The 2008 outburst of IGR J17473–2721: evidence for a disk corona?

Abstract

Context. The 2008 outburst of the atoll source IGR J17473-2721 was observed by INTEGRAL, RXTE and Swift. Tens of type-I X-ray bursts were found in this outburst. Aims. Joint observations by INTEGRAL, RXTE, and Swift provide sufficient data to look into the behavior of IGR J17473-2721 at the rising part of the 2008 outburst. The relation between the duration of the bursts and the accretion rate and the nature of the corona producing the observed power-law component can therefore be studied in detail. Methods. We analyze observational data of IGR J17473-2721, focusing on the spectral evolution during the state transition from quiescent to low hard state (LHS), and on the flux dependence of the type-I X-ray bursts along the outburst. Results. We find that the joint INTEGRAL, RXTE and Swift energy spectrum can be well fitted with a model composed of a blackbody and a cutoff power-law, with a cutoff energy decreasing from ∼150 keV to ∼40 keV as the source leaves the quiescent state toward the low hard state. This fits into a scenario in which the corona is cooled by the soft X-rays along the outburst evolution, as observed in several other atoll sources. Fifty-seven type-I bursts were reported in the 2008 outburst of IGR J17473-2721. By using the flux measured in the 1.5-30 keV band, we find that the linear relationship between the burst duration and the flux still holds for those bursts that occur at the decaying part of the low hard state, but with a different slope than the overall one that was estimated with the bursts happening in the whole extent of, and for the rest of the low hard state. The significance of such a dichotomy in the type-I X-ray bursts is ∼3σ under an F-test. Similar results are hinted at as well with the broader energy-band that was adopted recently. This dichotomy may be understood in a scenario where part of the accreting material forms a corona on the way of falling onto the surface of the neutron star during the decaying part of the low hard state.Based on the accretion rates of the preceding LHS, estimated from type-I X-ray bursts and from persistent emission, at least for IGR J17473-2721, most of the accretion material may fall on the neutron star (NS) surface in the LHS. Considering the burst behavior in the context of the outburst indicates a corona formed on top of the disk rather than on the NS surface.

Type-I X-Ray Bursts Reveal a Fast Co-Evolving Behavior of the Corona in an X-Ray Binary

“…Besides 4U 1908+005 (Aql X−1) and 4U 1608−522 (Gladstone et al 2007), IGR J17473−2721 is thus the third NS XRB observed with hysteresis. There are 32 bursts located in the preceding LHS and 10 bursts in the tail of the HSS and the lagging LHS (Chen et al 2010(Chen et al , 2011Chenevez et al 2011), constituting an entire sample of 42 bursts recorded by RXTE. The long-lived preceding LHS and the large number of burst events occurred in both the preceding and lagging LHSs make IGR J17473−2721 almost unique for probing its corona (Zhang et al 2009) under a circumstance of an outburst evolution.…”

“…Our previous reports showed that the corona of the system (Figure 3) is most likely located around a largely inclined disk and forms a relatively small opening angle with respect to the surface of neutron star, so that the type-I X-ray bursts can escape from the NS surface without suffering severe Comptonization (Zhang et al 2009;Chen et al 2011). The 30-50 keV decrement reaches a maximum of ∼2 cts/s at the 2-10 keV burst peak, accounting to about half of the 30-50 keV persistent flux.…”

Superexpansion as a possible probe of accretion in 4U 1820-30

“…Kuulkers et al (2009) discuss an X-ray burst from Cen X-4 which appears to have turned the accretion disk into a hot state, although it appears at odds with accretion disk theory. Other examples of influence are bursts that cool the accretion disk corona (Chen et al 2011) and bursts with superexpansion, such as the superburst from 4U 1820-30 (Strohmayer & Brown 2002;Ballantyne & Strohmayer 2004;Keek 2012) and a number of bursts from (candidate) ultracompact X-ray binaries (in 't Zand & Weinberg 2010). During these bursts, episodes exist when the flux is lower than the pre-burst accretion flux.…”

Fast co-evolving behavior of the corona with type-I X-ray bursts in Aql X-1

“…Plasmas may be energized to form a corona via either evaporation (Meyer et al 1994) or magnetic re-connections (Zhang et al 2000). Chen et al (2011) and Chen et al (2011) take advantage of the type-I X-ray bursts from the hard surface of a NS of the NS XRB IGR J17473−2721 to probe the purported corona, under a circumstance of outburst and accompanying spectral evolution, find that • The corona seems to be located above the disk, but not around the NS, so that emission from type-I X-ray bursts can escape from the NS surface without suffering Comptonization.…”

“…Among them, roughly one third of the bursts are PRE bursts (Galloway et al 2008). In this paper, we report the results by analysis all of the RXTE/PCA observations data on Aql X-1 using the similar procedure in Chen et al (2011).…”