Identification of XMMU J181227.8-181234 = XTE J1812-182 as a neutron star low-mass X-ray binary

ATel #10567; J. J.M. in 't Zand (SRON), D. K. Galloway (Monash Univ.), E. Kuulkers (ESA), A. Goodwin (Monash Univ.)
on 11 Jul 2017; 07:54 UT
Credential Certification: Jean in' t Zand (jeanz@sron.nl)

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We report on an analysis of archival Rossi X-ray Timing Explorer observations of the transient XMMU 181227.8-181234 = XTE J1812-182 = 1H 1812-182, taken during an outburst in 2008 (Cackett et al. 2006, MNRAS 369, 1965; Markwardt et al. 2008, Atel #1685; Torres et al. 2008, ATel #1696). We found seven burst-like events between MJD 54717 and 54736, in the data from the Proportional Counter Array (PCA) taken during 26 observations between MJD 54699 and 54758 (2008 August 21 and October 19) with a total exposure time of 45 ks. The events have rise times shorter than 2 s, durations above the noise of about 20 seconds, e-folding decay times of about 5 s and peak intensities of up to 180+/-10 c s^-1PCU^-1. The outburst-averaged recurrence time between events is 1.8 hr, while the shortest separation between any pair is 18 min.

Time-resolved spectral analysis of the two highest-quality bursts reveals a net spectrum consistent with black body radiation, with a peak temperature of kT=2.6+/-0.2 keV and a peak bolometric unabsorbed flux of (3.3+/-0.2)x10^-9 erg s^-1cm^-2. We found significant evidence for cooling of the spectrum throughout the bursts. All these features are commonly found in thermonuclear (type-I) X-ray burst from accreting neutron stars.

The nearest known burst source is the transient XTE J1810-189, 0.996 degrees away (Markwardt et al. 2008, ATel #1443). This source is just within the PCA field of view and, thus, could be the origin of the bursts. The instrumental response towards XTE J1810-189 would imply a peak intensity that is roughly consistent with our measurements of the bursts detected in 2008 September. However, XTE J1810-189 was not active over that period, having concluded an outburst 3 months earlier. The last significant detection of XTE J1810-189 with the PCA during the Galactic bulge scans (Swank & Markwardt 2001, ASP Conf. Proc. 251, 94) was on MJD 54627 (2008 June 10; see link below). Thus, we attribute the bursts to XMMU J181227.8-181234, identifying it as a bursting neutron-star in a presumed low-mass X-ray binary.

The peak bolometric burst flux translates, for a typical type-I X-ray burst peak luminosity of about 10³⁸ erg s^-1, to a distance of about 16 kpc. No photospheric expansion was detected, implying an upper limit for the distance of about 31 kpc for an Eddington limit of 3.8x10³⁸ erg s^-1 (Kuulkers et al. 2003, A&A 399, 663).

The peak persistent flux of the 2008 outburst of XMMU J181227.8-181234, according to the PCA Galactic bulge scans, is 97+/-3 c s^-1PCU^-1 which translates to approximately 45 mCrab (or 1.3x10^-9 erg s^-1cm^-2 for a Crab-like spectrum). IBIS/ISGRI observations from the INTEGRAL Galactic bulge monitoring program (ATel #438) do not yield any detection, with a typical 3-sigma upper limit of 6-10 mCrab in 18-40 keV for every 3.5 hr observation. A spectral analysis of a 1.9 ksec PCA observation close to the transient outburst peak yields a soft persistent spectrum with a plasma temperature, according to the Comptonized model of Titarchuk (1994, ApJ 434, 570), of 2.6+/-0.1 keV and an optical thickness of 5.6+/-0.2. The 3-25 keV flux is (1.13+/-0.02)x10^-9 erg s^-1cm^-2 and the estimated bolometric unabsorbed flux 3x10^-9 erg s^-1cm^-2. For >16 kpc, this translates to ~>25% of the Eddington limit. The 18-40 keV flux extrapolated from this spectrum is equivalent to 6 mCrab, consistent with the ISGRI upper limit. We found no evidence for burst or persistent pulsations in the data.

Given the comparatively short burst recurrence time, the low burst peak flux and the soft persistent spectrum, we tentatively qualify this source as a transient high-luminosity distant low-mass X-ray binary.

All light curves from the 1999-2011 PCA Galactic Bulge Scans (courtesy Craig B. Markwardt & Jean Swank)