Outbursts from IGR J17473-2721

¹ Key Laboratory for Particle Astrophysics, Institute of High Energy Physics, Beijing 100049, PR China e-mail: szhang@mail.ihep.ac.cn
² Theoretical Physics Center for Science Facilities (TPCSF), CAS, PR China
³ ICREA & Institut de Ciències de l'Espai (IEEC-CSIC), Campus UAB, Facultat de Ciències, Torre C5-parell, 2a planta, 08193 Barcelona, Spain
⁴ Center for Astrophysics,Tsinghua University, Beijing 100084, PR China

Received: 9 January 2009
Accepted: 23 April 2009

Abstract

Aims. IGR J17473-2721 was discovered by INTEGRAL as a neutron star low mass X-ray binary. To date, two outbursts have been detected in X-rays by RXTE/ASM and SWIFT: the one occurring in 2005 was weak and was characterized by a low/hard state spectrum; the one of March 2008 was strong and showed a 6-step evolution in its flux. We investigate their evolution, emphasizing the later outburst.

Methods. We analyzed all available observations carried out by RXTE on IGR J17473-2721 during its later outburst. We analyzed as well all the available SWIFT/BAT data on this source.

Results. The flux of the latter outburst rose in ~one month and then kept roughly constant for the following ~two months. During this time period, the source was in a low/hard state. The source moved to a high/soft state within the following three days, accompanied by the occurrence of an additional outburst at soft X-rays and the end of the preceding outburst in hard X-rays. During the decay of this soft outburst, the source went back to a low/hard state within 6 days, with a luminosity 4 times lower than the first transition. This shows a full cycle of the hysteresis in transition between the hard and the soft states. The fact that the flux remained roughly constant for ~two months at times prior to the spectral transition to a high/soft state might be regarded as the result of balancing the evaporation of the inner disk and the inward accretion flow, in a model in which the state transition is determined by the mass flow rate. Such a balance might be broken via an additional mass flow accreting onto the inner disk, which lightens the extra soft outburst and causes the state transition. However, the possibility of an origin of the emission from the jet during this time period cannot be excluded. The spectral analysis suggests an inclined XRB system for IGR J17473-2721. Such a long-lived preceding low/hard state makes IGR J17473-2721 resemble the behavior of outbursts seen in black hole X-ray binaries like GX 339-4.