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NICER discovers millisecond pulsations from the neutron star LMXB IGR J17379-3747

ATel #11507; T. E. Strohmayer (NASA/GSFC), P. S. Ray (NRL), K. C. Gendreau (NASA/GSFC), P. M. Bult (NASA/GSFC), S. Guillot (IRAP, CNES), S. Mahmoodifar (NASA/GSFC), G. K. Jaisawal (DTU Space), Z. Arzoumanian (NASA/GSFC), D. Altamirano (Univ. of Southampton), S. Bogdanov (Columbia), D. Chakrabarty (MIT), T. Enoto (Kyoto Univ.), C. B. Markwardt (NASA/GSFC), F. Ozel (Univ. of Arizona), S. M. Ransom (NRAO)
on 6 Apr 2018; 02:22 UT
Credential Certification: Tod Strohmayer (tod.strohmayer@nasa.gov)

Subjects: X-ray, Request for Observations, Binary, Neutron Star, Pulsar

Referred to by ATel #: 11520

Following a 2018 March 19 MAXI alert of a new outburst of the neutron star low-mass X-ray binary IGR J17379-3747 (ATel #11447), NICER has observed the source daily since 2018 March 29. From that date onward, the mean count rates detected each day through April 1 were 12.9, 11.0, 8.7, and 4.7 ct/s (0.5-12 keV), respectively. The background count rate in this band is less than 1 ct/s. After applying barycenter corrections to the event times using the radio position reported in ATel #11487, we computed a combined power spectrum of the full ~9 ks collected exposure and detected a clear pulsation (> 7 sigma significance for a single trial) at a frequency of 468.05 Hz. The data consist of 11 segments of varying uninterrupted exposure between 500 s and 1000 s duration. We searched these segments for pulsations individually using the PRESTO acceleration search code. We detected a power-spectrum excess near 468.05 Hz in seven of those segments (at a significance ranging from 6 to 8 sigma in each segment, with frequencies ranging from 468.05 to 468.12 Hz). Fitting the observed frequency modulation with a circular orbit model yielded an excellent fit, with an orbital period of 1.88 hrs and a minimum companion mass of 0.055 Msun (assuming a 1.4 Msun neutron star). The barycentric pulsar spin frequency is 468.0832 Hz. We extracted a 0.5-10 keV spectrum for each day, and find they are all consistent with an absorbed power-law model. However, adding a blackbody component significantly improves the fits (reduced Chi^2 = 0.97 for 1278 d.o.f.). The blackbody temperature decreases from 0.52+/-0.06 keV to 0.39+/-0.03 keV as the flux declines. At the same time, the power-law component softens slightly, with the photon index evolving from 2.1+/-0.1 to 2.4+/-0.1. The measured hydrogen column density is N_H=(0.7+/-0.1)e22 cm^-2, consistent with a previously reported value (ATel #1714), but significantly lower than the most recently reported value (ATel #11487). The unabsorbed flux decreases from 7.4e-11 ergs/cm^2/s (2018 March 29) to 2.5e-11 ergs/cm^2/s (2018 April 1). Our pulsation detection conclusively identifies IGR J17379-3747 as an accreting millisecond X-ray pulsar. NICER monitoring of the source continues. Given its rapidly declining X-ray flux, prompt follow up at other wavelengths is encouraged.