A hard X-ray transient discovered by INTEGRAL is revealed by XMM-Newton to be an obscured wind-fed pulsar
Among the main scientific results from INTEGRAL has been the discovery
of supergiant X-ray binaries that are highly obscured (column density:
nH > 1023 cm-2), and those that are extremely
variable, i.e. where the luminosity varies by several orders of magnitude
between quiescence and outburst.
One source discovered by INTEGRAL, IGR J18462-0223, represents an object
that displays both of these extreme characteristics. This candidate
supergiant fast X-ray transient was observed by XMM-Newton for 32 ks in
order to help clarify its nature. The image in the upper left panel is
in Galactic coordinates from XMM-Newton (MOS1, 0.5-10 keV), and it reveals
a bright X-ray point source located just inside the 1.6-arcmin error-radius
(90% confidence) circle from INTEGRAL-ISGRI (S. Grebenev & R. Sunyaev,
AstL 36, 533, 2010). The upper right panel shows the background-
corrected X-ray spectrum of IGR J18462-0223 as gathered by XMM-Newton
detectors (pn: black, MOS1: red, MOS2: blue), with each bin collecting a
minimum of 20 counts. An absorbed power law fit to the spectrum yields a
large absorbing column (nH = 3x1023 cm-2)
and a hard photon index (Gamma = 1.5). An iron line (at 6.4 keV) and a
break in the power law (at 3 keV) are also detected in the X-ray spectrum.
These spectral features are typical of wind-fed X-ray pulsars, and indeed,
we discovered a long, coherent pulsation at 997±1 s (lower left
panel) whose profile over two periods is shown in the lower right panel.
The pulsation is caused by a misalignment between the spin and magnetic
axes of a neutron star that accretes from the wind of a high-mass (O or
B type) stellar companion.
This illustrates the complementary nature of using hard X-ray observations
with INTEGRAL for discovering high-mass X-ray binaries, paired with
follow-up observations with XMM-Newton for characterizing the sources.
Discoveries by INTEGRAL continue to have an impact on the scientific
output of other X-ray observatories.
For more information, including the analysis of four other IGR sources,
please see:
"XMM-Newton observations of five INTEGRAL sources located towards the Scutum Arm,"
A. Bodaghee et al. 2012c,
ApJ, in press, 2012, ArXiv:1204.364
