NICER detection of the X-ray pulsation of Bursting Pulsar GRO J1744-28
ATel #14695; M. Ng (MIT), P. M. Bult (NASA/GSFC), A. Sanna (Univ. of Cagliari), D. Altamirano, A. C. Albayati (University of Southampton), J. M. Miller (Univ. Michigan), D. Chakrabarty (MIT), K. C. Gendreau, Z. Arzoumanian (NASA/GSFC), on behalf of the NICER team
on 10 Jun 2021; 00:09 UT
Credential Certification: Mason Ng (masonng@mit.edu)
Subjects: X-ray, Neutron Star, Transient, Pulsar
On 2021 June 04, Swift/BAT detected the X-ray bursting pulsar GRO J1744-28 in an outburst in the hard X-ray band (15-50 keV). Swift/XRT subsequently observed the source on 2021 June 08 and estimated an unabsorbed flux (0.3-10 keV) of about 1.5 × 10-9 erg/s/cm2 (ATel #14689). NICER began observing the source and collected 1.6 ks of exposure on 2021 June 09 between 18:35 UTC and 19:04 UTC. The 0.3-12 keV count rate is roughly 38 c/s (including a background of < 1 c/s).
We detected highly significant coherent pulsations, and from an analysis on four pulse arrival times in the 2-12 keV band, we find a barycentric pulse frequency of F0 = 2.141175(12) Hz at epoch T0 = MJD 59374.78 (TDB), or equivalently, a period of P = 0.467033(3) s. This is uncorrected for the pulsar's 11.8-day binary orbit, which will introduce a sinusoidal Doppler shift of amplitude around 3.5 × 10-5 Hz; our observation baseline is not yet long enough to determine the orbital phase. The pulse profile has a 12% fractional sinusoidal amplitude in the 2-12 keV band. The power density spectrum in the same energy band further shows an 8% rms amplitude QPO centered at 0.12 Hz.
The 1-10 keV spectrum can be fitted with a combination of cooler (disk blackbody kT = 1.1 keV) and hotter (blackbody kT = 2.5 keV) thermal components. This is only marginally preferred over a simple power-law (photon index Gamma = 1.1) within the initial snapshot. In both cases, the line-of-sight absorption is approximately N_H = 9.5 × 1022 cm-2. The model with two thermal components implies an absorbed flux of F = 6.8 × 10-10 erg/s/cm2, or an unabsorbed flux of F = 1.1 × 10-9 erg/s/cm2 (1-10 keV), consistent with the value observed with Swift/XRT (ATel #14689). In the Fe K band, emission lines consistent with He-like Fe XXV triplet are evident. They may plausibly be associated with the forbidden and resonance lines, and are of approximately equal strength (50 eV and 40 eV, respectively). There is weaker evidence of H-like Fe XXVI in emission. A qualitatively similar spectrum was observed with Chandra during the prior outburst of GRO J1744-28, and was better described in terms of a broad emission line imprinted with strong absorption (Degenaar et al. 2014, ApJ, 796, L9). Subsequent NICER spectra may be able to distinguish these alternatives.
NICER will continue to monitor GRO J1744-28. Further observations of this source are highly encouraged.
NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.
