AX J1754.2-2754: back in outburst

ATel #1643; P. G. Jonker (SRON & CfA), L. Keek (SRON)
on 3 Aug 2008; 10:05 UT
Distributed as an Instant Email Notice Transients
Credential Certification: P.G. Jonker (pjonker@cfa.harvard.edu)

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We report on a Chandra DDT and multi-epoch (9) Swift observations of the faint, bursting neutron star transient AX J1754.2-2754. We obtained a 29 ks-long Chandra ACIS-S observation on July 15, 2008 to determine the luminosity of the source in quiescence after a 2 ks Chandra ACIS-I observation did not detect the source down to a 0.5-10 keV luminosity of 3.0-5.8E32 erg/s (Bassa et al. 2008, Atel #1575).

To our surprise a bright point source was detected at the position of AX J1754.2-2754. With a 0.3-10 keV count rate of 0.155 the source spectrum is probably suffering from 10-20% pile-up. We modelled the spectrum with XSPEC12 taking into account the pile-up. The source spectrum could be described well by a simple absorbed power law with an N_H of (2.3+-0.2)E22 cm-2 and a power-law index of 2.9+-0.4. The best-fit source 0.5-10 keV absorbed flux was ~2.4E-12 erg cm-2 s-1, whereas the unabsorbed 0.5-10 keV flux was 1.2E-11 erg cm-2 s-1. These values are similar to those observed before the source went to quiescence. For a distance of 9.2 kpc the 0.5-10 keV luminosity of the source is 1.2E35 erg/s.

To monitor the evolution of the source Swift has observed the source for 9 times. Using the XRT data we determine that the source spectrum and flux are consistent with those derived from the 29 ks Chandra observation. We conclude that the source has started another outburst after a brief transition to quiescence. The non-detection during the 2 ks Chandra ACIS-I observation is unlikely to be due to an X-ray eclipse since that would imply an orbital period of several hours, in contradiction with the report of Bassa et al. (2008, Atel #1575) that the source probably harbors an ultra-compact X-ray binary. We have plotted the long-duration light curve in the attached figure (http://www.sron.nl/~peterj/images/axj1754-longterm-lc.pdf). The ASCA data is from Sakano et al. (1999), the INTEGRAL data is from Krivonos et al. (2007), the first Swift observation is reported in Del Santo et al. (2007; Atel #1143) and the 1st and 2nd Chandra data point in Bassa et al. (2008; Atel #1575). The arrow indicates the time of the detection of a type I X-ray burst by Chelovekov & Grebenev (2007; Atel #1094).

We would like to thank the Swift and Chandra team for making the Swift and Chandra DDT observations, respectively.