Free Access
Issue
A&A
Volume 517, July 2010
Article Number A14
Number of page(s) 9
Section Catalogs and data
DOI https://doi.org/10.1051/0004-6361/200913967
Published online 26 July 2010
A&A 517, A14 (2010)

Swift follow-up observations of 13 INTEGRAL sources

J. Rodriguez1 - J. A. Tomsick2 - A. Bodaghee2

1 - Laboratoire AIM, CEA/IRFU - CNRS/INSU - Université Paris Diderot, CEA DSM/IRFU/SAp, Centre de Saclay, 91191 Gif-sur-Yvette, France
2 - Space Sciences Laboratory, 7 Gauss Way, University of California, Berkeley, CA 94720-7450, USA

Received 24 December 2009 / Accepted 13 March 2010

Abstract
The various IBIS/ISGRI catalogues contain a large population of hard X-ray sources whose nature is still unknown. Even if the >20 keV positional uncertainty provided by ISGRI is unprecedented, it is still too large to pinpoint the counterpart at other wavelengths, which is the only secure way of obtaining a source identification. We continue the work of trying to reveal the nature of these hard X-ray sources, starting with analysis of X-ray data collected via focusing X-ray telescopes, in order to obtain arcsec accurate X-ray positions. We can then identify counterparts at infrared and optical wavelengths and try to unveil the nature of the sources. We analysed data from observations of 13 INTEGRAL sources made with the Swift satellite. The X-ray images obtained by the X-Ray Telescope instrument allowed us to find possible counterparts to the IGR sources with a positional accuracy of a few arcsec. We then browsed the online catalogues (e.g., NED, SIMBAD, 2MASS, 2MASX, USNO B1.0) to search for counterparts at other wavelengths. We also made use of the X-ray spectral parameters in trying to identify the nature of those objects. For the 13 objects, we found possible counterparts at X-ray energies and identified the IR/optical and/or UV counterparts as seen with Swift/UVOT. We also discuss the likelihood of association of the X-ray and INTEGRAL source in each case. We confirm the previously proposed classification of IGR J02524-0829 (Sey 2 AGN), J08023-6954 (RS CVn star), and J11457-1827 (Sey 1 AGN). For 7 of these sources we give the first identification of their nature: IGR J02086-1742, J12060+3818, J12070+2535, J13042-1020, and J13412+3022 are AGN, and J14488-5942 is a probable X-ray binary. For J03184-0014, although we question the association of the IGR and Swift sources, we classify the latter as an AGN. We suggest that IGR J15283-4443 is a Galactic source, but we cannot classify the source further. Finally, we question the association of IGR J11457-1827 and J23130+8608 with the X-ray sources we found, and go on to question the genuineness of the former IGR source.

Key words: astrometry - binaries: close - galaxies: Seyfert - X-rays: binaries - X-rays: galaxies

1 Introduction

The most recent version of the IBIS catalogue contains more than 700 hard X-ray sources (Bird et al. 2010). While a certain number were known as (hard) X-ray emitters prior to the launch of INTEGRAL, about half of them have been detected for the first time above 20 keV with IBIS/ISGRI (Lebrun et al. 2003). In this paper we refer to these sources as ``IGRs''[*]. Bodaghee et al. (2007) have collected known parameters (e.g., the absorption column density, $N_{\rm H}$, the pulse period for Galactic sources with X-ray pulsations, the redshift for AGN, etc.) of all sources detected by INTEGRAL during the first four years of activity. With this they could study the parameter spaces occupied by different families of sources and therefore deduce important aspects of the physics of high-energy sources. However, many of these IGRs have still not been identified, and therefore any attempt to study, understand, and model populations of high-energy sources will be incomplete. The determination of the nature of these object is therefore extremely important if one wants to have the most complete view of the content of our Galaxy and our Universe.

In this paper, we continue our work of identifying the unknown IGRs that we started soon after the discovery of the first IGRs. A first step is to provide an $\sim$arcsec position with soft X-ray telescopes such as XMM-Newton (Bodaghee et al. 2006; Rodriguez et al. 2003,2006) Chandra (Tomsick et al. 2009,2006,2008), and also Swift (Rodriguez et al. 2009a,2008,2009b). We then search for counterparts at a position consistent with the refined X-ray position of a given source. As in Rodriguez et al. (2008) and Rodriguez et al. (2009b) (Papers 1 and 2 in the remainder of this article), we report here the analysis of Swift observations (XRT imaging and spectral analysis and UVOT imaging) of 13 IGRs that still lacked precise arcsec X-ray positions at the time of the writing of the paper. We also present the identification of IR and optical counterparts obtained from online catalogues such as SIMBAD, the United States Naval Observatory (USNO), the 2 Micron All Sky Survey point source and extended source catalogues[*] (2MASS and 2MASX Skrutskie et al. 2006), and the NASA/IPAC Extragalactic Database (NED[*]). Although the presence of a bright Swift source within a given INTEGRAL error circle usually renders the association between the two sources likely, there is a slight probability that the two sources are not associated, especially in the case of dim X-ray sources. This is, also, exemplified by the few cases where several Swift sources are found within the INTEGRAL error circle. Given the wide range of association probabilities from possible associations to nearly certain associations, no general statement can be given for the probability of associations. A low Galactic latitude source will have a higher chance of spurious association than a high latitude one. For all sources, we discuss the likelihood of association between the INTEGRAL, Swift, and counterparts at other wavelengths. Dubious cases (such as multiple possible counterparts) are discussed in more detail.

We start by introducing the Swift observations, and we briefly present the data reduction techniques in Sect. 2. We then give the results of X-ray (Sect. 3) and IR/Optical/UV (Sect. 4) candidate counterparts identification. In Sect. 5 we describe the results for each source, including the results of the X-ray spectral analysis, and discuss their possible nature. We conclude the paper by summarising the results in Sect. 6.

Table 1:   Journal of the Swift observations analysed in this paper.

2 Observations and data reduction

We searched the Swift archive for observations at less than 10$^\prime $ of any IGR that has a position uncertainty greater than about 10 $^{\prime\prime}$. We excluded most of the new IGRs found in the Galactic centre by Bird et al. (2010), as too many possible X-ray counterparts can be found in the IBIS error. We also excluded sources for which a clear positional uncertainty is not given in Bird et al. (2010), because this may indicate source confusion in IBIS. We then only retained the pointings during which the XRT instrument was in photon-counting mode since it is the only mode that provides a fine position. We report on the results of the 13 sources for which we found an X-ray source within the IBIS error box. The observing log for these is reported in Table 1.

We reduced the Swift data with the HEASoft V6.7 software package and the calibration files issued on 2009 December 1 and 2009 October 7 for the XRT and UVOT instruments, respectively. The reduction procedure is identical to those presented in Papers 1 and 2. For XRT, level 2 cleaned event files were obtained with xrtpipeline with standard parameters[*].

The XRT individual pointings of a given source were then co-added with xselect. We extracted spectra and light curves with xselect from a circular region with a radius of 20 pixels centred on the best position, while we obtained the background products from a source-free circular region with a radius of 40 pixels. The presence of columns of dead pixels in the XRT meant we had to produce ``true'' exposure maps with xrtexpomap that were given as input to xrtmkarf to produce corrected ancillary response files. We rebinned the spectra to have at least 20 counts per channel, which allows for $\chi^{2}$-minimisation in the fitting with XSPEC 12.5.1. When this criterion was not achievable, the Cash statistic (hereafter C-statistic Cash 1976) was used instead.

When available, we analysed the UVOT level 2 data obtained from the Swift data archive. We first corrected the aspect for each individual UVOT exposure with the uvotskycorr tool, calculating the aspect correction via comparison to the USNO-B1.0 catalogue[*](Monet et al. 2003). Then, we summed the aspect-corrected individual exposures and individual exposure maps with uvotimsum and performed the UVOT astrometry using the summed images and exposure maps with the uvotdetect tool. The tool uvotsource was finally used to obtain the photometry of the sources. The UVOT magnitudes were estimated on a source region of 5 $^{\prime\prime}$ radius centred on the best source position obtained with uvotdetect, using a region of 20 $^{\prime\prime}$ radius free of sources as background. This tool corrects the source count rates for coincidence losses, makes an aperture correction, and converts the results to magnitudes following $m_{\rm {source}}=Z_{\rm pt}-2.5\times \log(CR)$ (Poole et al. 2008), where $Z_{\rm pt}$ is the photometric zero-point of the filter considered, and CR is the corrected source count rate. All corrections have been applied with the UVOT calibration files released on 2009 October 7. The complete description of the UVOT photometric calibration can be found in Poole et al. (2008).

3 X-ray astrometry with Swift/XRT

For each source we produced an image accumulating the maximum number of pc mode pointings available. We then searched for potential X-ray counterparts within the IBIS error box with XIMAGE. We retained only sources that had a signal-to-noise ratio (SNR) greater than 4$\sigma$ and went down to 3$\sigma$ when no significant excesses were found. We estimated the best source position and errors with xrtcentroid. The list of candidates counterpart, their number within the IBIS error box, X-ray position, and SNR are reported in Table 2.

Table 2:   List of sources for which X-ray counterparts have been found.

Table 3:   List of newly identified infrared counterparts in the 2MASS and 2MASX catalogues.

4 Counterparts at other wavelengths

We searched the NED, 2MASS, 2MASX, and the USNO-B1.0 online catalogues and UVOT images for infrared, optical, and UV counterparts within the Swift/XRT error circle of each of the potential X-ray counterpart reported in Table 2. Infrared counterparts that are newly identified from this search are reported in Table 3. The typical positional accuracy for the 2MASS sources is 0.5 $^{\prime\prime}$ (Skrutskie et al. 2006), while that of the USNO-B1.0 sources is typically 0.2 $^{\prime\prime}$ (Monet et al. 2003). The magnitudes and UV positions of the optical and UV counterparts are reported in Table 4. The USNO-B1.0 photometric accuracy is typically 0.3 mag (Monet et al. 2003). The lower limits on the UVOT magnitudes are given at the $3\sigma$ level. The UVOT positional uncertainties are dominated by a 0.5 $^{\prime\prime}$ systematic uncertainty (90% confidence) for each source.

5 X-ray spectral analysis and possible identifications of the thirteen IGRs

In this section we report the results for each source and discuss their possible nature. We also include the results of the X-ray spectral analysis of the XRT data. In all cases, we started to fit the source spectra with a simple model of an absorbed power-law. This provided an acceptable representation in the large majority of the cases. The absorption due to intervening material along the line of sight is obtained from the Leiden/Argentine/Bonn (LAB) surveys of Galactic H I in the Galaxy[*]. The LAB Survey is the most sensitive Milky Way H I survey to date, with the most extensive coverage both spatially and kinematically and an angular resolution of 0.6 degrees (Kalberla et al. 2005).

The spectral parameters we obtained are reported in Table 5. The errors on the X-ray spectral parameters (including upper limits) are given at the 90$\%$ confidence level. We also report there the extrapolated hard X-ray fluxes of these sources as measured by IBIS and obtained by extrapolating the best XRT spectrum in the same energy range. Note that while, in most cases, these fluxes are estimated over the 20-40 keV range, in some cases, they are obtained in different ranges to match published results on those sources. To estimate the luminosity of the candidate AGN we used H0=75 km s-1Mpc-1 to convert the redshift (of the suggested counterpart) to distance.

Table 4:   Magnitudes and UVOT positions of the newly identified optical and UV counterparts in the USNO-B1.0 catalogue (I, R, and B bands) and Swift/UVOT detector (V, B, U, UVW1, UVM2, and UVW2 bands).

5.1 J02086-1742

This source is one of the new IGRs reported in the most recent version of the IBIS catalogue (Bird et al. 2010), for which inspection of the Swift archives shows an observation aimed at a Swift object. Although the coordinates of Swift J0208.5-1738 and that of the IGR sources indicate an offset of about 5$^\prime $, both objects have error boxes that render their association likely. There is one single bright X-ray source with a position that is compatible with both the Swift and INTEGRAL ones indicating that the sources are the same (Table 2). The X-ray position is also at 22 $^{\prime\prime}$ from the position of 1RXS J020835.8-173950. Given the 20 $^{\prime\prime}$ error box given by Rosat, it is likely that the XRT source and the Rosat one are the same. The XRT error box contains NVSS J020834-173933 a $\sim$42 mJy 1.4 GHz radio source as reported in NED. The source is also clearly detected in the infrared, optical and UV bands (Tables 3 and 4).

The XRT spectrum is represented well by a hard power-law model with very little absorption ( $\chi^2_\nu$ = 1.05 for 41 d.o.f.) compatible with the value on the line of sight obtained from the LAB survey (Kalberla et al. 2005). The extrapolated 20-40 keV flux is compatible with the flux measured by IBIS (Bird et al. 2010), which further strengthens the associations of the X-ray source and the IBIS one. The high galactic latitude, the radio detection, and the brightness in all IR/optical and UV bands as well as the detection at soft and hard X-ray energies argue in favour of the source being an AGN. The low value of the absorption would argue for of a type 1 AGN. Masetti et al. (2010) also classify the source as a Sey 1.2 AGN through optical spectroscopy.

Table 5:   Results of the X-ray spectral analysis. Errors and upper limits are all given at the 90% level.

5.2 J02524-0829

This object was first reported by (Krivonos et al. 2007) and associated to MCG-02-08-014, a z=0.016721Sey 2 galaxy (Bikmaev et al. 2008); however, no refined X-ray position was published for this object, leaving the possibility of a chance association. We found one single X-ray source within the IBIS 3$^\prime $ error box. The XRT position (Table 2) is at 1 $^{\prime\prime}$ from the position of the Sey 2 galaxy, which confirms the association of these sources. The X-ray position is also clearly compatible with the nucleus of a spiral galaxy seen in the UVOT images (not shown). The magnitudes measured in the U-filter show some slight variability between the two pointings (it has $m_U=15.373\pm0.08$ during the second one).

The X-ray spectrum is well-fitted with an absorbed power-law ( $\chi^2_\nu$ = 1.3 dof 27 d.o.f.). The absorption significantly exceeds to the value measured on the line of sight, which indicates that the source is significantly absorbed, while the photon index, although quite poorly constrained, has a value typical of a type 2 AGN. All our results confirm that IGR J02524-0829 is associated with MCG-02-08-014, making it a Sey 2 AGN.

5.3 J03184-0014

This very poorly studied object was first reported in Bird et al. (2007), and a single Swift observation was analysed by us in Paper 2. We found a single X-ray source, however, outside the IBIS error, which led us to dismiss it as the counterpart to the IGR source (Paper 2). Here we used 3 additional Swift observations, which allowed us to obtain a total exposure on this region that is about 2.5 times longer than in Paper 2.

As previously reported, there is no X-ray source within the 4$^\prime $ IBIS error box of this source. The most significant X-ray source in this field is 4.1$^\prime $ from the best position and is consistent with the object mentioned in Paper 2, and the position we report here should be considered as the most accurate one. The IR counterpart, 2MASS J03181753-0017502 (see Paper 2) is classified as a quasar in SIMBAD. We, however, point out that this classification is based on the positional coincidence of a Quasar (dubbed SDSS J03184-0015) within the IBIS error box mentioned in Bird et al. (2007). This object is not associated to Swift and to the 2MASS source we report here either. Instead, the X-ray error box contains SDSS J031817.53-001750.0 reported as an extended source and classified as a galaxy in NED. There is no source in the USNO B1.0 catalogue within the XRT error box. There is no obvious UVOT counterpart in the two pointings during which the X-ray position in the UVOT field of view (fov). The source position is, however, close to a very bright source and confusion with this nearby object cannot be completely excluded.

The X-ray spectrum has a very low statistical quality. It is fitted with an absorbed power-law (C=23 for 27 d.o.f.), but the parameters are poorly constrained (Table 5). The parameters are compatible with any X-ray emitting source, and the extrapolated 20-40 keV flux is not compatible with the 20-40 keV flux from Bird et al. (2007).

The X-ray position of the source may indicate that it is an active galaxy, although the low statistical quality of the data leads to some caution. In Paper 2 we did not favour an association of the Swift and IGR source. The finding of a Galaxy within the Swift error box may, however, question this conclusion. In any case we tentatively classify the Swift source as an active galaxy and suggest that it could be associated to the IGR. In this case, the source is not the quasar mentioned in SIMBAD. One should also keep in mind that the IGR object could be a spurious IBIS source since it is not reported in the last IBIS catalogue of Bird et al. (2010) and Krivonos et al. (2007)[*].

5.4 J08023-6954

This source was discovered by Revnivtsev et al. (2006), who have already suggested the source was an chromospherically active star (RS CVn type). Optical spectroscopy led Masetti et al. (2008) to confirm the suspected type of this object. These authors, however, point out that this identification is only tentative since they had just a marginal, possible X-ray counterpart. Re-analysis of the Swift data shows the presence of a $3.2\sigma$ source within the error box. The X-ray position is compatible with that of the counterpart analysed by Masetti et al. (2008). The XRT error box contains IR, optical, and UV counterparts with positions that are compatible. This gives strength to the reality of the X-ray source. The UVW1 magnitudes had compatible values between the two pointings. The average of these are reported in Table 4.

The spectrum has an extremely low statistical quality (only 13 net counts), and basically has no count above 2 keV, which prevents any sound spectral analysis. That all source counts are below 2 keV indicates that this source is not highly absorbed. This and the detection of the source in the UV and optical domains may indicate a nearby source. A tentative fit of the 0.2-2 keV spectrum shows that a simple power-law with $\Gamma=2.4_{\rm -0.7}^{+0.8}$ provides a good representation (C=7.2 for 13 d.o.f.). The 2-10 keV flux is $2.6\times10^{-14}$ erg cm-2 s-1. The extrapolated 20-40 keV flux is at least a factor of 3400 lower than the flux reported by Revnivtsev et al. (2006). This source is mentioned in the online version of the Krivonos' catalogue, but not in Bird et al. (2007) and Bird et al. (2010). This may indicate a variable source that is compatible with the proposed RS CVn classification, since these sources are known to be variable. We also note that RS CVn are known X-ray emitters (e.g. Osten et al. 2007), and they can sometimes be detected at hard X-rays, as exemplified by the Swift/BAT detection of the RS CVn star II Peg (Osten et al. 2007; Barbier et al. 2005) during a flare. The detection of the latter is a short-lived event and any comparison with the INTEGRAL source may be questioned. Osten et al. (2007) report their analysis of the full flare of II Peg as seen with Swift. Hard X-ray detection of II Peg by the BAT telescope is visible over more than 8 ks, and less than 12 ks. Over this period, the 10-200 keV flux is found around $1.2\times10^{-9}$ erg cm-2 s-1 (Osten et al. 2007), which is easily detectable by INTEGRAL in a short exposure. While such a flare would be missed in a total 2 Ms observation such as the one reported by Revnivtsev et al. (2006), one should note that IGR J08023-6954 lies close to a border of the covered field. Using Fig. 1 of Revnivtsev et al. (2006) and assuming they reached a 0.16 mCrab sensitivity for the full 2 Ms exposure, one can roughly estimate an effective exposure of $\sim$140 ks at the position of the IGR source. A II Peg-like flare (assuming a constant flux during 10 ks) would be detected by INTEGRAL during this accumulation time. It is therefore possible that Revnivtsev et al. (2006) caught the source during an outburst. Not-detecting this source in the long term IBIS catalogues is also compatible with the source being highly variable and showing short-lived hard X-ray activity. We do not offer any definite proof, the source could be a fake detection by the IBIS telescope; if true, we suggest that the IGR source and the Swift one could possibly be associated, which would then confirm that the hard X-ray source is an RS CVn.

5.5 J11427+0854

This source was first reported in Paltani et al. (2008), who, however, give a low probability of genuineness. They tentatively identify it with a 2MASX source. We found one single faint X-ray source within the IBIS error box even when extending the error box to the $\sim$5$^\prime $ value expected for a $5\sigma$ source (Gros et al. 2003). There are no 2MASS or USNO B1.0 source within the Swift/XRT error box of this potential counterpart, and it lies at more than 4$^\prime $ from the 2MASX source mentioned in Paltani et al. (2008). Therefore an association of the two is ruled out. The XRT error box contains SDSS J114227.17+085448.2, which is classified as a point source in NED. We find a UV counterpart in the UVW1 and UVM2 UVOT filters, which may indicate that this object emits mostly at short wavelengths.

The X-ray spectrum has only 15 counts and is consistent with no counts above 2 keV, which could indicate a very soft source. Given the low statistical quality, a fit cannot be performed. The low probability of reality for the INTEGRAL source, and the faintness of the Swift renders the question of the association dubious, although we cannot conclude much about the nature or reality of this source.

5.6 J11457-1827

This source was first reported in Paltani et al. (2008) who suggest an association with 1H 1142-178 based on the location of the latter within the IBIS error box; therefore, the INTEGRAL source is a Sey 1 AGN. A Swift refined position of the source is given by Winter et al. (2009), who associate the source to 2MASX J11454045-1827149 reported as a $z=0.0329\pm0.0001$ galaxy in NED. We also found a possible counterpart in the USNO B1.0 catalogue (Table 4).

Although the source is quite bright, it seems that only the inner 5 pixels may be affected by pile up. We therefore extracted the spectrum from an annulus of 5 pixel inner radius and 30 pixel outer radius. The spectrum is well-fitted with an absorbed power-law ( $\chi^2_\nu$ = 0.99 for 154 d.o.f.) with parameters that are well constrained. The value of $N_{\rm H}$ is lower than what is obtained through the LAB survey (Kalberla et al. 2005). The latter value is measured from a pointing that is $\sim$17$^\prime $ from the best position of the source, and spatial variations of the Galactic $N_{\rm H}$ could result in this apparent discrepancy. In a second run we, however, froze $N_{\rm H}$ to the best value returned by the LAB survey. We obtained a good fit ( $\chi^2_\nu$ = 1.07 for 155 d.o.f.) and $\Gamma=2.04\pm0.04$ slightly softer than when leaving $N_{\rm H}$ free to vary. The parameters are clearly compatible with those of a Sey 1 AGN.

5.7 J12060+3818

This object was first reported in Paltani et al. (2008) who give a probability of only 59% that the IGR source is real. There is no X-ray source within the IBIS error, but we notice a rather significant ($6.6\sigma$) source (Swift J120617.2+381837) at $\sim$6$^\prime $ from the centre of the IBIS error. First, one should note that Paltani et al. (2008) give an error of $\sim$3.96$^\prime $, which is a bit underestimated compared to the expected 4.72$^\prime $ error of a $5.26\sigma$ source as obtained from Gros et al. (2003). In this case the Swift source is within the $97\%$ (i.e. ${<}3{-}\sigma$) of the IBIS confidence radius. Now, given that only $1 > 3{-}\sigma$ source is detected over the whole detector, the mean number of source within a 6$^\prime $ radius from the IBIS position is $\sim$0.26. By assuming a Poisson distribution, we can estimate a probability of 19% to find a source by chance in this area. Although chance association is non-negligible, we study the possibility here that the sources are related. IGR J12060+3818 has been associated to 2MASX J12055104+3819308 by Paltani et al. (2008). Since this IR source is not the IR counterpart to Swift J120617.2+381837, if the IGR and Swift sources are the same, then the former source is wrongly identified with the IR source in Paltani et al. (2008). The error box of XRT contains two quasars, SDSS J120617.35+381234.9 at $z=0.8379\pm0.0011$ and B2 1203+38 at z=0.8380. We consider it very likely that those two objects are the same. A possible optical and UV counterpart is detected in all filters (Table 4), indicating that the source is not absorbed.

The X-ray spectrum is represented well by an absorbed power-law (C=36.6 for 42 d.o.f.). As the parameters are poorly constrained, and since the value of the absorption (and the detection of the source at UV wavelengths) is compatible with very little absorption in the direction of this source, we froze $N_{\rm H}$ to the value returned by the LAB survey in a second run. The photon index tends to a harder value ( $\Gamma=1.8\pm0.4$), a value more compatible with those usually found in AGN. The extrapolated 20-60 keV flux is far below the value found by Paltani et al. (2008), which could indicate that the Swift and INTEGRAL sources are not related. If they are related, then this source could be significantly variable. Our results lead us to tentatively classify Swift J120617.2+381837 as an AGN most probably of type 1. While we cannot completely rule out a chance association of the IGR and Swift sources, that AGN are known hard X-ray emitters and the 81% probability of true association between the 2 sources also make us conclude that the sources are likely the same.

\begin{figure}
\par\includegraphics[width=16cm,clip]{13967fig1.eps}
\end{figure} Figure 1:

9.6$^\prime $$\times $6.0$^\prime $ a) UVM2 b) UVW1 c) UVW2 UVOT images around IGR J13042-1020. The white circle shows the best XRT position for Src #1, identifying it with the centre of the face-on spiral galaxy NGC 4939.

Open with DEXTER

5.8 J12070+2535

As for the previous object while there is no X-ray object in the error IBIS box given by Paltani et al. (2008), we note a possible underestimation of this positional uncertainty. According to Gros et al. (2003), the IBIS error for a $5.37\sigma$ source is 4.64$^\prime $. In that case, a $8.3\sigma$ source, Swift J120705.3+253906, is found within the error. The XRT error box contains one 2MASS object, at a position compatible with a USNO B1.0 source, itself detected at UV wavelengths with UVOT (Tables 3 and 4). The XRT position is also compatible with two objects reported in NED, SDSS J120705.29+253906.0 and MAPS-NGP O_377_0077115, which are very probably a unique object (separation of 1 $^{\prime\prime}$). Both these objects are extended and classified as galaxies. Our refined X-ray position rules out the association of the IGR source with IRAS 12046+2554 (also identified as LEDA 38453 in SIMBAD) suggested by Paltani et al. (2008).

The XRT spectrum of this source is well-fitted with an absorbed power-law (C=56 for 47 d.o.f.). The spectral analysis shows some absorption in excess of the value of the line of sight; however, its level remains low, and this object is clearly not a member of the heavily obscured sources. The extrapolated 20-60 keV flux is much lower than that reported in Paltani et al. (2008), which, again, may indicate that the sources are either not related or that we are observing a variable X-ray source. Here again we cannot make more conclusions about the association of the two sources. We nevertheless tentatively classify Swift J120705.3+253906 as an AGN, most probably one of type 1.

5.9 J13042-1020

The first mention of this object can be found in Bird et al. (2010). There are four ${>}4\sigma$ XRT sources within the IBIS error box. Two of these (Src #2 and Src #4 in Table 2) are coincident with HII regions related to the galaxy NGC 4939 according to NED. We do not discuss them further here as it seems quite unlikely that they are at the origin of the hard X-ray source. Src #3 has 2MASS J13040977-1019414 in its error box. It is detected in the optical filters of UVOT but not at UV wavelengths. Src #1, on the other hand, is positionally compatible with an extended 2 MASS source, reported as being NGC 4939 a z=0.010374 Sey 2. The X-ray position falls right on the centre of a face-on spiral Galaxy, as can be seen in Fig. 1. We consider this object as the most promising counterpart. It is also reported in the 2nd XMM catalogue (Watson et al. 2009) as 2XMM J130414.3-102021 at a position consistent with the Swift one.

\begin{figure}
\par\includegraphics[width=16cm,clip]{13967fig2.eps}
\vspace*{-1mm}\end{figure} Figure 2:

2.3$^\prime $$\times $1.5$^\prime $ a) UVM2 b) UVW1 c) UVW2 UVOT images around IGR J13412+3022. The white circle shows the best XRT position, identifying it with the centre of the spiral galaxy Mrk 268.

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To avoid contamination by Src #2 and #4, the region of extraction of the X-ray spectrum was slightly shifted. The X-ray spectrum has a shape typical of an absorbed source showing a soft excess, and, indeed, a simple absorbed power-law does not provide an acceptable fit. The origin of soft excesses in AGN is subject to debate (see e.g. Porquet et al. 2004). Amongst other possibilities it could, for example, be the signature of partial absorption of the inner flow, or the high-energy tail of thermal emission from the accretion disc. A spectral fit with a partial covering absorber and a power-law leads to a rather good fit ( $\chi^2_\nu$ = 0.95 for 9 d.o.f.). The value of the absorption is clearly in excess of the value of the line of sight $N_{\rm H}$ (Table 5). The covering fraction is $97\pm1\%$, and the photon index is typical of an AGN (Table 5). The extrapolated 20-40 keV flux is about 4 times lower than the value obtained with IBIS (Bird et al. 2010). While this is not definite proof against the partial covering model (could simply indicate that the source is variable), and since thermal emission is also a commonly suggested hypothesis for the origin of soft excesses (Porquet et al. 2004; Pounds et al. 2001), we also fitted the spectrum with an absorbed power-law and a black body only absorbed by the intervening material on the line of sight (i.e., $N_{\rm H}$ is frozen to $=0.033\times 10^{22}$ cm-2). A good fit is also achieved ( $\chi^2_\nu$ = 0.79 for 8 d.o.f.) with this model. The parameters are poorly constrained, but seem to, however, indicate a rather hard spectrum ( $\Gamma=0.1_{-0.8}^{+1.2}$) and some intrinsic absorption ($N_{\rm H}$ $=10_{-9}^{+7} \times 10^{22}$ cm-2). The extrapolated 20-40 keV flux is $2.2\times10^{-11}$ erg cm-2 s-1, which is a factor of $\gtrsim$6 higher than found with IBIS. The black body has a temperature kTbb=0.24-0.04+0.05 keV and luminosity $L_{bb}=2.2\pm0.4 \times 10^{40}$ erg/s at z=0.0104. The black body temperature is compatible with those reported by Porquet et al. (2004), and the luminosity is compatible with an AGN nature for this source. In both cases (partial covering and black body), our results tend to further confirm the association of the XRT source with the IGR, and the high absorption would argue in favour of a Sey 2.

5.10 J13412+3022

The first mention of this object can be found in Bird et al. (2010). The authors, however, give the source a wrong name (IGR J13415+3033) when considering the source coordinates obtained by IBIS. We therefore propose to rename the source IGR J13412+3022. The source is also referred to as IGR J13415+3023 in SIMBAD. There is one single bright XRT source within the IBIS error box. There are two objects in the XRT error box according to SIMBAD. One is Mrk 268, a Sey 2 galaxy at $z=0.03986\pm0.00004$ according to NED, the other SN1994o an SN Ia. We disregard the second object as a possible counterpart to the IGR source and suggest that Mrk 268 is the true counterpart. This object is reported in the IBIS catalogue of Krivonos et al. (2007), but the position they obtained has a 6.5$^\prime $ offset compared to the XRT one. There are two possible IR counterparts within the XRT error box. The first is the one reported in Table 3. It is an extended source and is clearly the IR counterpart to the Sey 2. The second one is 2MASS J13411114+3022410, a point source at 0.8 $^{\prime\prime}$ from the centre of the Swift position. We favour the first as the infrared counterpart to the high-energy source. The source is also clearly detected in the USNO B1.0 catalogue and in all UVOT filters (Table 4, Fig. 2). Note the appearance of spiral arm-like structures clear at shorter UV wavelengths, compatible with the classification of the host galaxy as a SBb as reported in NED.

As for the previous IGR source, the spectrum, although of poor quality, may show two well separated broad structures. An absorbed power-law does not provide an acceptable fit, and large residuals are seen at very soft X-rays. Similar to the previous source, a fit with a partial covering absorber and a power-law provides an improvement over the simple absorbed power-law (C=81 for 45 d.o.f.). The source shows intrinsic absorption with a large covering fraction ( $98_{-6}^{+2}\%$) and a rather hard photon index (Table 5). With this model the extrapolated 20-40 keV flux is erg cm-2 s-1, a value clearly compatible with the IBIS flux (Table 5). Adding a black body (only absorbed by the intervening material on the line of sight) to a fully absorbed power-law also leads to a good representation of the spectrum (C=71 for 44 d.o.f.). The black body has a temperature kTbb=0.31-0.09+0.15 keV and a luminosity $L_{bb}=2.9_{-1.0}^{+1.7} \times 10^{41}$ erg/s at z=0.04, compatible with the values obtained for other AGN (e.g. Porquet et al. 2004). The power-law slope is, however, very poorly constrained and very soft ( $\Gamma=4.3_{-2.4}^{+3.9}$) for an AGN, but this model also indicates a high level of intrinsic absorption. In this case the extrapolated 20-40 keV flux is incompatible with the IBIS flux reported in Bird et al. (2010). However, freezing the photon index to a more common value of 1.8 leads to a 20-40 keV flux just slightly below what is reported in the 4th IBIS catalogue. These results are clearly consistent with the source being a Sey 2 AGN. We therefore conclude that IGR J13412+3022 is the hard X-ray counterpart to Mrk 268, hence a Sey 2.

5.11 J14488-5942

This source was first mentioned in Bird et al. (2010) as a transient. There are two X-ray sources within the IBIS error box. Src #1 (Swift J144843.3-594216) has the highest significance. Its X-ray error box contains a 2MASS but no USNO B1.0 source. It is not detected by the UVOT telescope with lower limits (based on the faintest object detected in the images) mU>22.1 and $m_{{\it UVW1}}>21.6$. This is expected given that the object is in the Galactic plane and tends to suggest that it lies at a rather large distance in our Galaxy.

Src #2 (Swift J144900.5-594503) has a rather low significance (Table 2). It has no counterpart in any of the online catalogues, although it is about 3$^\prime $ from G 317.3-0.2-41.5, a molecular cloud. An association of the two cannot be dismissed. It is not detected in the UVOT filters.

An absorbed power-law gives a marginally acceptable fit ( $\chi^2_\nu$ = 1.8 for 14 d.o.f.) to the spectrum of source #1. This fit indicates that significant intrinsic absorption shields the intrinsic emission of the source. An absorbed black body provides a good description of the spectrum as well with a temperature kT=1.2-0.2+0.3 keV and a luminosity of $6\times10^{34}$ erg/s/( $(\rm {d}/10\rm {kpc})^2$). The extrapolated 20-40 keV flux obtained with the absorbed power-law model is a factor of $\gtrsim$5 below the IBIS flux (Table 5). Flux difference would not be surprising if the Swift and IGR sources were associated, as the IGR is reported as a transient and is therefore a variable source. In this case, the soft spectrum and the black body fit may indicate that we are seeing a neutron star binary in quiescence.

The spectrum of source #2 has a very low significance. An absorbed power-law provides a reasonable fit (C=24.5 for 20 d.o.f.), but the parameters are very poorly constrained (Table 5). An absorbed black body also provides a good fit (C=25.5 for 20 d.o.f.). In this case, we obtain $N_{\rm H}$ $<1.8\times10^{22}$ cm-2 (therefore at maximum consistent with the value of Galactic absorption on the line of sight), kT=0.5-0.2+0.5 keV and a luminosity ${<}6.3\times10^{33}$ erg/s/( $(\rm {d}/10\rm {kpc})^2$).

Bird et al. (2010) classify this source as a transient, since it was only detected during Rev 520. The analysis of both possible X-ray counterparts indicates that they are likely to be Galactic sources (which is strengthened by their low galactic latitude), and, in that case most probably X-ray binaries. While it is not possible to further confirm which (if any) of the 2 X-ray sources is the true counterpart to the IGR, source #1 is intrinsically absorbed, which is reminiscent of many of the IGR X-ray binaries. We also note that the amplitude of the variations of the 20-40 keV flux between the extrapolated value obtained from our fits, and the maximum reported in Bird et al. (2010) is of ${\sim}3\times10^4$ for source #2. Again while this is not definite proofs that object #2 is not the IGR, all those points, and the fact that source #1 is the closest in position to the IGR make us slightly prefer source #1 as the counterpart to the IGR source. In either case, we conclude that IGR J14488-5942 has a Galactic origin and that, if it is indeed associated to source #1, could be a member of the absorbed X-ray binaries.

5.12 J15283-4443

This source was discovered by Paizis et al. (2006), and apart from its detection in a single INTEGRAL pointing, nothing is known about it. We detect a single faint X-ray source at a position compatible with that of IBIS. There is one IR, optical, and UV counterpart within the XRT error box (Tables 3 and 4). The source is know as TYC 7847-975-1 and classified as a star. It is very bright in the UVOT filters and saturates the V, B, and U filters. The magnitudes in these filters should then be taken with caution.

The spectrum has too few counts (9 net cts) to be exploitable. However, to compare the possible flux from the Swift source in the INTEGRAL range, we assumed a power-law spectrum whose photon index was frozen to different values, and let normalization free to vary. The highest extrapolated 17-40 keV flux is obtained with $\Gamma=0.2$. It is, however, still a factor of $\sim$1000 lower than the flux obtained with IBIS by Paizis et al. (2006). We therefore can only conclude something about the Galactic nature of this source if the IGR source is associated to TYC 7847-975-1. The TYC source has a high proper motion, which indicates a probable nearby object.

5.13 J23130+8608

This source was first reported in Bird et al. (2007), but is absent in the last version of the IBIS catalogue. We found one faint X-ray source within the IBIS error box. The positional uncertainties of the XRT and the 2MASS catalogue render the IR and X-ray sources marginally compatible (Table 2 and 3). The IR position is also compatible with that obtained at UV wavelengths with UVOT. The optical counterpart mentioned in Table 4 is more than 2 $^{\prime\prime}$ away from the position of the IR (and UV) source, which indicates that these sources are probably not related.

The X-ray spectrum has very low statistical quality and contains no count above 3 keV. As the parameters are very poorly constrained, and given the probable detection of the source at UV wavelengths, it probably has a very low intrinsic absorption. This point is further confirmed by the low value taken by $N_{\rm H}$ when fitted with an absorbed power-law. A simple power-law provides an adequate fit (C=7 for 14 d.o.f.). A simple black body also provides a good fit (C=6.3 for 14 d.o.f.), with kT=0.3-0.1+0.2 keV and $L=4\pm2 \times10^{32}$ erg/s/( $(10~{\rm kpc})^2$). With the results in hand we cannot conclude anything more about the nature of the source, although the high Galactic latitude would tend to dismiss a Galactic compact object such as an X-ray binary. We do not exclude the Swift and INTEGRAL sources being unrelated.

6 Summary and conclusions

In this paper, we reported the Swift X-ray analysis of the field of thirteen IGRs that still lacked an arcsec accurate position. The refined X-ray positions provided by the Swift observations (Table 2) allowed us to pinpoint the possible IR, optical, and UV counterparts in most of the cases. We also analysed the X-ray spectra of the sources and used these results as additional arguments to confirm or refute the association of the Swift source with the INTEGRAL one. This also helped us to tentatively give a possible classification for the X-ray source. Table 6 reports the conclusions of our analysis

Table 6:   Summary of the possible type for each counterpart of the thirteen sources, obtained through our analysis.

We can summarise our results as follows.

  • We identify IGR J02086-1742, IGR J12060+3818, IGR J12070+2535, IGR J13042-1020, and IGR J13412+3022 as AGN. We, however, do question the associations of IGR J12060+3818 and IGR J12070+2535 with the X-ray counteparts we found. We suggest that IGR J02086-1742 is a possible Sey 1 and that IGR J13042-1020 is a possible Sey 2. Our analysis permits us to clearly identify IGR J13412+3022 as a Sey 2.
  • We confirm the previously proposed associations of IGR J02524-0829, IGR J08023-6954, and IGR J11457-1827. These objects are respectively classified as a Sey 2 AGN, an RS CVn star, and a Sey 1 AGN.
  • We classify IGR J14488-5942 as a probable XRB,
  • Apart from classifying IGR J15283-4443 as a Galacric source, we cannot conclude much more about the nature of this source .
  • We provide new data for IGR J03184-0014. We confirm the presence of a source found in Paper 2, but we discuss its possible association with the IGR source. We provide a new SDSS identification for the counterpart, which is classified as a galaxy. The X-ray source is therefore an AGN, which leads us to tentatively associate it with the IBIS source, although we notice that the latter could be spurious.
  • We are not able to give a classification for IGR J11427+0854 and IGR J23130+8608. In both cases, the association of the X-ray and the IGR can be questioned. We further question the genuineness of the former IGR source.
Caution is needed with these proposed identifications, as definitive conclusions will only come from optical/IR spectroscopy. We are, however, confident wbout the objects proposed as AGN, because they come from identifying extended counterparts within the XRT error box. Note that IGR J13412+3022 is also a known Sey 2 object.

Acknowledgements
J.R. thanks S. Soldi, I. Caballero, and P. Ferrando for useful discussions. JAT acknowledges partial support from a NASA INTEGRAL Guest Observer INTEGRAL grant NNX08AX91G. A.B. acknowledges support from a NASA Chandra grant GO8-9055X. We warmly thank the referee for fruitful comments that helped to improve this paper. We acknowledge the use of data collected with the Swift observatory. This research has made use of the USNOFS Image and Catalogue Archive operated by the United States Naval Observatory, Flagstaff Station (http://www.nofs.navy.mil/data/fchpix/) This research made use of the SIMBAD database, operated at the CDS, Strasbourg, France. It also made use of data products from the Two Micron All Sky Survey, 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 made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

References

Footnotes

... ``IGRs''[*]
An up-to-date online catalogue of all IGRs can be found at http://irfu.cea.fr/Sap/IGR-Sources/ note the new address for the site
... catalogues[*]
http://www.ipac.caltech.edu/2mass/
... (NED[*]
http://nedwww.ipac.caltech.edu/index.html
... parameters[*]
see http://heasarc.gsfc.nasa.gov/docs/swift/analysis/
... catalogue[*]
http://tdc-www.harvard.edu/software/catalogs/ub1.html
... Galaxy[*]
http://www.astro.uni-bonn.de/ webaiub/english/tools_labsearch.php
...Krivonos et al. (2007)[*]
see also http://hea.iki.rssi.ru/rsdc/catalog/index.php for an updated version of the Krivonos et al. (2007) catalogue.

All Tables

Table 1:   Journal of the Swift observations analysed in this paper.

Table 2:   List of sources for which X-ray counterparts have been found.

Table 3:   List of newly identified infrared counterparts in the 2MASS and 2MASX catalogues.

Table 4:   Magnitudes and UVOT positions of the newly identified optical and UV counterparts in the USNO-B1.0 catalogue (I, R, and B bands) and Swift/UVOT detector (V, B, U, UVW1, UVM2, and UVW2 bands).

Table 5:   Results of the X-ray spectral analysis. Errors and upper limits are all given at the 90% level.

Table 6:   Summary of the possible type for each counterpart of the thirteen sources, obtained through our analysis.

All Figures

  \begin{figure}
\par\includegraphics[width=16cm,clip]{13967fig1.eps}
\end{figure} Figure 1:

9.6$^\prime $$\times $6.0$^\prime $ a) UVM2 b) UVW1 c) UVW2 UVOT images around IGR J13042-1020. The white circle shows the best XRT position for Src #1, identifying it with the centre of the face-on spiral galaxy NGC 4939.

Open with DEXTER
In the text

  \begin{figure}
\par\includegraphics[width=16cm,clip]{13967fig2.eps}
\vspace*{-1mm}\end{figure} Figure 2:

2.3$^\prime $$\times $1.5$^\prime $ a) UVM2 b) UVW1 c) UVW2 UVOT images around IGR J13412+3022. The white circle shows the best XRT position, identifying it with the centre of the spiral galaxy Mrk 268.

Open with DEXTER
In the text


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