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ATEL # 1982; J. P. Halpern, J. P. Schroeder (Columbia U.), L. Vican (Barnard)
on 22 Mar 2009; 0:32 UT
Password Certification: Jules Halpern (firstname.lastname@example.org)
Subjects: Optical, X-ray, Transients
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 IGR J18284-0345.
Swift XRT Light Curve