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Register a new userGiant outburst from the supergiant fast X-ray transient IGR J17544-2619: accretion from a transient disc?
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P. Romano
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Supergiant fast X-ray transients (SFXTs) are high mass X-ray binaries associated with OB supergiant companions and characterised by an X-ray flaring behaviour whose dynamical range reaches 5 orders of magnitude on timescales of a few hundred to thousands of seconds. Current investigations concentrate on finding possible mechanisms to inhibit accretion in SFXTs and explain their unusually low average X-ray luminosity. We present the Swift observations of an exceptionally bright outburst displayed by the SFXT IGR J17544-2619 on 2014 October 10 when the source achieved a peak luminosity of $3times10^{38}$ erg s$^{-1}$. This extends the total source dynamic range to $gtrsim$10$^6$, the largest (by a factor of 10) recorded so far from an SFXT. Tentative evidence for pulsations at a period of 11.6 s is also reported. We show that these observations challenge, for the first time, the maximum theoretical luminosity achievable by an SFXT and propose that this giant outburst was due to the formation of a transient accretion disc around the compact object.
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The supergiant fast X-ray transient (SFXT) system IGR J17544-2619 has displayed many large outbursts in the past and is considered an archetypal example of SFXTs. A search of the INTEGRAL/ISGRI data archive from MJD 52698-54354 has revealed 11 outbursts and timing analysis of the light curve identifies a period of 4.926$pm$0.001 days which we interpret as the orbital period of the system. We find that large outbursts occasionally occur outside of periastron and place an upper limit for the radius of the supergiant of <23R$_{sun}$.
We present NuSTAR spectral and timing studies of the Supergiant Fast X-ray Transient (SFXT) IGR J17544-2619. The spectrum is well-described by a ~1 keV blackbody and a hard continuum component, as expected from an accreting X-ray pulsar. We detect a cyclotron line at 17 keV, confirming that the compact object in IGR J17544-2619 is indeed a neutron star. This is the first measurement of the magnetic field in a SFXT. The inferred magnetic field strength, B = (1.45 +/- 0.03) * 10^12 G * (1+z) is typical of neutron stars in X-ray binaries, and rules out a magnetar nature for the compact object. We do not find any significant pulsations in the source on time scales of 1-2000 s.
One of the most recent discoveries of the INTEGRAL observatory is the existence of a previously unknown population of X-ray sources in the inner arms of the Galaxy. IGR J17544-2619, IGR J16465-4507 and XTE J1739-302 are among these sources. Although the nature of these systems is still unexplained, the investigations of the optical/NIR counterparts of the two last sources, combined with high energy data, have provided evidence of them being highly absorbed high mass X-ray binaries with blue supergiant secondaries and displaying fast X-ray transient behaviour. In this work we present our optical/NIR observations of IGR J17544-2619, aimed at identifying and characterizing its counterpart. We show that the source is a high mass X-ray binary at a distance of 2-4 kpc with a strongly absorbed O9Ib secondary, and discuss the nature of the system.
We present the first direct evidence for dense clumps of matter in the companion wind in a Supergiant Fast X-ray Transient (SFXT) binary. This is seen as a brief period of enhanced absorption during one of the bright, fast flares that distinguish these systems. The object under study was IGR J17544-2619, and a total of 236 ks of data were accumulated with the Japanese satellite Suzaku. The activity in this period spans a dynamic range of almost 10000 in luminosity and gives a detailed look at SFXT behavior.
Supergiant fast X-ray transients (SFXTs) are X-ray binary systems with a supergiant companion and likely a neutron star, which show a fast ($sim 10^3$ s) and high variability with a dynamic range up to $10^{5-6}$. Given their extreme properties, they are considered among the most valuable laboratories to test accretion models. Recently, the orbital parameters of a member of this class, IGR J08408-4503, were obtained from optical observations. We used this information, together with X-ray observations from previous publications and new results from X-ray and optical data collected by INTEGRAL and presented in this work, to study the accretion mechanisms at work in IGR J08408-4503. We found that the high eccentricity of the compact object orbit and the large size of the donor star imply Roche lobe overflow (RLO) around the periastron. It is also likely that a fraction of the outer layers of the photosphere of the donor star are lost from the Lagrangian point $L_2$ during the periastron passages. On the basis of these findings, we discuss the flaring variability of IGR J08408-4503 assuming the presence of an accretion disc. We point out that IGR J08408-4503 may not be the only SFXT with an accretion disc fueled by RLO. These findings open a new scenario for accretion mechanisms in SFXTs, since most of them have so far been based on the assumption of spherically symmetric accretion.
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