Since its launch on October 2002, the INTEGRAL satellite has revolutionized our knowledge of the hard X-ray sky thanks to its unprecedented imaging capabilities and source detection positional accuracy above 20 keV. Nevertheless, many of the newly-detected sources in the INTEGRAL sky surveys are of unknown nature. The combined use of available information at longer wavelengths (mainly soft X-rays and radio) and of optical spectroscopy on the putative counterparts of these new hard X-ray objects allows pinpointing their exact nature. Continuing our long-standing program running since 2004, here we report the classification, through optical spectroscopy, of 19 more unidentified or poorly studied high-energy sources detected with the IBIS instrument onboard INTEGRAL.
One of the main objectives of the INTEGRAL [18] is a regular survey of the whole sky in the hard X-ray band. This makes use of the unique imaging capability of the IBIS instrument [17] which allows the detection of sources at the mCrab level with a typical localization accuracy of a few arcmin. The latest (4 th one) IBIS catalogue published up to now by [3] shows that, among more than 700 sources detected above 20 keV, a substantial fraction (∼30%) had no obvious counterpart at other wavelengths and therefore could not yet be associated with any known class of high-energy emitting objects.
With the task of identifying them, our group started in 2004 an optical spectroscopy program which used data from ten telescopes worldwide up to now and which proved extremely successful, leading to the identification of about 150 INTEGRAL objects 1 , that is about three quarters of the unidentified sources sample of the first three IBIS catalogues.
It is found that more than half of the sources identified by us are Active Galactic Nuclei (AGNs), followed by X-ray binaries (∼30%) and Cataclysmic Variables (CVs; ∼13%); see [13] and references therein.
Here we present the continuation of this follow-up and identification work on INTEGRAL objects with the use of data collected in more than one year, between March 2010 and May 2011.
Using the criterion applied in our past works (see [13] and references therein), we positionally cross-correlated the unidentified sources belonging to recently published IBIS surveys ( [2,3,8,14]) with available X-ray (ROSAT, XMM-Newton) and radio (NVSS, SUMSS, MGPS) on-line catalogues, and with archival X-ray observations (Swift, Chandra) to reduce the arcminute-sized source error circle to a radius of better than a few arcsec, thus pinpointing the putative optical counterpart.
This process eventually allowed us to select 19 new candidate counterparts to be observed through optical spectroscopy. The list of selected objects is reported in the first column of Table 1.
While it is known [15] that the presence of a single, bright soft X-ray object within the IBIS error circle indicates that it is with a high probability the lower-energy counterpart of the corresponding INTEGRAL source, the same cannot be said with high confidence for radio objects (see however [11]). Thus, for the IBIS sources which were singled out using catalogues at wavelengths longer than soft X-rays, we caution the reader that the association with the selected optical object should await confirmation via a pointed observation with a soft X-ray satellite such as XMM-Newton, Chandra or Swift.
We also stress that in a few cases the putative soft X-ray (and optical) counterpart lies outside the nominal IBIS 90% confidence level error circle (but within the 99% one).
To remark the above uncertainties, we put an asterisk beside the source name in Table 1.
In this work, 6 telescopes located in Northern and Southern Hemisphere observatories were used; moreover, a spectrum from the 6dF on-line archive [7] was also employed. In the following, the list of the telescopes used for the classifications reported in Table 1 is given:
• 1.5m telescope at CTIO, Chile;
• 1.5m “Cassini” telescope in Loiano, Italy;
• 1.8m “Copernico” telescope in Asiago, Italy;
• 2.1m telescope in San Pedro Mártir, Mexico;
• 3.58m Telescopio Nazionale Galileo, Canary Islands, Spain;
• 3.9m AAT telescope of the Anglo-Australian Observatory, Siding Spring, Australia (for the 6dF spectrum);
• 10.4m Gran Telescopio Canarias, Canary Islands, Spain.
Table 1 reports, for each selected source, the corresponding classification obtained through optical spectroscopy, its redshift and the name of the telescope with which the observation was performed; in Fig. 1 the spectra of the optical counterparts of some of these 19 selected IBIS sources are shown.
As one can see from Table 1, most objects (14 out of 19) are AGNs, with a large majority (12) of Type 1 (i.e. broad-line) ones. Among them, 5 lie at large (z > 0.5) redshift: this confirms the trend that new, deeper IBIS surveys are able to detect more distant AGNs. In particular, we were able to identify IGR J12319-0749 (at z = 3.12) as the second most distant object ever detected by INTEGRAL up to now after the blazar IGR J22517+2218, which lies at redshift z = 3.668 ( [1] and references therein).
Within our AGN identifications we moreover refine the classifications of IGR J06523+5334 and IGR J19491-1035 made by [5] and [9], respectively. We also confirm the double Seyfert 2 nature of source IGR J03249+4041 first suggested by [10].
Only 5 of our identifications are instead of Galactic nature: two of them are (likely magnetic) CVs, one is a Symbiotic Star, one is a magnetic flare star (possibly of RS CVn type), and one is a High-Mass X-ray Binary (HMXB). We confirm the CV nature of sources RX J0525.3+2413 and AX J1740.2-2903 as first proposed by [16] and [6], respectively; we also note that the spectra of th
This content is AI-processed based on open access ArXiv data.