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ATEL # 1686; C. B. Markwardt (CRESST/U.Md./NASA/GSFC)
on 30 Aug 2008; 0:15 UT
Password Certification: Craig B. Markwardt (firstname.lastname@example.org)
Subjects: X-ray, Binaries, Neutron Stars, Pulsars, Transients
After the report of a 120 second pulsar, XTE J1824-141 (ATEL #1679), and of a possible INTEGRAL counterpart (IGR J18246-1425; Stephen et al., ATEL #1680), we obtained preliminary Swift XRT observations of a small part of the RXTE PCA error circle (ATEL #1681). In that observation, only a single source was found, consistent with the IGR source position.
We obtained further Swift XRT observations which almost completely cover the PCA error circle (observed mostly on 2008-08-28). We can now confirm that the previously reported Swift/IGR source is the only reasonable counterpart in the entire PCA error circle. The next brightest XRT source is a factor of 10 fainter, completely inconsistent with the PCA flux. We also obtained RXTE PCA observations on 2008-08-28 at 11:20 UT which confirmed that the RXTE pulsar was still active at approximately the same flux level, ~1 mCrab, i.e. it has not become quiescent.
We re-examined the XRT data for pulsations. We do detect a periodic modulation at the expected pulse period of 120.0 seconds, with 99% confidence. We had disregarded this signal previously because of a processing error related to the CCD frame read-out time, which had produced some spurious aliases.
Given the detection of pulsations, we are convinced that XTE J1824-141 and IGR J18246-1425 are the same source. We note that a slightly revised position based on the 'xrtcentroid' software is RA,Dec = 276.0990, -14.4154 (J2000) with a systematic error radius of 4.3 arcsec. (We thank A. Kong, priv. comm., for his comment on this.)
I would like to thank the Swift planners for their assistance in planning this observation and their help afterward, especially J. Kennea and M. Stroh.