seyfert 1.8–1.9, that is, they are more similar to type 2 AGNs because of the progressive disappearance of broad line regions. PBC J2035.2+2604 is the only NLS1 found; it has an optical counterpart in the USNO-A2.0 catalogue (USNO-A2.0 1125−16409384) with magnitude R = 13.8, a redshift of 0.05, and significant optical extinction (E(B − V) = 0.834 mag). NLS1 are rare among galaxies detected above 20 keV since their fraction is typically 5% of type 1 (Panessa et al. 2011) and 2% of all AGNs; this perfectly matches our findings.

Of the 33 type 2 AGNs, 22 are Seyfert 2 galaxies, 2 are LINERs, one is a starburst galaxy, 3 are transition objects, and 2 are X-ray bright, optically normal galaxies (XBONG, Comastri et al. 2002). PBC J0747.7−7326 and PBC J1321.1+0858 are classified as LINERs. The first object is a pure LINER, according to Ho et al. (1997); the second source is a less clear case since in the Kauffmann et al. (2003) diagram it is placed in the LINERs region, but its nature is ambiguous according to the Ho et al. (1997) diagrams. We decided to classify this source as a LINER because it has some features typical of this class. Clear detection of both these objects at high energies points to an AGN (and not a burst of star formation) as the source that excites the ionized gas in these galaxies.

PBC J1231.4+5759 is classified as a starburst galaxy. In the diagrams of Ho et al. (1997) it is placed in the left part with a low [O iii]/Hβ ratio and a very low value [N ii]/Hα ratio. It is a blue compact galaxy also known as NGC 4500. The source is characterized by intense far-infrared and UV emission typical of a recent burst of star formation. The soft X-ray counterpart of this BAT source is quite absorbed, which suggests that a low-luminosity AGN maybe hidden behind the strong signature of the starburst emission.

PBC J0859.5+4457, PBC J1540.3+1415-1 and PBC J1540.3+1415-2 are transition objects. These sources are likely LINERs whose integrated spectra are diluted or contaminated by neighbouring H ii regions. Recent radio and mostly X-ray observations have suggested that these objects probably harbour low-luminosity AGNs (Ho 2008); they could represent a late stage of AGN activity when the accretion rate is lower than at the Seyfert stage. The detection of two such objects among BAT sources furthermore supports the assumption that there are low-luminosity AGN in transition galaxies. PBC J1540.3+1415-1 and PBC J1540.3+1415-2 have about the same redshift (~0.05) and are probably in interaction; only PBC J1540.3+1415-1 has some associated radio emission, which makes it a more likely counterpart for the BAT source. We recall that the other candidate counterpart for the same BAT source is PBC J1540.3+1415-3, which is a Seyfert 1.2 at a redshift of 0.12 that displays strong radio emission (see the following section). Using the statistical method of Tomsick et al. (2012), we estimated the probability for each of these three sources to be contained by chance within the BAT hard X-ray error box and found that PBC J1540.3+1415-1 and PBC J1540.3+1415-2 have a probability of ~0.14, with a 2–10 keV flux of 0.1 × 10-12 erg s-1 cm-2 (power law with Γ = 1.8), while PBC J1540.3+1415-3 has a probability lower than 0.02, with a 2–10 keV flux of 1.4 × 10-12 erg s-1 cm-2 (power law with Γ = 1.8), which makes it the most likely soft X-ray counterpart. On the other hand, an analysis of the XRT image at energies above 3 keV suggests that the X-ray source associated with optical objects PBC J1540.3+1415-1 and PBC J1540.3+1415-2 is the harder of the two objects detected inside the BAT positional uncertainty and is also the only one still visible above 6 keV. PBC J1540.3+1415 is clearly a complicated object that deserves more detailed studies, possibly in multiwavebands if one aims at understanding which source is responsible for the hard X-ray emission and ultimately the real nature of the BAT object.

Finally, PBC J1034.2+7301 and PBC J1355.5+3523 are both classified as XBONGs, which are X-ray bright galactic nuclei without emission lines in their optical spectra. While the presence of weak Hα and [O iii] emission lines in the first source makes it more similar to a Seyfert 2, the second shows no evidence of emission lines in its optical spectrum. From a quick analysis of their X-ray spectra, both objects seem to be absorbed in X-rays with column densities above 1022 cm-2, which suggests that they may be AGNs whose obscuration covers almost 4π of the nuclear source (see Malizia et al. 2012, for details). Additional investigation in X-rays, but also in other wavebands, can help in clarifying the nature of both sources.

Table 6

Broad-line region gas velocities, central black-hole masses and apparent Eddington ratios for 29 broad-line AGNs.

5. Additional comments

We note that a number of AGNs in our sample display evidence of interaction or clustering: they either belong to a small group of objects (such as a galaxy pair or triple or a compact group) or are being disturbed by nearby galaxies. In addition to PBC J1540.3+1415-1 and PBC J1540.3+1415-2 that were discussed in the previous section, the counterpart of PBC J0116.3+3102 (also known as NGC 452) also makes a pair with NGC 444 around 131.8 kpc away.

Other examples of galaxy pairs discovered in the present sample are NGC 5100 NED 02 and MCG +09−19−015 NED 02, the optical counterparts of PBC J1321.1+0858 and PBC J1115.3+5425; these two objects are associated with a nearby galaxy with similar redshift that is located at 23.7 kpc and 113.7 kpc distance, respectively. The AGNs associated with PBC J0223.4+4549 belong instead to a triple system (VZW232) according to NED, although no redshift is available for the other two members of the group. Finally, the active galaxy associated with PBC J1254.8−2655 belongs to a group of objects and more specifically is listed as AM 1252−264 in the catalogue of southern peculiar galaxies and associations assembled by Arp & Madore (1987); interestingly, another member of this group, 2MASX J12544294−2657107, is also visible in X-rays, but is softer and less luminous than the BAT counterpart. We also have a few cases where the galaxy associated with the high-energy source seems to interact with a nearby companion; one such clear case is that of the counterpart of PBC J2035.2+2604.

Overall, we estimate that the fraction of AGNs in the present sample that display evidence of interaction or clustering is around 20%, which is a value very close to that found by Koss et al. (2010) in a more accurate analysis of a set of known active galaxies detected by BAT. This high rate of apparent mergers suggests that AGN activity and merging are critically linked for the moderate-luminosity AGNs in the BAT sample. We also note that a few sources display the properties of radio-loud AGN: at least four (the counterparts of PBC J0459.8+2705, PBC J0602.5+6522, PBC J0654.5+0703, and PBC J1540.3+1415 (specifically object N.3)) display strong radio emission and are characterized by a flat spectrum at these frequencies; their radio morphology is that of a compact source. Two remaining objects are clearly radio galaxies. PBC J0709.2−3601, also known as PKS 0707−35, is a complex and extended source, an edge-brightened double, with two compact outer components of which the south-east one is stronger; two weaker inner components extend away from the axis that joins the outer components and give a slight twist to the structure (Jones & McAdam 1992). PBC J0950.0+7315, also named 4C 73.08, is also a giant double-lobed radio-galaxy, with 13 arcmin angular size between hotspots and a clear FRII morphology (Aretxaga et al. 2001; Hardcastle & Worrall 1999).

We conclude that around 9% of the AGNs in the present sample are radio loud. This percentage is close to what is generally observed among active galaxies. Moreover, we note that the black hole masses listed in Table 6 cover quite a wide range from a few 106 to around 7 × 108  M; the Eddington ratio also spans from 0.001 to 0.2, suggesting that BAT AGNs tend to cover a broad range in the parameter space.

Finally, even if this matter is beyond the scope of this paper because it would need a more careful selection of the sample of sources, we checked possible correlations between optical emission line fluxes (Hα, Hβ and Oiii) and hard X-ray flux (15–150 keV). Using the least-squares bisector method (Isobe et al. 1986), we found no evident correlation between the above optical quantities and the 15–150 keV hard X-ray flux, obtaining correlation coefficients R2 < 0.15, as previously suggested by Winter et al. (2010). We do not expect a correlation between line strength and source class, because the latter is defined through the ratio of lines and through their width, and not through their strength. Moreover, to the best of our knowledge, there is no known correlation between line strength and redshift, because AGNs are not standard candles; also, the fact that almost all sources in our sample lie at low z (<0.2) does not grant a wide enough baseline for the redshift range to properly explore this point.

6. Conclusions

We have either provided for the first time or confirmed or corrected the optical spectroscopic identifications of 73 sources belonging to the Palermo 54-month Swift/BAT catalogue (Cusumano et al. 2010).

This was achieved by performing a multisite observational campaign in Europe and Central and South America. Only for PBC J1540.3+1415 we found more than one optical counterpart, specifically, three objects that most likely emit at high energies.

We found that our sample is dominated by extragalactic objects with only six sources of galactic nature. The extragalactic sample is composed of 69 AGNs (35 of type 1, 33 of type 2, and 1 QSO), with redshifts between 0.006 and 1.137. Among them we highlighted some peculiar objects, such as two galaxies displaying LINER features, one starburst galaxy, two XBONGs, three transition objects, and one object with the properties of an NLS1. For 29 type 1 AGNs we estimated the BLR size, velocity, and the central black-hole mass as well as their Eddington ratio. The AGNs sample presented in this work shows a large portion (around 20%) of objects that display evidence of interaction or clustering, while only 9% show indications of being radio loud.

For the six galactic sources we found two that are most likely magnetic CVs belonging to the IP subclass, three objects with a tentative IP classification, and only one source with non-magnetic properties. Finally, we determined possible correlations between optical emission line fluxes (Hα, Hβ and Oiii) and hard X-ray flux (15–150 keV), as well as correlations for source class or redshift, but no evident correlation was found.

These results point out the efficiency of using catalogue cross-correlation and/or follow-up X-ray observations from satellites such as Chandra, ROSAT, or Swift that are capable of providing arcsec-sized error boxes followed by optical spectroscopy to determine the actual nature of still unidentified BAT sources.


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/

5

We were unable to estimate the mass of the central black hole of PBC J0116.3+3102, PBC J0602.5+6522, and PBC J1926.6+4131 because they all lack the Hβ emission line, and of PBC J0000.9−0708, PBC J0917.2−6454, and PBC J1824.2+1846 because only the narrow component of the Hβ line was observed in their spectra.

Acknowledgments

We thank John Stephen for the useful comments and suggestions, Silvia Galleti for Service Mode observations at the Loiano Telescope, and Roberto Gualandi for night assistance, Giorgio Martorana for Service Mode observations at the Asiago Telescope and Luciano Traverso for coordinating them, Manuel Hernández, Rodrigo Hernández and Josè Velasquez for Service Mode observations at the CTIO telescope, and Fred Walter for coordinating them. We also acknowledge the use of public data from the Swift data archive. This research has made use of 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 6dF Galaxy Survey and the Sloan Digitized Sky Survey archives; the SIMBAD database operated at the CDS, Strasbourg, France, and of the HyperLeda catalogue operated at the Observatoire de Lyon, France. The authors acknowledge the ASI and INAF financial support via grants Nos. I/033/10/0, I/009/10/0; P.P. is supported by the INTEGRAL ASI-INAF grant No. I/033/10/0. L.M. is supported by the University of Padua through grant No. CPDR061795/06. G.G. is supported by FONDECYT 1085267. V.C. is supported by the CONACyT research grants 54480 and 151494 (Mexico). D.M. is supported by the Basal CATA PFB 06/09, and FONDAP Center for Astrophysics grant No. 15010003.

References

Online material

Table 2

Main results obtained from the analysis of the optical spectra of the 35 type 1 AGNs.

Table 3

Main results obtained from the analysis of the optical spectra of the 33 type 2 AGNs.

thumbnail Fig. 2

Spectra of the optical counterpart of AGNs presented in this work (not corrected for the intervening Galactic absorption). For each spectrum the main spectral features are labelled.

Open with DEXTER

All Tables

Table 1

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

Table 4

Main results obtained from the analysis of the optical spectrum of the QSO PBC J0030.5−5902.

Table 5

Main optical results concerning sources identified as cataclysmic variables (see Fig. 1).

Table 6

Broad-line region gas velocities, central black-hole masses and apparent Eddington ratios for 29 broad-line AGNs.

Table 2

Main results obtained from the analysis of the optical spectra of the 35 type 1 AGNs.

Table 3

Main results obtained from the analysis of the optical spectra of the 33 type 2 AGNs.

All Figures

thumbnail Fig. 1

Spectra (not corrected for the intervening Galactic absorption) of the optical counterparts of the six CVs belonging to the sample of BAT sources. For each spectrum the main spectral features are labelled.

Open with DEXTER
In the text
thumbnail Fig. 3

Spectrum (not corrected for the intervening Galactic absorption) of the optical counterpart of the starburst galaxy belonging to the sample of BAT sources. For this spectrum the main spectral features are labelled.

Open with DEXTER
In the text
thumbnail Fig. 4

Spectra (not corrected for the intervening Galactic absorption) of the optical counterparts of the three transition objects belonging to the sample of BAT sources. For each spectrum the main spectral features are labelled.

Open with DEXTER
In the text
thumbnail Fig. 5

Spectra (not corrected for the intervening Galactic absorption) of the optical counterparts of the two LINERs belonging to the sample of BAT sources. For each spectrum the main spectral features are labelled.

Open with DEXTER
In the text
thumbnail Fig. 2

Spectra of the optical counterpart of AGNs presented in this work (not corrected for the intervening Galactic absorption). For each spectrum the main spectral features are labelled.

Open with DEXTER
In the text

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