Chandra detection of increased X-ray activity from SAX J1747.0-2853

ATel #9115; M. Clavel, J. A. Tomsick (SSL/UCB), R. Terrier, A. Goldwurm (APC/Paris 7)
on 4 Jun 2016; 17:04 UT
Credential Certification: Maïca Clavel (maica.clavel@ssl.berkeley.edu)

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We report the detection of a bright halo in the Chandra ACIS-I observation obtained on 2016 May 17 (ObsID 18852, MJD 57525). The shape of this diffuse emission is consistent with a dust scattering halo surrounding the neutron star LMXB SAX J1747.0-2853, which is outside of the field of view.

The monitoring of this source by MAXI (http://maxi.riken.jp/top/index.php?cid=1&jname=J1747-288) highlights a recent 0.2-0.4 cts/cm2/s rise in the 2-20 keV energy range, which would correspond to SAX J1747.0-2853 having an average absorbed 2-20keV flux of (2+/-1)e-9 erg/cm2/s during our 2016 Chandra observation.

The 2016 Chandra spectrum of the halo is well fitted (chi2/dof = 274/292) by an absorbed power law: Nh = (9.1+/-0.2)e22 cm-2, Gamma = 4.1+/-0.1, with a surface brightness of ~3e-12 erg/cm2/s/arcmin2. Such a soft spectrum is expected in case of a dust scattering halo.

To confirm that the surface brightness of the dust scattering halo is consistent with the flux derived from the MAXI light curve, we investigated previous Chandra observations covering this region. We report the detection of a similar but ten times fainter halo in an ACIS-I observation which was obtained in 2000 July 7 (ObsID 945, MJD 51732) and with SAX J1747.0-2853 being outside of the field of view. At that time, the source was in an active phase (e.g. Werner et al. 2004). Using the RXTE/ASM simultaneous count rate and the conversion factor provided by Wijnands et al. (2002), we derive an absorbed flux of ~1e-10 erg/cm2/s in the 0.5-10 keV range at the time of the 2000 Chandra observation. Therefore, the brightness of the scattering halo observed in 2016 is consistent with SAX J1747.0-2853 having a 0.5-10 keV flux of ~1e-9 erg/cm2/s, which is fully consistent with the 2-20 keV flux inferred from the MAXI count rate.

Furthermore, the flux of SAX J1747.0-2853 has increased recently since the source was not detected in the Swift/XRT observation obtained in 2016 March 15 (ObsID 00034427001, MJD 57462). Assuming the spectral shape reported by Wijnands et al. (2002), we derive a 90% confidence upper limit of 3e-13 erg/cm2/s on the source X-ray flux at that time. SAX J1747.0-2853 is also not detected by the INTEGRAL monitoring of the region (http://integral.esac.esa.int/BULGE/SOURCES/1A_1743-288/1A_1743-288.html), whose last observation was on 2016 April 21 (MJD 57499).

X-ray observations of SAX J1747.0-2853 are encouraged in order to confirm its renewed activity.