Review on latest progress on Supergiant Fast X-ray Transients and future direction

📝 Abstract

In the recent years, the discovery of a new class of Galactic transients with fast and bright flaring X-ray activity, the Supergiant Fast X-ray Transients, has completely changed our view and comprehension of massive X-ray binaries. These objects display X-ray outbursts which are difficult to be explained in the framework of standard theories for the accretion of matter onto compact objects, and could represent a dominant population of X-ray binaries. I will review their main observational properties (neutron star magnetic field, orbital and spin period, long term behavior, duty cycle, quiescence and outburst emission), which pose serious problems to the main mechanisms recently proposed to explain their X-ray behavior. I will discuss both present results and future perspectives with the next generation of X-ray telescopes.

💡 Analysis

In the recent years, the discovery of a new class of Galactic transients with fast and bright flaring X-ray activity, the Supergiant Fast X-ray Transients, has completely changed our view and comprehension of massive X-ray binaries. These objects display X-ray outbursts which are difficult to be explained in the framework of standard theories for the accretion of matter onto compact objects, and could represent a dominant population of X-ray binaries. I will review their main observational properties (neutron star magnetic field, orbital and spin period, long term behavior, duty cycle, quiescence and outburst emission), which pose serious problems to the main mechanisms recently proposed to explain their X-ray behavior. I will discuss both present results and future perspectives with the next generation of X-ray telescopes.

📄 Content

arXiv:1101.2835v2 [astro-ph.HE] 2 Mar 2011 Review on latest progress on Supergiant Fast X–ray Transients and future direction Lara Sidoli INAF/IASF-Milano, Via Bassini 15, 20133 Milano (Italy) Abstract In the recent years, the discovery of a new class of Galactic transients with fast and bright flaring X–ray activity, the Supergiant Fast X–ray Transients, has com- pletely changed our view and comprehension of massive X–ray binaries. These ob- jects display X–ray outbursts which are difficult to be explained in the framework of standard theories for the accretion of matter onto compact objects, and could represent a dominant population of X–ray binaries. I will review their main obser- vational properties (neutron star magnetic field, orbital and spin period, long term behavior, duty cycle, quiescence and outburst emission), which pose serious prob- lems to the main mechanisms recently proposed to explain their X–ray behaviour. I will discuss both present results and future perspectives with the next generation of X–ray telescopes. Key words: X–ray binaries, X–ray sources, accretion and accretion disks, supergiants 1 Main observational properties of a new sub-class of High Mass X–ray Binaries: the Supergiant Fast X–ray Transients High Mass X–ray Binaries (HMXBs) contain a compact object (a neutron star or a black hole) accreting matter from a massive companion star. They can be divided into three different sub-classes, depending on both the donor type (OB supergiant or Be star) and the X–ray activity (persistent or transient): (1)-supergiant HMXBs with persistent emission (divided into (1a)–wind-fed and (1b)–disk-fed accretors), (2)-Be/X–ray transients (although there are also Email address: sidoli@iasf-milano.inaf.it (Lara Sidoli). Preprint submitted to Elsevier Science 9 November 2018 a few Be/X–ray binaries with persistent low luminosity X–ray emission) and, more recently, the (3)- supergiant HMXBs with fast transient emission, the so-called Supergiant Fast X–ray Transients (SFXTs). Liu et al. (2006) list 114 HMXBs located in our Galaxy, 66 of which are classified as accreting X–ray pulsars. Most of them are in binaries with Be stars, although the number of HMXBs with supergiant companions is continuously growing thanks to the INTEGRAL discoveries in the energy range 17–100 keV: indeed, about 70% of the HMXBs discovered with INTEGRAL host OB supergiants (Bird et al. 2010). SFXTs are hard X–ray transients displaying a high dynamic range of ∼103– 105, with sporadic, recurrent, bright and short (a few hour long) flares (Sguera et al. 2005, 2006; Negueruela et al. 2006a), reaching 1036–1037 erg s−1. This fast flaring activity is superimposed on outburst phases lasting a few days, shorter than those displayed by Be/X–ray transients (Romano et al. 2007; Sidoli et al. 2009; Rampy et al. 2009). Their optical association with blue supergiants has led to the identification of ten members (e.g. Halpern et al. 2004, Pellizza et al. 2006, Masetti et al. 2006, Negueruela et al. 2006b, Nespoli et al. 2008), together with several candidates with fast hard X–ray flaring activity but still unknown optical/IR counterparts. Their long term properties (on timescales of months) consist of a large flux variability at an average intermediate X–ray luminosity of 1033–1034 erg s−1 (Sidoli et al. 2008), between the quiescence and the flare peaks. The lowest luminosity level detected in a few SFXTs, 1032 erg s−1, sometimes shows a very soft spectrum (and, likely, no accretion; e.g. in IGR J17544–2619, in’t Zand 2005), sometimes a harder X–ray emission together with mild flux variability (indicative of ongoing accretion at a very low level, e.g. IGR J08408–4503, Sidoli et al. 2010). A common property of accreting pulsars in HMXBs is the X–ray spectral shape, typically characterized by a flat hard power law below 10 keV (photon index ∼0–1), together with a high energy cut-offin the range 10–30 keV, sometimes strongly absorbed at soft energies (Walter et al., 2006). SFXTs display a similar spectral shape when they are in outburst, thus it is usually assumed that most of these sources harbour neutron stars, although X–ray pulsations have been detected only in about half of them (5 of about 10 members of the class) with spin periods ranging from 4.7 s (AX J1841.0–0536, Bamba et al. 2001) to 228 s (IGRJ 16465–4507, Lutovinov et al. 2005) and 1246 s (for the SFXT candidate IGRJ 16418–4532, Walter et al. 2006). The possibility of the presence of a black hole in non-pulsating SFXTs cannot be completely ruled out. SFXTs orbital periods have been determined in 8 sources, spanning a large range as well, between 3.3 days (IGRJ 16479–4514; Jain et al. 2009) and 165 days (IGRJ 11215–5952; Sidoli et al. 2006, 2007; Romano et al. 2009). The SFXTs for which both the orbital and spin periods are known can be overplot- 2 Fig. 1. Corbet diagram of Galactic accreting pulsars, together with the new sources discovered with INTEGRAL (red squares), and a few SFXTs where

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