INTEGRAL observations of recurrent fast X-ray transient sources

Abstract

Fast X-ray Transients (FXTs) are believed to be non-recurrent bright X-ray sources lasting less than a day and occuring at serendipitous positions, they can best be detected and discovered by instruments having a sufficiently wide field of view and high sensitivity. The IBIS/ISGRI instrument onboard INTEGRAL is particularly suited to detect new or already known fast X-ray transient sources. We report on IBIS/ISGRI detection of newly discovered outbursts of three fast transient sources located at low Galactic latitude: SAX J1818.6−1703; IGR J16479−4514; IGR J17391−302/XTE J1739−302. The reported results confirm and strengthen the very fast transient nature of these sources, given that all their newly detected outbursts have a duration less than ∼3 h. Additionally, they provide the first evidence for a possible recurrent fast transient behaviour as all three sources were detected in outburst by ISGRI more than once during the last 2 years.

First hard X-ray detection and broad-band X-ray study of the unidentified transient AX J1949.8+2534

“…dynamic range ≥ 620 and duration of ∼1.5 days and ≥ 4 days from the two IBIS/ISGRI detected activities, respectively) could be typical of the Be HMXB class. In addition we note that mentioned X-ray characteristics are compatible as well with a Supergiant Fast X-ray Transients nature (SFXTs, Sguera et al 2008), which are a newly discovered class of HMXBs (Sguera et al 2005, Negueruela et al 2005 Although classical SFXTs usually display above 20 keV hard X-ray outbursts lasting much less than a day, a few other SFXTs are known to show unusually longer hard X-ray activity, exceptionally lasting several days (e.g. IGR J18483−0311, ; IGR J17354−3255, Sguera et al 2011), i.e.…”

High-mass X-ray binaries in the Milky Way

“…Several outbursts from this source were detected with ASCA (Sakano et al, 2002), RXTE (Smith et al, 2006), INTEGRAL (Sunyaev et al, 2003a;Lutovinov et al, 2005b;Sguera et al, 2005Sguera et al, , 2006bBlay et al, 2008), and Swift /BAT (Sidoli et al, 2009a,c;Romano et al, 2009bRomano et al, , 2011b. The source was also observed during faint X-ray states by Chandra and XMM-Newton, revealing the typical variability of the SFXT sources (Smith et al, 2006;Bozzo et al, 2010;Bodaghee et al, 2011b).…”

Giant outburst from the supergiant fast X-ray transient IGR J17544−2619: accretion from a transient disc?

“…Supergiant fast X-ray transients (SFXTs) are high mass X-ray binaries (HMXBs) hosting a neutron star (NS) and an OB supergiant companion (Sguera et al 2005;Negueruela et al 2006) which display X-ray flares reaching, for a few hours, 10 36 −10 37 erg s −1 (see Romano et al 2014c, for a recent review). This is at odds with normal supergiant HMXBs, which display a fairly constant average luminosity with typical variations of a factor of 10−50 on time scales of a few hundred to thousands of seconds; SFXTs are also significantly subluminous with respect to classical Sg-HMXBs like Vela X-1 (Bozzo et al 2015), and show a dynamical range up to 5 orders of magnitude, as their luminosities can be as low as ∼10 32 erg s −1 during quiescence (e.g.…”