Unveiling the nature of INTEGRAL objects through optical spectroscopy

Abstract

We present further results from our onging optical spectrophotometric campaign at the Astronomical Observatory of Bologna in Loiano (Italy) on unidentified hard X-ray sources detected by INTEGRAL. We observed spectroscopically the putative optical counterparts of the INTEGRAL sources IGR J00234+6141, IGR J01583+6713, IGR J06074+2205, IGR J13091+1137 and IGR J20286+2544. We find that the first two are Galactic objects, namely a Cataclysmic Variable at a distance d ∼ 300 pc and a Be/X transient High-Mass X-ray Binary (HMXB) located at ∼6.4 kpc, respectively, whereas the last one is identified with MCG +04-48-002, a Starburst/H ii galaxy at redshift z = 0.013 hiding a Seyfert 2 nucleus. We identify IGR J13091+1137 as the (likely Seyfert 2 type) active nucleus of galaxy NGC 4992, which we classify as an X-ray Bright, Optically Normal Galaxy; this is the first example of this type of object to be detected by INTEGRAL, and one of the closest of this class. We moreover confirm the possible Be/X nature of IGR J06074+2205, and we estimate it to be at a distance of ∼1 kpc. We also reexamine the spectrum of the z = 0.087 elliptical radio galaxy PKS 0352−686, the possible counterpart of the INTEGRAL source IGR J03532−6829, and we find that it is a BL Lac. Physical parameters for these sources are also evaluated by discussing our findings in the context of the available multiwavelength information. These identifications further stress the importance of INTEGRAL in the study of the hard X-ray spectrum of Active Galactic Nuclei, HMXBs and Cataclysmic Variables.

High-mass X-ray binaries in the Milky Way

“…IGR J08408-4503 was discovered in the Vela region on 2006 May 15 with INTEGRAL during a short flare lasting less than 1000 s . Its optical counterpart was later identified as the supergiant star HD 74194 located at 3 kpc, thus confirming that this source belongs to the SFXT class Masetti et al, 2006a). IGR J08408-4503 was observed in outburst several times with INTEGRAL and Swift Leyder et al, 2007;Sidoli et al, 2009c;Barthelmy et al, 2009).…”

“…A number of studies were dedicated to the search of the optical counterpart of this source (Halpern and Tyagi, 2005;Tomsick et al, 2006b;Masetti et al, 2006a;Reig and Zezas, 2009;Reig et al, 2010). Finally Reig et al (2010) identified it with a relatively bright (V = 12.3) B0.5Ve star located at a distance of ∼ 4.5 kpc.…”

“…The companion star was identified as the B0IIIe star HD100199 located at about 3 kpc (Masetti et al, 2006a). The broad-band X-ray spectrum, moderate absorption, and transient X-ray activity led La to classify the source as a Be X-ray binary.…”

“…The soft Xray counterpart was detected first with Swift /XRT and later confirmed by Chandra (Tomsick et al, 2009b). Masetti et al (2008b) suggested that the spectral type of the companion star is BIII/BV, but this classification is still a matter of debate .…”

“…It was soon understood that all SFXT systems were hosting a compact object accreting from the wind of a massive companion, typically a super-giant O-B star (Tomsick et al, 2006aChaty et al, 2008;Masetti et al, 2008b;Negueruela et al, 2008b;Tomsick, 2009;Tomsick et al, 2009b;Bodaghee et al, 2012a). SFXTs were thus classified as a subclass of windaccreting super-giant X-ray binaries.…”