Supergiant Fast X-ray Transients - A short review

Abstract

I present a brief up-to-date review of the current understanding of Supergiant Fast X-ray Transients, with an emphasis on the observational point of view. After more than a decade since their discovery, a remarkable progress has been made in getting the picture of their phenomenology at X-ray energies. However, a similar in-depth investigation of the properties of the supergiant companions is needed, but has started more recently. A multifrequency approach is the key to fully understand the physical mechanism driving the SFXT behaviour, still under debate.In this short review paper I will concentrate on the most recent achievements about Supergiant Fast X-ray Transients (SFXTs), emphasizing an observational standpoint. The X-ray window will be looked at in particular, but with an eye also to observations at other frequencies, especially for what concerns the future directions. I will mainly focus on advances on the topic obtained thanks to systematic investigations of SFXT class as a whole, while I refer the reader to [1] for a summary of the properties of the single sources. For a unique, comprehensive and detailed review of the current understanding of the interplay between the outflowing wind from massive stars and accretion processes in wind-fed supergiant HMXBs, I refer the reader to [2].SFXTs ([3], [4]) are a sub-class of High Mass X-ray Binaries (HMXBs) where a compact object (typically a neutron star) accretes a fraction of the clumpy wind from the blue supergiant companion, triggering rare and short (less than a few days) outbursts, made of a number of bright X-ray flares (each flare with a duration of ∼thousands seconds). Most SFXTs were discovered by the INTEGRAL satellite (launched in 2002; [5]) as hard (above 17 keV) X-ray sources detected only during a short duration (a few hours) X-ray activity. Thanks to X-ray sky positions refined at arcsec level at soft X-rays (1-10 keV), they were quickly associated with O or B-type supergiants. The identification of these X-ray transients with massive X-ray binaries was somehow surprising, since the classical HMXBs with supergiant companion (SgHMXBs) known since the birth of Xray astronomy are persistent X-ray emitters, with a limited range of intensity variability (around a factor of ∼10). Fig. 1 shows the comparison between long-term X-ray INTEGRAL light curve of an SFXT and Vela X-1, the prototype of persistent HMXBs.Since their discovery, more than a decade ago, a huge observational effort has involved both long-term, X-ray monitoring investigations (summarized by [6] using RXT E, by [7] and [8] using INTEGRAL, by [9] and [10] using Swift) and deep, very sensitive, X-ray pointings (by Chandra, e.g. [11], and XMM-Newton, starting from the pioneering observations discussed by [12], to the more recent works by [13], [14], [15], ending with the latest published paper on SFXTs with XMM-Newton [16]).These X-ray observations led to the characterization of the SFXT properties, as follows:1. low duty cycle ( < ∼ 5%) in bright X-ray flares (at L X > ∼ 10 36 erg s −1...

X-ray and optical monitoring of the December 2017 outburst of the Be/X-ray binary AXJ0049.4–7323

“…SFXTs are a sub-class of high-mass X-ray binaries (HMXBs) hosting a compact object (likely a neutron star) that is accreting from the stellar wind of a massive OB supergiant. These objects show typical dynamic ranges of ∼ 10 4 − 10 5 (up to 10 6 , Romano et al 2015) and sporadic flares with durations of ∼ 10 3 s. The recorded peak X-ray luminosities during these events are usually of 10 36 − 10 37 erg s −1 (Romano 2015;Sidoli 2017;Romano et al 2023). Several accretion mechanisms have been proposed to explain the fast and strong variability of SFXTs.…”

Supergiant Fast X-ray Transients uncovered by the EXTraS project: flares reveal the development of magnetospheric instability in accreting neutron stars

“…Supergiant Fast X-ray Transients (SFXTs) are a kind of high mass X-ray binaries (HMXBs) where a neutron star (NS) accretes a fraction of the wind of an early-type supergiant donor (see Sidoli 2017, Martínez-Núñez et al 2017, Walter et al 2015, for the most recent reviews). They were recognized as a new class of massive binaries thanks to rare, short and bright flares, reaching a peak luminosity L X ∼10 36 -10 37 erg s −1 , caught during INT EGRAL observations (Sguera et al 2005(Sguera et al , 2006Negueruela et al 2006).…”

A search for the presence of magnetic fields in the two supergiant fast X-ray transients, IGR J08408−4503 and IGR J11215−5952

“…Because of the faintness of SFXTs -most of them have a visual magnitude m V 12, up to m V 31 (Sidoli 2017;Persi et al 2015) -no high-resolution spectropolarimetric observations were carried out for them so far and the presented FORS 2 observations are the first to explore the magnetic nature of the optical counterparts. Future spectropolarimetric observations of a representative sample of SFXTs are urgently needed to be able to draw solid conclusions about the role of magnetic fields in the wind accretion process and to constrain the conditions that enable the presence of magnetic fields in massive binary systems.…”

“…Since, typically, the orbital separation between the neutron star and the supergiant is a few R * of the latter, the expected required magnetic field on the stellar surface should be of the order of 100 − 1000 G. To investigate the magnetic nature of the optical counterparts in these systems, we recently observed the two optically brightest SFXTs, IGR J08408−4503 (m V = 7.6) and IGR J11215−5952 (m V = 10.0), using the FOcal Reducer low dispersion Spectrograph (FORS 2; Appenzeller et al 1998) mounted on the 8 m Antu telescope of the VLT. The optical counterpart of the system IGR J08408−4503 is the O-type supergiant star HD 74194 with spectral classification O8.6 Ib-II(f)p (Sota et al 2014). The orbital solution (P orb = 9.5436 ± 0.002 d, e = 0.63 ± 0.03) was for the first time determined by Gamen et al (2015).…”