RXTE Detects a Transient, XTE J1812-182 ( = XMMU J181227.8-181234 ? )
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
Credential Certification: Craig B. Markwardt (craigm@lheamail.gsfc.nasa.gov)
Subjects: X-ray, Request for Observations, Binary, Black Hole, Neutron Star, Transient, Pulsar
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.