Type-I X-ray bursts from SAX J1806.5-2215 with RXTE
ATel #3381; Guo-Bao Zhang(Groningen), Mariano Mendez(Groningen)
on 27 May 2011; 12:57 UT
Credential Certification: Guo-Bao Zhang (zhang@astro.rug.nl)
Subjects: X-ray, Binary, Neutron Star, Transient
We report the detection of two type-I X-ray bursts in RXTE/PCA observation of the neutron star transient SAX J1806.5-2215, which is currently in outburst (ATEL #3193, #3202, #3210, #3218, #3268). The first X-ray burst took place on 2011-04-01 01:59:33.256 UTC, and the second one on 2011-05-06 17:28:51.878 UTC. This is the first time that X-ray bursts are detected from this source with RXTE/PCA, after the only previous detection with BeppoSAX (in't Zand et al. 1999, Nucl. Phys. B, Proc. Suppl., 69, 228).
We analyzed time-resolved spectra of the two bursts, fitting a blackbody model to the net burst spectra in the 3.0-25.0 keV range. We also include the effect of interstellar absorption towards the source, with a column density of 5.6e22 cm-2 (ATEL #
3202). None of the two bursts shows photospheric radius
expansion (e.g., Galloway et al. 2008 ApJS 179 360). According to the bolometric flux light-curve, the first one is a typical type-I X-ray burst with a fast rise (2.4s) and exponential decay (21s). The second burst shows a double-peaked profile, in which the second peak is sharper and has higher flux than the first one. The time interval between these two peaks is about 7s. We also inspected 0.5-s, 1-s and 2-s long Fourier power density spectra of the two bursts in the 10-1000 Hz frequency range, but we found no significant oscillations, with an upper limit of 18.7% rms.
During the current outburst, the source evolves in the color-color diagram like other transient accreting neutron-star systems (the so-called atoll sources). The double-peaked burst took place when the source was in the soft state, which is consistent with what happens in another similar (albeit persistent) source, 4U 1636-53 (Zhang et at. 2009 MNRAS 398 368). Multi-peaked X-ray bursts are a rare phenomenon; about 5% of all X-ray bursts of 4U 1636-53 observed with RXTE (about 320 bursts) show multiple peaks. There are other RXTE observations planned.
