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The faint neutron star and probable ultra-compact transient AX J1754.2-2754: quiescent after a (long) outburst

ATel #1575; C. Bassa (SRON & Nijmegen), P. G. Jonker (SRON & CfA), G. Nelemans (Nijmegen), D. Steeghs (Warwick), M. A.P. Torres (CfA), L. Kuiper (SRON), J. J.M. In 't Zand (SRON), N. Rea (UvA), T. Maccarone (Southampton), E. Kuulkers (ESA/ESAC), J. Grindlay (CfA), R. Wijnands (UvA), M. Mendez (Groningen)
on 13 Jun 2008; 08:05 UT
Distributed as an Instant Email Notice Transients
Credential Certification: P.G. Jonker (pjonker@cfa.harvard.edu)

Subjects: Optical, X-ray, Request for Observations, Neutron Star, Transient

Referred to by ATel #: 1643, 4109, 10195

We report on multi-epoch Chandra and optical observations of the faint, bursting (Chelovekov & Grebenev 2007, Atel #1094) neutron star transient AX J1754.2-2754. The source has been detected every time since 1999 (Sakano et al. 1999) when an X-ray satellite observed the source. However, we do not detect the source in Chandra Galactic Bulge Survey observations obtained in May 2008. The Galactic Bulge Survey is an optical and X-ray survey of two strips each 6 by 1 degrees centered 1.5 degrees above and below the Galactic plane towards the Galactic center.

We derive an accurate source position for AX J1754.2-2754 using a 1.18 ks Chandra HRC-I observation that we obtained on Aug. 14, 2007. The source position is: R.A.: 17h54m14.5s Dec: -27d54m35.6s (J2000.0). The positional uncertainty is dominated by the Chandra aspect offset which, for the HRC-I, has a 90% confidence radius of 0.45 arcsec. This location is inside the Swift error circle of the source (Del Santo et al. 2007; Atel #1143). Using the spectral parameters found by Del Santo et al. (2007) and the source distance given by Chelovekov & Grebenev (2007; 6.6/9.2 kpc) we get a source 0.5-10 keV luminosity of 1.6-3.2E35 erg/s.

In the Chandra X-ray Galactic Bulge Survey observations obtained in May 2008, we detect more than a 1000 sources. However, AX J1754.2-2754 is not one of them. The Chandra ACIS-I observation of the field of AX J1754.2-2754 took place on May, 14, 2008 and lasted 2.19 ks. We do not find X-ray photons in a circular region with radius of 2 arcsec centered on the source position. Using the formulation of Gehrels (1989) this yields a 95% confidence upper limit to the source 0.5-10 keV luminosity of 3.0-5.8E32 erg/s for a power-law spectrum with photon index 2 and an extinction as found by ASCA and Swift observations of the source in outburst (N_H=2.3E22 cm-2). Similar limits are derived assuming a blackbody spectrum with temperature of 0.35 keV.

As part of our Galactic Bulge Survey we have also obtained optical images with the Blanco 4m telescope in June 2006 of the area covered by the X-ray observations. We do not detect a source in the Chandra error circle. This provides an upper limit to the outburst SDSS i-band magnitude of i>23.5. Using the distance and the fact that the interstellar extinction found in outburst X-ray spectra corresponds to A_i~7 we calculate an upper limit to the outburst absolute i-band magnitude of 2.4/1.7 for a distance of 6.6/9.2 kpc. The low outburst luminosity, the burst properties (cf. In't Zand, Jonker & Markwardt 2007) together with the low absolute magnitude suggest that AX J1754.2-2754 is an ultra-compact X-ray binary.

Due to the faintness of the source in outburst the historic light curve of the source is sketchy, but probably the source has been in outburst at least since the discovery by ASCA in October 1999. Depending on the exact date that the source went to quiescence (some time between Aug. 14, 2007 and May 15, 2008) deep follow-up observations could find the cooling neutron star.