Discovery of a Fourth Accreting Millisecond Pulsar, XTE J1807-294
ATel #122; C. B. Markwardt (U. Maryland & GSFC), E. Smith (GSFC) & J. H. Swank (GSFC)
on 26 Feb 2003; 02:55 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Craig B. Markwardt (craigm@lheamail.gsfc.nasa.gov)
Subjects: X-ray, Request for Observations, Binary, Neutron Star, Transient, Pulsar
C. B. Markwardt, University of Maryland and Goddard Space Flight
Center (GSFC), and E. Smith and J. H. Swank, GSFC, report that RXTE
PCA monitoring observations of the galactic center region have
revealed a new millisecond pulsar, designated XTE J1807-294. PCA
cross scans on Feb 21.6 (UT) determined the position to be R.A. =
18h07m00s, Dec. = -29o24' (equinox 2000.0) with an uncertainty of
approx. 1', due to the variable nature of the source and diffuse
galactic emission. The Feb 21.6 observation also contained pointed
data with a time baseline of 3600 s, during which coherent pulsations
were detected at a barycentered frequency of 190.625 Hz. While the
pointing is interrupted by scans, there appears to be a significant
sinusoidal modulation of the pulse frequency, at a period of 35 +/- 3
minutes, and with a half-amplitude of 2.2 +/- 0.6 mHz. This source is
the fourth known accreting millisecond pulsar, and the orbital period,
if confirmed, is the shortest yet of that group. The r.m.s. pulse
amplitude is approximately 7% (2-30 keV), consistent with previous
such pulsars. The 2-10 keV X-ray fluxes on the following dates were
(in mCrab): Feb 16.7, 33; 19.8, 38; 21.6, 58; 22.6, 41; 25.4, 32;
which suggests that the X-ray peak has passed. The spectrum is
consistent with a cut-off power law with an photon index of 2, and a
hydrogen column density of 3 x 10**21 cm-2. Further RXTE observations
are planned. Observations at other wavelengths are encouraged.
