Swift/XRT observations of the neutron-star X-ray transient SAX J1810.8-2609
N. Degenaar, M. Klein-Wolt and R. Wijnands (University of Amsterdam)
Following reports of renewed activity of the neutron-star X-ray transient SAX J1810.8-2609 (GCN 6706 and 6707), we triggered our Swift monitoring program on very-faint X-ray transients when in outburst. As part of this program, SAX J1810.8-2609 is currently observed on a daily basis with Swift using ~1 ksec exposures, starting August 6, 2007. In all our observations so far, the source X-ray spectrum can be fit using an absorbed powerlaw model with a hydrogen column density of Nh~0.5E22 cm2 and a powerlaw index of ~2. The source flux shows a gradual increase from ~4.7E-11 erg/cm2/s on August 6, to ~8.0E-11 erg/cm2/s on August 8 and up to ~1.1E-10 erg/cm2/s on August 9 (2-10 keV unabsorbed fluxes). Assuming a distance to SAX J1810.8-2609 of 5 kpc (inferred from type-I X-ray burst analysis; Natalucci et al., 2000, ApJ 536, 891), the derived fluxes translate into X-ray luminosities of ~1.4E35 erg/s, ~2.4E35 erg/s and ~3.3E35 erg/s, respectively. Swift monitoring observations of SAX J1810.8-2609 are ongoing.

We thank the Swift team for a prompt scheduling of our monitoring observations.
x-ray,binary,neutron star,star,transient