NICER detects X-ray pulsations from the rapidly brightening SAX J1808.4-3658

ATel #13001; P. M. Bult, K. C. Gendreau, Z. Arzoumanian (NASA/GSFC), D. Chakrabarty (MIT), P. S. Ray (NRL), S. Guillot (IRAP/CNRS), M. T. Wolff (NRL), S. Bogdanov (Columbia), R. M. Ludlam (Michigan)
on 7 Aug 2019; 23:49 UT
Credential Certification: Peter Bult (p.m.bult@nasa.gov)

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Following recent reports of an optical brightening at the position of the accreting millisecond X-ray pulsar SAX J1808.4-3658 (ATel #12964), NICER began regular monitoring of this source. The first NICER observations of SAX J1808.4-3658 were collected on 2019 July 30 14:46 UTC, after which we returned to the source for an average of 2 ks every day. Between July 30 and August 5 the 0.5-10 keV NICER count rate fluctuated between 0.5 and 2.0 c/s, which is consistent with the estimated background rate in the direction of SAX J1808.4-3658. On August 6 the observed count-rate first exceeded 3 c/s and started rising rapidly to 15 c/s on August 7 between 19:17 UTC and 19:36 UTC (1.2 ks total exposure).

After barycenter-correcting the August 7 event times and correcting for the Doppler delays due to the known binary orbit (Hartman et al. 2009, ApJ, 702, 1673), we attempted to recover the expected 401 Hz pulsations by folding the data on the pulsar spin period. Given the uncertainty in the extrapolated orbital solution, we conservatively attempted this recovery using an orbital epoch grid spanning 200 seconds centered on the predicted time of ascending node (T_asc), using steps of 1 second. We obtained a positive pulse detection at 6 sigma significance. The pulsations were detected in the 1-10 keV band at a fractional sinusoidal amplitude of 9% with a best-fit orbital epoch time of T_asc = 58699.91979 +/- 0.00001 MJD (TDB). This T_asc value is 27 seconds (3.3 sigma) earlier than the predicted value, indicating that the orbital expansion is proceeding more slowly than expected from a simple quadratic model (see Patruno et al. 2017, ApJ, 841, 98 and Sanna et al. 2017, MNRAS, 471, 463 for more detailed modeling).

The X-ray spectrum of the August 7 data is well described by an absorbed blackbody plus nthcomp model (reduced chi-squared of 1.17 for 220 dof) with the electron temperature fixed at kT = 30.0 keV, the seed temperature linked to the blackbody, and an absorption column density of N_H = 2.1e21 cm^-2 (see Di Salvo et al. 2019, MNRAS, 483, 767). We find a blackbody temperature of kT = 0.10 +/- 0.02 keV and a power-law index of Gamma = 2.3 +/- 0.1. The 1-10 keV unabsorbed source flux was (1.66 +/- 0.07)e-11 erg / cm^2 / s on MJD 58702.82, and is rapidly increasing.

The rapid rise in X-ray flux and detection of 401 Hz pulsations confirm the recent reports from Swift (ATel #12993, ATel #13000) that SAX J1808.4-3658 has now begun a new outburst.

Further high-cadence NICER monitoring of this source is underway. Additional monitoring and multiwavelength follow-up of this accreting millisecond pulsar is 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.