Detection of a spin derivative in GRO J1744-28 with Swift/XRT

ATel #5901; Pintore F., Sanna A., Riggio A., Burderi L. (University of Cagliari), Di Salvo T., D'Ai A., Iaria R., Robba N. R. (University of Palermo)
on 18 Feb 2014; 19:21 UT
Credential Certification: Alessandro Riggio (riggio@dsf.unica.it)

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We report a timing solution for the accreting Neutron Star GRO J1744-28, observed during its ongoing outburst (ATel #5790, #5810, #5845, #5858, #5883). We analyzed all the available Swift-XRT observations taken in Window Timing mode, from 2nd to 17th February 2014. These observations have an effective exposure time of ~13 ks. The intervals in which X-ray bursts are clearly detected (Atel #5883) were removed. The times of arrival (ToA) of photons detected by XRT on-board Swift were reported 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. We epoch-folded data on time intervals of 1 ks, using the frequency reported by the GBM Pulsar Project at the time of the Swift observations. We modeled the pulse profile with a sinusoid. Since the data cover slightly more than one orbital period, we subsequently fitted the pulse phase delays keeping orbital period and projected semi major axis fixed, with a model that takes into account a spin frequency derivative. We obtained a good fit (reduced chi squared ~0.8 for 9 d.o.f., see figure 1) with a Time of ascending node passage of 50241.839(7) MJD, a spin frequency of 2.1411162(4) Hz at 56693.0 MJD, and a spin frequency derivative of 1.7(3)e-12 Hz/s. Errors are given at the 1 sigma confidence level. Our frequency measurement is consistent within the errors with that reported by Finger et al. (ATel #5810). In figure 2, we show the pulse profile obtained by folding all the Swift observations.