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GRO J1744-28 active as X-ray pulsar

ATel #5858; DAi A, T. Di Salvo, R. Iaria (Uni Palermo), A. Riggio, L. Burderi, A. Sanna, F. Pintore (Uni Cagliari)
on 6 Feb 2014; 23:55 UT
Credential Certification: Antonino D'Ai (antonino.dai@unipa.it)

Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar

Referred to by ATel #: 5883, 5895, 5901, 5904, 5910, 5997, 5999

We report on a preliminary analysis of two Swift/XRT observations of the outgoing outburst of GRO J1744-28 [Atel #5845, Atel #5810, Atel #5790]. The source was observed on 2014-02-03 20:40:17 UT and on 2014-02-05 19:05:17 Ut in WT Mode for 980 s and 973 s, respectively. In both cases, the spectrum can adequately be fitted with an absorbed power-law model. The photon-index of the power-law, the 1-10 keV absorbed flux, and the NH absorption column (adopting the abundances from Wilms at al. 2000, ApJ 542, 914) are 1.2(1), 4(1)E-09 erg/cm^2/s, 11.2(7) E22 cm^-2 for the observation of Feb.3 and 1.0(1), 5.8(1.0)E-09 erg/cm^2/s, 10.5(8)E22 cm-2 for the Feb.5 observation. During the Feb.5 observation, coherent pulsed emission has been clearly detected in the power spectrum with significativity >8 sigma. After having corrected the photon arrival times at the solar system barycentre, a folding search around the FERMI gamma-ray period of 0.467047 s [Atel #5810], revealed a clear peak at P(spin) 0.46708(5) s, with a pulsed fraction 6.2(7)%. The large uncertainty in the spin estimate due to the lack of the satellite orbit file. The pulsar is therefore again X-ray active.