NICER observations of reflares from SAX J1808.4-3658
ATel #15647; G. Illiano, A. Papitto, A. Miraval Zanon, F. Ambrosino (INAF OAR), M. C. Baglio (NYU Abu Dhabi), A. Sanna, L. Burderi (Univ. of Cagliari), P. M. Bult (NASA/GSFC), M. Ng, D. Chakrabarty (MIT), T. Di Salvo (Univ. of Palermo), D. Altamirano (Univ. of Southampton), on behalf of a larger collaboration
on 8 Oct 2022; 11:00 UT
Credential Certification: Alessandro Papitto (alessandro.papitto@inaf.it)
Subjects: X-ray, Neutron Star, Transient, Pulsar
On 2022 August 19 (MJD 59810), NICER confirmed the beginning of a new outburst of the accreting millisecond X-ray pulsar SAX J1808.4-3658 (Atel #15559), after being alerted by the MAXI/GSC nova alert system (Atel #15563). Radio and optical observations of the outburst were also performed by MeerKAT (Atel #15584) and the Las Cumbres Observatory Network of telescopes (LCO; Atel #15603), respectively. According to the optical monitoring (Atel #15603), the source has entered the typical reflaring period that characterizes the last phases of its outbursts (see e.g. Hartman et al. 2008, ApJ, 675, 1468; Patruno & Watts 2021, ASSL, 461, 143) around 2022 September 9 (MJD 59829). Here, we report on NICER observations performed on 2022 October 5 at 17:02 UT (MJD 59857.7), which show that SAX J1808.4-3658 is still reflaring after more than 30 days since the outburst onset.
NICER has carried out a high-cadence monitoring campaign of the source in the past six weeks. The usually short-living peak of SAX J1808.4-3658 lasted ~5-6 days during this 10th outburst, reaching a maximum NICER 0.2-12 keV count rate of ~300 c/s on 2022 August 24 (MJD 59815). Close to the peak, the 0.5-10 keV spectrum observed by NICER could be described by a 1.0(1) keV black body and a Comptonized component with a photon index of 1.5(1) and electron temperature exceeding 20 keV, giving a 0.5-10 keV unabsorbed flux of 7.0(2)E-10 erg/cm2/s. The decay phase lasted about three days, after which the source started its typical flaring tail (see e.g., Hartman et al. 2008, ApJ, 675, 1468; Patruno & Watts 2021, ASSL, 461, 143).
Similarly to previous outbursts, the flaring state shows variations in luminosity over a few hours (see e.g., Wijnands et al. 2001, ApJ, 560, 892; Hartman et al. 2008, ApJ, 675, 1468; Patruno et al. 2009, ApJ, 707, 1296). During this last phase, SAX J1808.4-3658 shows a NICER 0.2-12 keV count rate ranging between ~5 c/s and ~110 c/s and a pulse fractional amplitude in the 0.2-12 keV energy range as high as 6%, similar to the values observed throughout the outburst.
No type-I X-ray bursts have been recorded so far during NICER observations, unlike other outbursts (see e.g., in't Zand et al. 2001, A&A, 372, 916; Chakrabarty et al. 2003, Nature, 424, 42; Galloway & Cumming 2006, ApJ, 652, 559; Bult et al. 2020, ApJ, 898, 38).
NICER monitoring of the latest outburst of SAX J1808.4-3658 is still ongoing. 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.
