The 401 Hz accreting millisecond X-ray pulsar SAX J1808.4-3658 has been in outburst since 2019 August 6 (ATel #12964, ATel #12993, ATel #13000, ATel #13001). Over the past four weeks, NICER has been extensively monitoring this source, the analysis of which is ongoing. SAX J1808.4-3658 reached a peak intensity of 270 c/s in the 0.3-10.0 keV energy band on Aug 13 and has been declining since. On Aug 24 this downward trend was interrupted as the source transitioned into its flaring state . This state is characterized by cyclic swings in luminosity during which the source oscillates between luminosities of 10^32 erg/s and 10^33 erg/s on a timescale of 3 to 5 days (Wijnands et al 2001, ApJ, 560, 892; Patruno et al 2009, ApJ, 707, 1296).
On 2019 Sep 3 we observed an unusually bright and abrupt reflare from SAX J1808.4-3658. Over the span of 5.7 hours, the observed count-rate increased fivefold, going from 22 c/s at 00:14 UTC to 115 c/s at 06:04 UTC, and peaking at 140 c/s at 21:24 UTC. At the time of our latest observation, on Sep 4 02:06 UTC, however, the observed count-rate dropped back to 107 c/s, suggesting that the peak luminosity of this event has already passed.
In addition to being unusually abrupt, the energy spectrum at the peak of the reflare is unusually soft. Extracting a spectrum from the 789 seconds of good time exposure collected across the reflare, we find that the emission above 1 keV is well described (reduced chisq of 1.04 for 356 dof) by an absorbed (N_H = 2.1e21 cm^-2) comptonized component (nthcomp, Gamma=2.16 +/- 0.03) plus a blackbody (diskbb, kT = 0.16 +/- 0.03 keV). Below 1 keV, however, a large excess is observed (reduced chisq of 29). This excess could be reasonably well described with a blackbody component (bbodyrad), yielding a reduced chisq of 1.24 for 425 dof. The blackbody had a temperature of 0.066 +/- 0.003 keV and a radius of 550 +/- 70 km (for d=3.5 kpc). The unabsorbed flux was (2.16 +/- 0.03)e-10 erg/cm^2/s, corresponding, at the source distance, to a luminosity of 3.2e35 erg/s -- an order of magnitude brighter than any reflare previously recorded from SAX J1808.4-3658.
Folding all photon event times collected during the Sep 3 reflare on the preliminary ephemeris of the 2019 outburst (ATel #13001, ATel #13022), we detected significant (9 sigma) pulsations at 401 Hz. The pulse amplitude in 0.3-10 keV energy range was 3.8%. For comparison, folding the 4.3 ks of exposure collected between Aug 31 and Sep 3 gives a fractional pulse amplitude of 7.3% for the same energy range. Excluding all events below 1 keV increased these amplitudes to 5% and 9%, respectively. No QPOs were observed in the 0.3-10 keV power spectrum.
The abrupt rise and soft spectrum mark this reflare from SAX J1808.4-3658 as a highly unusual event. Changes in pulse amplitude by a factor of 2 in this source are known to depend critically on the kHz QPO frequency (Bult & van der Klis, 2015, ApJL, 798, L29), suggesting that the accretion disk may have undergone a sudden shift to smaller radii, possibly releasing the observed burst of soft emission. NICER will continue to monitor this source to determine if this event marks the start of renewed outburst activity, or whether it will proceed in the flaring state.
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.
|Publication date||4 Sep 2019|
|Publication status||Published - 4 Sep 2019|
|Series||The Astronomer's telegram|
- Neutron star