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ATEL # 1681; C. B. Markwardt (CRESST/U.Md./NASA/GSFC)
on 27 Aug 2008; 2:40 UT
Password Certification: Craig B. Markwardt (email@example.com)
Subjects: Infra-Red, X-ray, Binaries, Neutron Stars, Pulsars, Transients
Regarding the 120 sec X-ray pulsar XTE J1824-141 (Markwardt et al., ATEL #1679), we requested follow-up observations with the Swift observatory. The Swift planners concentrated on the INTEGRAL candidate position, IGR J18246-1425 (Stephen, ATEL #1680). The observation started on 2008-08-26 at 15:40 UT, for an exposure of 1 ks. The Swift pointing direction was RA, Dec = 276.1536, -14.39392 (J2000), and the XRT field of view is 24 x 24 arcmin^2.
In the XRT image, only one significant source appears in the image, at position RA, Dec = 276.09833, -14.415778 (J2000), with a position error of < 6 arcsec. The XRT candidate position is consistent with the IGR and XTE source positions. The XRT source has a 2-10 keV flux of about 1.4e-11 erg/cm^2/s, or ~0.6 mCrab, which is quite a bit fainter than the recent PCA fluxes (ATEL #1679). Also, no significant pulsations were present at the expected period of 120 seconds. The spectrum is highly absorbed, with a neutral column density of about 8e22 cm^2 (for a power law continuum, with photon index 1.15). There is also a rather bright infrared 2MASS catalog source within about 6 arcsec of the XRT position (K=10), designated 2MASS J18242381-1424509, which may be a plausible X-ray binary counterpart. (Thanks to J. Halpern for noting this; priv. comm.)
Given the lack of pulsations, the correspondence between the XTE and IGR candidates is suggestive, but we cannot directly confirm the association yet. The source flux is already known to be variable based on PCA data (ATEL #1679), so the low flux observed in the XRT source may be consistent with the PCA behavior.