Optical and X-ray Observations of the New Transient IGR J18284-0345
ATel #1982; J. P. Halpern, J. P. Schroeder (Columbia U.), L. Vican (Barnard)
on 22 Mar 2009; 00:32 UT
Credential Certification: Jules Halpern (email@example.com)
Subjects: Optical, X-ray, Transient
We observed the position of the INTEGRAL hard X-ray transient IGR J18284-0345
and its Swift XRT counterpart (Blanchard et al., ATel #1981) in the
I-band, using the MDM 1.3m telescope on 2009 March 21 11:48 UT.
No optical counterpart
is detected to a limiting magnitude of I=20 in a 15 minute exposure.
The surrounding region is largely devoid of stars, which can be related to the
high extinction estimates, E(B-V) = 3.49, A(V)=11.57, A(I)=6.77
derived by Schlegel et al. (1998, ApJ, 500, 525) for this location.
Searches for an infrared counterpart are therefore encouraged.
We also analyzed the Swift XRT spectrum described in ATel #1981, but
explicitly fitting for column density N(H) rather than
fixing it at the Galactic 21 cm value of 5.6e21 cm-2.
For a blackbody model we find
kT = 2.9(-0.8,+1.8) keV and
N(H) = 2.4(-1.3,+1.7)e22 cm-2 (90% confidence),
with a 2-10 keV flux of 3.2e-11 ergs cm-2 s-1.
We prefer our larger value of X-ray N(H),
~4 times the 21 cm value, because it is more
consistent with the optical extinction. In contrast,
the Galactic 21 cm N(H)
would correspond to A(V)=3.1 according to the relation
of Predehl & Schmitt (1995, A&A, 293, 889).
Other continuum models fitted to the Swift XRT data,
such as power-law and bremsstrahlung,
require even larger values of N(H) than a blackbody,
but probably unrealistic slopes.
The Swift XRT light curve shows variability by a factor of 2 on time scales
as short as 100 s.
There is a faint X-ray source in an XMM-Newton
observation of this region from 2005 March 22, at coordinates
R.A. = 18h28m30.1s, decl. = -03d45'44" (J2000), which is consistent
with the Swift XRT position. It may be the quiescent counterpart of
Swift XRT Light Curve