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Swift detects increased activity of the neutron star transient SAX J1750.8-2900

ATEL # 1777; M. Linares, N. Degenaar, R. Wijnands, D. Altamirano (University of Amsterdam)
on 10 Oct 2008; 18:08 UT
Distributed as an Instant Email Notice (Transients)
Password Certification: Manuel Linares (

Subjects: X-ray, Binaries, Neutron Stars, Transients

After our report on the flux decay of the neutron star transient SAX J1750.8-2900 (ATel #1662) we have continued monitoring the source with Swift-XRT in order to study its transition to quiescence. In these observations (performed between August 14th. and October 8th.) we detect the source at different flux levels, with the corresponding luminosity (at 6.3 kpc; Kaaret et al 2002, ApJ) swinging in the range ~ 10^34 - 10^35 erg/s. A detailed analysis will be reported elsewhere.

In the latest Swift-XRT pointings taken on October 1st and 8th (~2 kiloseconds each) we detect the source back to a luminosity of approximately 10^36 erg/s. From background-and-pile-up corrected PC mode data we measure absorbed(unabsorbed) 2-10 keV fluxes of about 1.4(2.0)*10^-10 erg/s/cm2 (October 1st) and 1.6(2.1)*10^-10 erg/s/cm2 (October 8th). This luminosity indicates that SAX J1750.8-2900 is in outburst again, consistent with the X-ray burst detected by SuperAGILE on October 9th (Pacciani et al., ATel #1775). Using our measured fluxes and applying standard (yet uncertain) bolometric corrections (e.g. in't Zand et al. 2007, A&A) we estimate that the burst occurred at about 1% of the Eddington luminosity (for a 1.4 Mo neutron star).

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