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Swift-XRT confirms the renewed X-ray activity of the Bursting Pulsar "GRO J1744-28"

ATel #10079; A. Sanna (UNICA), A. D`Ai (INAF/IASF-Palermo), E. Bozzo (ISDC, Switzerland) , A. Riggio (UNICA), F. Pintore (INAF/IASF-Milano), L. Burderi (UNICA), T. Di Salvo (UNIPA), R. Iaria (UNIPA)
on 15 Feb 2017; 10:21 UT
Credential Certification: Andrea Sanna (andrea.sanna@dsf.unica.it)

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

Referred to by ATel #: 10106

Triggered by the X-ray enhancement reported by INTEGRAL on 2017, Feb. 13 at a position compatible with the X-ray binary GRO J1744-28 (Atel #10073), a 1 ks observation with Swift-XRT was promptly carried out. Swift-XRT, operated in Window Timing mode, detected a bright X-ray source with mean count rate of approximately 3 cts/s. The X-ray spectrum can adequately be fitted with an absorbed power-law model (chi^2 = 99 with 112 degrees of freedom). The photon-index of the power-law, the 0.3-10 keV absorbed flux, and the NH absorption column are 1.2(1), 3.4(2)E-10 erg/cm^2/s and 7(1) E22 cm^-2, respectively.

We reported the detected times of arrival (ToA) of photons to the Solar system barycentre, using the source coordinates reported by Wijnands & Wang 2002 (ApJ, 568, L93), consistent with the recently reported Chandra position (ATel #5895). ToAs were subsequently corrected for orbital delays using the orbital parameters provided by the GBM Pulsar Project for the 2014 outburst and consistent with that reported by Sanna et al 2017 (submitted). We epoch-folded the 1ks observation around the spin frequency reported by the D`Ai et al. 2015 (MNRAS, 449, L4288), with frequency bin steps of 1E-5 Hz. We detected a significant (7 sigma) X-ray pulsation at the frequency nu=2.1413(1) Hz, consistent within the errors with the spin frequency observed during the previous outburst (ATel #5810, #5901, D`Ai et al. 2015 MNRAS, 449, L4288). We modelled the pulse profile with a sinusoid, detecting a pulse fraction of 16.8(2)%. The detection of X-ray pulsation consistent with the spin frequency of the Bursting Pulsar, confirms that GRO J1744-28 is again X-ray active.

We thank the Swift team for the prompt scheduling of the reported observation. Additional Swift/XRT observations have been requested. Multi-wavelength observations are encouraged.