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ATEL # 1685; C. B. Markwardt (CRESST/U.Md./NASA/GSFC), D. Pereira (WIS/NASA/GSFC), J. H. Swank (NASA/GSFC)
on 28 Aug 2008; 22:02 UT
Distributed as an Instant Email Notice (Transients)
Password Certification: Craig B. Markwardt (email@example.com)
Subjects: X-ray, Request for Observations, Binaries, Black Holes, Neutron Stars, Pulsars, Transients
In PCA scans of the galactic ridge region on 2008-08-21, RXTE detected a transient source. A follow-up scanning observation determined the position to be (RA,Dec) = 273.117,-18.26 (J2000) with an error radius of 2.1 arcmin (95% confidence). Thus, we designate this new source XTE J1812-182. We note that this source is distinct from the 120 second pulsar, XTE J1824-141, which was discovered around the same time and in the same region.
The source flux is rising. Monitoring PCA scans found 2-10 keV fluxes of 6.3, 14.4 and 20 mCrab on August 21.4, 24.6 and 28.2 respectively. The source was not detectable before that time (from Feb-Aug 2008), with a typical 95% confidence flux upper limit of about 1.5 mCrab.
In a pointed observation on 2008-08-22 at 02:53 UT, the source was detected. However, there is some contamination from diffuse galactic X-rays, which was estimated phenomenologically with a Raymond spectral model. The intrinsic spectrum is consistent with a highly absorbed power law (NH = 7e22 cm-2; photon index = 2.6). The fluxes in the 2-10, 10-20 and 20-40 keV bands were 6.6e-11, 2.6e-11 and 1.8e-11 erg cm-2 s-1 respectively (note that the flux has at least tripled since that time). No variability signatures such as pulsations, QPOs or bursts were detected.
Just outside the PCA error circle lies a transient source previously detected by XMM-Newton and RXTE ASM in 2003 (offset 3.2 arcmin), designated XMM J181227.8-181234 (Cackett, Wijnands & Remillard 2006, MNRAS, 369, 1965). The XMM source had a similar spectrum (highly absorbed power law), similar flux level, and similar lack of variability. We are thus reasonably confident that the XMM and XTE sources are the same. If true, this PCA detection would establish a recurrence period of about 5 years. Cackett et al. also note that the XMM source may match a HEAO 1 source from the 1980s, 1H1812-182.
In their paper, Cackett et al. speculated that the XMM source was a high-mass X-ray binary, although no pulsations were detected. One might also interpret the energy spectrum and variability behavior as being a black hole system in a softer spectral state.
Follow-up observations in the IR at the position of the XMM counterpart may reveal activity, and may confirm the nature of the source. As a guide, the previous XMM transient outburst in 2003 lasted 60-100 days.