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NEW RESULTS ON OPTICAL IDENTIFICATIONS OF INTEGRAL SOURCES
N. Masetti1 , L. Morelli2 , E. Palazzi1 , G. Galaz2 , L. Bassani1 , A. Bazzano3 , A.J. Bird4 , A.J. Dean4 , G.L. Israel5 , R. Landi1 , A. Malizia1 , D. Minniti2 , J.B. Stephen1 , F. Schiavone1 , P. Ubertini3 , and R. Walter6
1 2

INAF-IASF di Bologna, Bologna, Italy
3

Pontificia Universidad Catolica de Chile, Santiago, Chile ´ INAF-IASF di Roma, Rome, Italy

4 5

University of Southampton, Southampton, United Kingdom
6

INAF-Osservatorio Astronomico di Roma, Monteporzio Catone, Italy INTEGRAL Science Data Centre, Versoix, Switzerland

ABSTRACT Optical spectroscopic identification of the nature of 21 unidentified southern hard X­ray objects is reported here in the framework of our campaign aimed at determining the nature of newly-discovered and/or unidentified sources detected by INTEGRAL. Our results show that 12 of these objects are Active Galactic Nuclei (AGNs), 5 are magnetic Cataclysmic Variables (CVs), and 4 are High-Mass X­ray Binaries (HMXBs), one of which is in the Large Magellanic Cloud. These identifications further underscore the importance of INTEGRAL in the study of the hard X­ray spectrum of AGNs, HMXBs and CVs, and the usefulness of a strategy of catalogues crosscorrelation plus optical spectroscopy to unveil the nature of the X­ray sources detected with INTEGRAL. Key words: Galaxies: Seyfert -- Stars: novae, cataclysmic variables -- X­rays: binaries -- Techniques: spectroscopic -- X­rays: individuals. 1. INTRODUCTION Since its launch in October 2002, the INTEGRAL satellite [35] is boosting our knowledge of the hard X­ray sky above 20 keV in terms of both sensitivity and positional accuracy of the detected sources. Thanks to the capabilities of the IBIS instrument [28], INTEGRAL is effectively detecting hard X­ray objects at the mCrab level with a typical localization accuracy of 2­3 [6]. This has made it possible, for the first time, to obtain all-sky maps in the 20­100 keV range with arcminute accuracy and down to mCrab sensitivities (e.g., [2]). Most of the sources detected by INTEGRAL are known Galactic X­ray binaries (50% of the total number of detected objects), plus a fraction of known Active Galactic Nuclei (AGNs; 10%) and Cataclysmic Variables (CVs; 5%). However, a large majority of the remaining objects (about 25% of all detections achieved with IBIS) has no obvious counterpart at other wavelengths and therefore cannot immediately be associated with any known class of high-energy emitting objects.

Recently, in order to fill this identification gap, we started a campaign aimed at identifying the nature of these still unknown sources through optical spectroscopy at northern and southern telescopes [13, 14, 15, 16, 17]. Our results indicate that, despite INTEGRAL doubled the number of Galactic High Mass X­ray Binaries (HMXBs; see [32]) and despite the expectation according to which most of these unidentified objects should be HMXBs [5], about < half of them are identified in the optical as nearby (z 0.1) AGNs [13, 14, 15, 16]. In the framework of our continuing effort to identify unknown INTEGRAL sources, we present here the optical spectroscopic observations obtained on 21 southern objects at the 1.5-metre telescope of the Cerro Tololo Interamerican Observatory (CTIO) located in Cerro Tololo (Chile). Preliminary analysis of part of the data presented here can be found in Masetti et al. [18]. A detailed presentation of the results reported here is available elsewhere [17]. 2. SAMPLE SELECTION AND OBSERVATIONS In order to continue our program [13, 14, 15, 16] of optical spectroscopic identifications of INTEGRAL sources with unknown nature, we first collected all objects belonging to the 2nd IBIS Galactic Plane Survey [2], to the Crux arm Tangent Survey [22], to the AGN minisurvey of Sazonov et al. [25] and to the Circinus-Carina arm Survey [12], and which are visible from the southern hemisphere. We then positionally cross-correlated the IBIS error circles of the selected southern unidentified INTEGRAL objects with catalogues of soft (<10 keV) X­ray sources. This was made in order to reduce the X­ray error box < size to some ( 10) arcsec at most. For the present sample, we selected INTEGRAL objects with a single ROSAT source [30, 31, 24], or a single Swift/XRT archival X­ray source (available at http://www.asdc.asi.it), or a single Chandra source [7, 11, 25] within the IBIS error box. This approach was chosen because Stephen et al.


[26] showed that, from a statistical argument, these are very likely to be the soft X­ray counterparts of the positionally corresponding INTEGRAL sources; besides, the results of Masetti et al. [13, 14, 15, 16] prove that this approach is very effective, when combined with optical spectroscopy. For the cross-correlation searches, we considered 90% confidence level INTEGRAL/IBIS error circles. To this aim, a conservative 90% confidence level error box radius of 2 was assumed for the objects belonging to the 2nd IBIS Galactic Plane Survey [2], and of 6 for the Crux arm Survey objects (as stated in Revnivtsev et al. [22]). For the Circinus-Carina Survey sources, the 90% confidence level error radius as reported in Kuiper et al. [12] regarding each object was considered. In this way we could select 18 unidentified INTEGRAL sources associated with a single arcsec-sized soft X­ ray error box which, when overlaid onto the corresponding DSS-II-Red survey1 images, is seen to contain a single or few (3 at most) relatively bright (R < 18) possible optical counterparts. Three additional sources (IGR J14175-4641, IGR J14552-5133 and IGR J18244-5622) were added to our sample as their IBIS error circle includes bright field objects which were suggested as their possible counterparts [22, 23]. The list of selected INTEGRAL sources is shown in the first column of Table 1. All objects were observed spectroscopically between March 21 and April 6, 2006, with the 1.5-metre CTIO telescope of Cerro Tololo (Chile) equipped with the R-C spectrograph. Spectra were extracted and analyzed using IRAF2 . Wavelength calibration was performed using He-Ar lamps acquired soon after each spectroscopic exposure; the spectra were then flux-calibrated using the spectrophotometric standards LTT 3218 and LTT 7379 [9]. Finally, and when applicable, different spectra of the same object were stacked together to increase the S/N ratio. 3. RESULTS Optical spectroscopy allowed us to find that our sample of 21 unidentified objects is composed of 12 AGNs, 5 CVs and 4 HMXBs. The classification for each object is reported in Table 1. 3.1. AGNs It is found that 12 objects of our sample show optical spectra which are dominated by redshifted broad and/or narrow emission lines typical of AGNs. For their classification, we used the criteria of Veilleux & Osterbrock [29] and the line ratio diagnostics of Ho et al. [10]; moreover, for the subclass assignation of Seyfert 1 nuclei, we used the H /[OI I I]5007 line flux ratio criterion as in [34]. In detail, we found that, of the 12 AGNs of the sample, 6 of them are classified as Seyfert 2 galaxies, and
1 2

the other 6 are Seyfert 1 galaxies. Of these latter objects, 2 are classified as Seyfert 1.2, one as Seyfert 1.9 and two as Narrow-Line (NL) Seyfert 1; for the case of IGR J17488-3253 only a general Seyfert 1 classification can be given due to the lower quality of the spectrum. Their redshifts range between 0.016 and 0.076, indicating that with INTEGRAL we are sampling the hard X­ray emission from new AGNs in the nearby Universe. Assuming a cosmology with H0 = 65 km s-1 Mpc-1 , = 0.7 and m = 0.3, these redshifts correspond to distances between 74.7 and 370.6 Mpc. This in turn implies that their hard X­ray luminosities detected with INTEGRAL are between 1â1043 and 3â1044 erg s-1 . 3 . 2 . CVs Five objects of our sample were identified as CVs through the appearance of their optical spectra, which show Balmer emissions up to at least H , as well as several He I and He I I lines in emission. All of the detected lines are consistent with being at z = 0, indicating that these objects belong to our Galaxy. The facts that, in the spectra of all these 5 objects, the Balmer decrement clearly appears negative, the > HeI I4686/H Equivalent Width (EW) ratio is 0.5 and the EWs of He I I and H are around (or larger than) 10 ° A indicate that these sources are magnetic CVs belonging to the Intermediate Polar (IP) subclass (see [33]). Assuming for the CVs an absolute magnitude MV 9 and an intrinsic color index (V - R)0 0 mag [33], from their USNO-A2.03 optical magnitudes we determined their distances to be in the range 110­220 pc; this implies hard X­ray fluxes observed with INTEGRAL in the (0.4­1.4)â1032 erg s-1 . 3.3. HMXBs Finally, we identify the remaining 4 INTEGRAL sources of our sample as HMXBs by their overall spectral appearance, which is typical of this class of objects (see e.g. [15]), with narrow H emission at a wavelength consistent with that of the laboratory frame, superimposed on an intrinsically blue continuum with Balmer absorptions. In three cases, however, the stellar continuum appears strongly reddened and almost undetected blueward of ° 5000 A, implying the presence of substantial interstellar dust along the line of sight. This also is quite typical of HMXBs detected with INTEGRAL (e.g., 2RXP J130159.6-635806 [15]) and indicates that these objects are relatively far from Earth. In the remaining case (IGR J05007-7047) we detect the H line in emission, and the rest of the Balmer series (up ° to H ) in absorption, all redshifted of 6 A with respect to the corresponding laboratory wavelengths. This is consistent with the redshift of the Large Magellanic Cloud (LMC; see [3]). This information implies for this source an X­ray luminosity of 3.6â1036 erg s-1 in the 17­60 keV band as seen by INTEGRAL.
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http://archive.eso.org/dss/dss http://iraf.noao.edu/

http://archive.eso.org/skycat/servers/usnoa/


Table 1. at the 1. with the these ob

List of unidentified INTEGRAL sources observed 5m CTIO telescope on March-April 2006, along spectroscopic optical classification derived from servations. Object I GR I GR I GR I GR I GR XSS I GR I GR I GR I GR I GR I GR I GR I GR I GR I GR I GR I GR I GR I GR I GR J05007-7047 J07565-4139 J07597-3842 J10101-5654 J12026-5349 J12270-4859 J14175-4641 J14471-6319 J14515-5542 J14536-5522 J14552-5133 J15094-6649 J16167-4957 J16185-5928 J16207-5129 J16558-5203 J17195-4100 J17200-3116 J17488-3253 J17513-2011 J18244-5622 Classification HMXB in LMC Sy2 AGN Sy1.2 AGN HM XB Sy2 AGN IP CV Sy2 AGN Sy2 AGN Sy2 AGN IP CV NL Sy1 AGN IP CV IP CV NL Sy1 AGN HM XB Sy1.2 AGN IP CV HM XB Sy1 AGN Sy1.9 AGN Sy2 AGN

Figure 1. Histogram, subdivided into source types, showing the percentage of INTEGRAL objects of known nature and belonging to the 2nd IBIS Galactic Plane Survey ([2]; left-side, darker columns), and INTEGRAL sources from various surveys and identified through optical spectroscopy (right-side, lighter columns). The latter columns also show (as shaded areas) the percentage of sources identified thanks to optical spectroscopy and which belong to 2nd IBIS Galactic Plane Survey. CVs, with at least 6 of them of magnetic nature (IPs or Polars). From these numbers, graphically reported in percentage terms in the histogram in Fig. 1, one can immediately see that the CV sample detected with INTEGRAL has been doubled thanks to the optical spectroscopy identification approach. This also stresses INTEGRAL's sensitivity in detecting hard X­ray emission from this class of objects. One of the reasons for this may be found in the fact that the bulk of the X­ray emission from magnetic CVs falls in the 20­40 keV band (e.g., [1, 4]), which is the one in which INTEGRAL has the strongest sensitivity. The number of INTEGRAL-detected AGNs also has nearly doubled thanks to these optical studies; besides, it is apparent that an important fraction of the INTEGRAL sources identified by means of optical spectroscopy and lying on the Galactic Plane is composed of background AGNs. This once again underscores the extraordinary capabilities of INTEGRAL of piercing through the Zone of Avoidance of the Galaxy for the exploration of this part of the extragalactic sky. As a final corollary, we would like here to stress the extreme effectiveness of the strategy of catalogues crosscorrelation plus optical spectroscopy we are pursuing to securely pinpoint the actual nature of the X­ray sources detected with INTEGRAL: for instance, of the 56 unidentified objects belonging to the 2nd IBIS Galactic Plane Survey [2], this observational approach led to the discov-

Likewise, from the available multiwavelength information on IGR J16207-5129 [27], we could determine a distance of 4.6 kpc and a 20­100 keV INTEGRAL X­ ray luminosity of 1â1035 erg s-1 . Unfortunately, due to the lack of reliable optical photometry for the optical counterparts of IGR J10101-5654 and IGR J17200-3116, no significant information concerning distance, spectral type and X­ray luminosity can be determined for these two objects. 4. STATISTICAL CONSIDERATIONS We can now briefly recover the statistical approach made in Masetti et al. [14], updating the numbers presented there with recent discoveries [8, 15, 16, 17, 19, 20, 21] and with the sample of sources illustrated in the present work. It is now found that, presently, of the 54 INTEGRAL sources identified through optical spectroscopy, 22 (41%) are X­ray binaries (with a large majority, i.e. more than 90%, of HMXB), 24 (44%) are AGNs (half of which were presented in this paper for the first time) and 8 (15%) are CVs (5 of which were shown in Masetti et al. [17] for the first time), with at least 6 of them belonging to the IP subclass (see [16, 17]). One can compare, for instance, these numbers with those for the group of the 153 identified objects belonging to the largest catalogue of INTEGRAL sources published up to now, i.e., the 2nd IBIS Galactic Plane Survey [2]. In this survey we have 107 (70%) X­ray binaries (of which, only one third are HMXBs), 27 (18%) AGNs and 8 (5%)


ery of the nature of 18 sources (10 AGNs, 6 HMXBs and 2 CVs), i.e., nearly one third of the total, 15 of which being reported in Masetti et al. [14, 15, 16, 17]. The corresponding source type percentages are represented as shaded areas in Fig. 1. The lack of known accurate (up to 10 ) soft X­ray position is the main cause of failure in this identification task; therefore, observations with high-resolution imaging X­ray satellites (such as Chandra, XMM-Newton and/or Swift) are of paramount importance for the continuation of this program aimed at identifying the nature of unknown INTEGRAL hard X­ray sources. 5. CONCLUSIONS In our continuing work of identification of sources by means of optical spectroscopy [13, we have identified and studied 21 southern objects of unknown nature by means of the telescope of Cerro Tololo (Chile). INTEGRAL 14, 15, 16], h a r d X­ r a y 1.5m CTIO

We found that the selected sample is made of 12 AGNs (6 of which are of Seyfert 1 type and 6 are Seyfert 2 AGNs), 5 magnetic CVs and 4 HMXBs (one of which in the LMC). In terms of relative sizes of the three groups, we notice the absolute majority of AGNs in the sample, and a comparatively large fraction (25%) of CVs. We recall that in three cases (IGR J14175-4641, IGR J14452-5133 and IGR J18244-5622), all identified as AGNs, only a tentative albeit likely optical counterpart was given because of the the lack of an univocal arcsecond-sized soft X­ray position. Thus, for them an observation with soft X­ray satellites affording arcsecond localizations (such as Chandra, XMM-Newton or Swift) is needed to confirm the proposed association. The results presented in pabilities of INTEGRAL mass X-ray binaries, but objects in the nearby U dwarf novae. this work further indicate the cato reveal not only high- and lowalso (if not mostly) extragalactic niverse (z < 0.1) and magnetic

ACKNOWLEDGEMENTS We thank Claudio Aguilera and Arturo Gomez for the ´ assistance at the telescope. NM thanks the Departamento de Astronom´a y Astrof´sica of the Pontificia Universidad i i Catolica de Chile for the pleasant hospitality during the ´ preparation of this paper. REFERENCES [1] Barlow, E.J., Knigge, C., Bird, A.J., et al. 2006, MNRAS, 372, 224 [2] Bird, A.J., Barlow, E.J., Bassani, L., et al. 2006, ApJ, 636, 765 [3] Cusumano, G., Israel, G.L., Mannucci, F., et al. 1998, A& A, 3 3 7 , 7 7 2 [4] de Martino, D., Matt, G., Belloni, T., Haberl, F., & Mukai, K. 2004, A&A, 415, 1009 [5] Dean, A.J., Bazzano, A., Hill, A.B., et al. 2005, A& A, 4 4 3 , 4 8 5

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