Unveiling the nature of six HMXBs through IR spectroscopy

Abstract

Context. The International Gamma-Ray Astrophyiscs Laboratory (INTEGRAL) is discovering a large number of new hard X-ray sources, many of them being HMXBs. The identification and spectral characterization of their optical/infrared counterparts is a necessary step to undertake detailed study of these systems. In particular, the determination of the spectral type is crucial in the case of the new class of Supergiant Fast X-ray Transients (SFXTs), which show X-ray properties common to other objects. Aims. Our goal is to perform spectral analysis and classification of proposed counterparts to HMXBs in order to characterize the system they belong to. Methods. We used the ESO/NTT SofI spectrograph to observe proposed IR counterparts to HMXBs, obtaining K s medium resolution spectra (R = 1320) with a S /N 100. We classified them through comparison with published atlases. Results. We were able to spectrally classify the six sources. This allowed us to ascribe one of them to the new class of SFXTs and confirm the membership of two sources to this class. We confirmed the spectral classification, derived from optical spectroscopy, of a known system, 4U 1907-09, showing for the first time its infrared spectrum. The spectral classification was also used to estimate the distance of the sources. We compared the extinction as derived from X-ray data with effective interstellar extinction obtained from our data, discussing the absorption component due to the circumstellar environment, which we observed in four systems; in particular, intrinsic absorption seems to emerge as a typical feature of the entire class of SFXTs.

High-mass X-ray binaries in the Milky Way

“…This behavior led to the association of IGR J1841.0-0536 with the SFXT class. This association was strengthened by the identification of the supergiant companion through infrared observations (Nespoli et al, 2008b). A 45 ks-long XMM-Newton observation performed in 2011 in the direction of IGR J1841.0-0536 caught the source undergoing a bright X-ray flare, which could be interpreted in terms of sudden "ingestion" of accreting material from the dense wind environment.…”

“…The companion star (Masetti et al, 2006a;Negueruela et al, 2007a) was classified as a B1 Ia star at ∼6.1 kpc by Nespoli et al (2008b). Due to its relatively high persistent flux a and the lack of prominent outbursts, Walter and Zurita Heras (2007b);Tomsick et al (2009c) suggested that IGR J16207-5121 belong to the class of the highly absorbed HMXBs, rather than to the SFXT class.…”

“…Ia supergiant (located at 2-6kpc) in 8.37d with an eccentricity of 0.28 (Makishima et al, 1984;in 't Zand et al, 1998b;Cox et al, 2005;Nespoli et al, 2008a). Its X-ray emission is highly variable and feature cyclotron absorption features Cusumano et al, 1998;Rivers et al, 2010;Fürst et al, 2011a;Hemphill et al, 2013).…”

4U 1907+09: an HMXB running away from the Galactic plane

“…van Kerkwijk et al 1989;Iye 1986). This classification was refined by Nespoli et al (2008), who got the infrared spectrum of 4U 1907+09 and derived the spectral type of O9.5 Iab.…”

INTEGRAL and Swift/XRT observations of IGR J19405-3016

“…Among them, 171 are Galactic accreting systems, 122 extragalactic objects, and 113 are sources of unknown nature. Most of the unclassified sources were identified through optical and near-infrared spectroscopy (Masetti et al 2004(Masetti et al , 2006a(Masetti et al ,b,c,d, 2008(Masetti et al , 2009Chaty et al 2008;Nespoli et al 2008). For the newly identified sources, about 55% are AGNs (almost equally divided into Seyfert 1s and 2s), 32% are X-ray binaries, and 12% are CVs (Masetti et al 2009).…”