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Swift/XRT observations of SAX J1828.5-1037 in outburst

ATEL # 1831; N. Degenaar and R. Wijnands (University of Amsterdam)
on 6 Nov 2008; 13:05 UT
Distributed as an Instant Email Notice (Request for Observations)
Password Certification: Rudy Wijnands (

Subjects: X-ray, Request for Observations, Binaries, Neutron Stars, Transients

On October 31, 2008, Swift observed the bursting neutron star X-ray transient SAX J1828.5-1037 (Cornelisse et al. 2002, A&A 392) with the X-ray Telescope (XRT) for ~3.7 ksec. The source is clearly detected during this observation, which demonstrates that SAX J1828.5-1037 is exhibiting a new outburst. The XRT spectrum can be described by an absorbed powerlaw model with a hydrogen column of ~4.1(-2.4,+3.4)E22 cm-2 and a photon index of 1.7(-1.0,+1.3). The resulting absorbed (unabsorbed) 2-10 keV flux is 4.3E-12 (5.8E-12) erg cm-2 s-1. Assuming a source distance of 6.2 kpc (Cornelisse et al. 2002, A&A 392), this corresponds to a luminosity of Lx~2.7E34 erg s-1, which is over two orders of magnitude higher than the quiescent luminosity of SAX J1828.5-1037 (Lx<9E31 erg s-1; Hands et al. 2004, MNRAS 351). Comparable levels of outburst activity were observed with ROSAT in 1993 (Cornelisse et al. 2002, A&A 392) and with XMM-Newton in 2001 (Hands et al. 2004, MNRAS 351). We note that SAX J1828.5-1037 was not detected during Swift/XRT observations performed in February, April and November 2007 (with upper limits on the 2-10 keV source luminosity of a few times 1E33 erg s-1).

We requested additional Swift/XRT observations to monitor the current outburst of SAX J1828.5-1037, the first of which was carried out on November 5, 2008. During this ~0.9 ksec observation, SAX J1828.5-1037 is detected at an absorbed (unabsorbed) 2-10 keV flux of 3.6E-12 (4.9E-12) erg cm-2 s-1, for an absorbed powerlaw model with the hydrogen column fixed at 4.1E22 cm-2 and a photon index of 1.8 (-1.5,+2.2). The unabsorbed flux translates into a source luminosity of Lx~2.2E34 erg s-1 for a distance of 6.2 kpc, which is comparable with the level observed on October 31, 2008, and demonstrates that the outburst is ongoing. Observations in other wavelength bands are encouraged.

We thank the Swift team for approving and scheduling our Too observations.

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