NICER detects pulsations from a new outburst of the accreting millisecond X-ray pulsar IGR J17379-3747

ATel #14056; P. M. Bult (NASA/GSFC), A. Sanna (Univ. of Cagliari), K. C. Gendreau, Z. Arzoumanian, T. E. Strohmayer (NASA/GSFC), L. Burderi (Univ. of Cagliari), T. Di Salvo (Univ. of Palermo), Deepto Chakrabarty (MIT)
on 1 Oct 2020; 01:43 UT
Credential Certification: Peter Bult (p.m.bult@nasa.gov)

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On 2020 September 29, the all-sky monitor ART-XC detected a brightening in the direction of IGR J17379-3747, suggesting that this known 468 Hz accreting millisecond pulsar is undergoing a new X-ray outburst (ATel #14051). Following this report we began observing IGR J17379-3747 with NICER. Between 2020-09-29 23:09 UTC and 2020-09-30 08:40 UTC we collected 4.6 ks of good time exposure. The source is clearly detected in these data, showing a steady drop in count-rate from 8 ct/s to 6 ct/s in the 1-10 keV energy band. The estimated background contribution in this same energy band is 1 ct/s.

The 0.5-10 keV energy spectrum is well described as an absorbed blackbody with a thermally comptonized continuum. We model the spectrum in XSPEC as tbabs (bbodyrad + nthcomp), keeping the blackbody temperature linked between the two model components and holding the electron temperature fixed at 30 keV. Our best fit solution has a chi-square of 472 for 442 degrees of freedom. The measured absorption column is (9.5 +/- 0.9) x10²¹ cm^-2, while the blackbody temperature and power law index are measured at kT = 0.10 +/- 0.01 keV and Gamma= 2.1 +/- 0.1, respectively. Neither emission lines nor reflection components are required to improve the spectral model. The unabsorbed 1-10 keV flux is found to be Fx = (3.4 +/- 0.2) x10^-11 erg cm^-2 s^-1. These results are consistent with previous observations of IGR J17379-3747 (Sanna et al. 2018, A&A, 616, L17; Bult et al. 2019, ApJ, 877, 70).

After barycenter-correcting the photon event times, we computed a power density spectrum of the 1-10 keV data. We find a clear pulsed signal at the known 468 Hz spin frequency, confirming definitively that IGR J17379-3747 is indeed the active source. We then removed the Doppler modulation imposed by the binary orbit based on the best-known ephemeris (Sanna et al., 2018; Bult et al., 2019) and computed a pulse profile for each continuous NICER pointing (~900 seconds exposure each). By fitting the phase residuals for the pulse frequency and time of ascending node only, we measure the local time of ascending node at Tasc = 59120.945847(3) TDB; which is earlier than predicted by the long-term binary orbit solution of Sanna et al. (2018). Fitting the long-term evolution in the residual Tasc values with a quadratic model (see, e.g., Hartman et al. 2008, ApJ, 675, 2), we find significant evidence for a binary period derivative of Pdot = (-2.9 +/- 0.5) x 10^-12 s s^-1. Finally, we computed the average pulse profile, which yielded a fundamental pulse amplitude of 20% fractional sinusoidal amplitude, and a second harmonic at 7.5% fractional amplitude.

Because IGR J17379-3747 is currently subject to visibility constraints, we will resume NICER monitoring of this source after October 7. Multi-wavelength observations of the source are strongly 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.