Unveiling the nature of INTEGRAL objects through optical spectroscopy. VII. Identification of 20 Galactic and extragalactic hard X-ray sources

Abstract

Within the framework of our program of assessment of the nature of unidentified or poorly known INTEGRAL sources, we present here spectroscopy of optical objects, selected through positional cross-correlation with soft X-ray detections (afforded with satellites such as Swift, ROSAT, Chandra and/or XMM-Newton) as putative counterparts of hard X-ray sources detected with the IBIS instrument onboard INTEGRAL. Using 6 telescopes of various sizes and archival data from two on-line spectroscopic surveys we are able to identify, either for the first time or independent of other groups, the nature of 20 INTEGRAL hard X-ray sources. Our results indicate that: 11 of these objects are active galactic nuclei (AGNs) at redshifts between 0.014 and 0.978, 7 of which display broad emission lines, 2 show narrow emission lines only, and 2 have unremarkable or no emission lines (thus are likely Compton thick AGNs); 5 are cataclysmic variables (CVs), 4 of which are (possibly magnetic) dwarf novae and one is a symbiotic star; and 4 are Galactic X-ray binaries (3 with high-mass companions and one with a low-mass secondary). It is thus again found that the majority of these sources are AGNs or magnetic CVs, confirming our previous findings. When possible, the main physical parameters for these hard X-ray sources are also computed using the multiwavelength information available in the literature. These identifications support the importance of INTEGRAL in the study of the hard X-ray spectrum of all classes of X-ray emitting objects, and the effectiveness of a strategy of multi-catalogue cross-correlation plus optical spectroscopy to securely pinpoint the actual nature of unidentified hard X-ray sources.

Identifications of Five INTEGRAL Sources via Optical Spectroscopy

“…Though we did not observe all possible sources within the INTEGRAL error circle, the identification of the one we did observe as an AGN allows us to conclude that this is very likely the counterpart of the INTEGRAL source. We note that Masetti et al (2009) came to similar conclusions from their spectroscopic analysis. Although we identify it as a Seyfert 1.5, while they find that it is a Seyfert 1, there were several months between the observations, and this level of variability is not unheard-of (e.g., Tran et al 1992).…”

“…The optical spectrum shows Balmer, Paschen, HeI and HeII emission lines at z = 0 (Figure 2d). These lines, along with the hard X-ray spectrum, suggest a CV nature (e.g., Masetti et al 2009). Furthermore, the HeII line and large equivalent width of Hβ (49 ± 2) suggest a magnetic CV (Silber 1992) 8 .…”

“…We first consider the possibility that the source is a CV. Combining the apparent magnitudes with typical absolute magnitudes for CVs (M V ∼ 9 and (V − R) 0 ∼ 0 (Masetti et al 2009)) and a range of extinctions from A V = 5 − 15 mag results in distances that range from d = 4 − 440 pc. This distance range is far too close to account for the observed amount of extinction (assuming the optical extinction is interstellar).…”

“…Combining the magnitude and visual extinction with the typical values for CV absolute magnitudes listed above, we find d = 220 +160 −190 pc. Masetti et al (2009) also identify this as a CV. They find a larger distance, although it agrees within the uncertainties, of d = 320 pc as a result of using USNO-A2.0 magnitudes (which are fainter for this source) and a different method of calculating extinction (which resulted in lower extinction).…”

“…Since this last census, much work has been done to identify the ∼50% of unclassified IGR sources (corresponding to ∼25% of all sources detected by INTEGRAL). Many have been identified as AGN, LMXBs and HMXBs, as well as relatively rare systems such as heavily absorbed (i.e., "obscured") supergiant HMXBs, supergiant fast X-ray transients, and Intermediate Polar (IP) CVs (e.g., Masetti et al 2009;Chaty et al 2008).…”