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ATEL # 2215; M.A. Nowak (MIT-CXC), A. Paizis (INAF-Milano), J. Wilms (Remeis-Observatory & ECAP), J. Rodriguez (CEA), S. Chaty (CEA),K. Ebisawa (ISAS), M. Del Santo (IASF-Roma), R. Farinelli (U. Ferrara), P. Ubertini (IASF-Roma), T. Courvoisier (ISDC)
on 25 Sep 2009; 16:08 UT
Password Certification: Joern Wilms (firstname.lastname@example.org)
Subjects: X-ray, Binaries, Neutron Stars, Pulsars
Referred to by ATEL #: 2216
Chandra-HETG observations were performed on the bursting accreting milli-second X-ray pulsar IGR J17511-3057 (Baldovin et al. ATEL #2196, and further reported upon in ATEL #2197, #2198, #2199). We have used these observations to refine the X-ray position. By intersecting the image of the Medium Energy Grating (MEG) dispersed spectra with the CCD readout streak from the 0th order image, we find a J2000.0 position of:
RA: 17 51 08.66
Dec: -30 57 41.0
We estimate that the 1sigma statistical error on the position is 0.1 arcsec, while the 1sigma systematic error on the position is 0.6 arcsec. As there are no other X-ray sources observed within the field of view, the latter is determined by the absolute accuracy with which the Chandra aspect solution is typically known.
Note that this position is slightly more than 5 arcsec away from the position reported by Bozzo et al. (ATEL #2198), which was determined from a Swift observation. We have reexamined the Swift data and find that the Chandra error circle lies within the Swift source image, and furthermore that due to pileup effects, the Swift error circle was considerably larger than the originally reported 3.5 arcsec.
The Chandra-HETG observations began on 2009 Sept. 22 7:40 UT and lasted for 20 ksec. We have performed simple phenomenological (absorbed disk plus powerlaw) fits to the gratings spectra. The spectra are consistent with being absorbed by a column of approximately 1.3x1022 cm-2. The absorbed 0.5-5 keV flux is 1.9x10-10 ergs cm2 sec-1 (6 mCrab), and the absorbed 2-10 keV flux is 3.2x10-10 ergs cm-2 sec-1 (13 mCrab).
The Chandra lightcurve shows an X-ray burst occurring approximately 17.5 ksec into the observation, rising in less than a few 10s of seconds and completely decaying in less than 200 seconds. (These data have been excluded from the above spectral fits.) Owing to the CCD pileup present during the burst, further details of the burst properties await a more careful analysis.
We would like to thank the Chandra team for their rapid response in both scheduling this Target of Opportunity observation and in delivering the data.