NICER observes the tail of a long-duration X-ray burst in the direction of MAXI J0911-655
ATel #13760; P. M. Bult, Z. Arzoumanian, K. C. Gendreau (NASA/GSFC)
on 24 May 2020; 17:36 UT
Credential Certification: Peter Bult (p.m.bult@nasa.gov)
Subjects: X-ray, Neutron Star, Transient
Referred to by ATel #: 16358
In response to a MAXI/GSC nova alert of a sudden X-ray brightening in the direction of the intermittent accreting millisecond X-ray pulsar MAXI J0911-655 (also known as Swift J0911.9-6452; ATel #13754), we executed rapid follow-up observations with NICER. We observed MAXI J0911-655 between 2020-05-22 18:57 and 22:16 UTC, collecting 2.5 ks exposure. Over the course of these observations we measured an exponentially decaying count-rate; the 0.5-10 keV rate dropped from 300 c/s to about 100 c/s with an e-folding time of 43 minutes. We returned to the target a day later, observing between 2020-05-23 22:49 UTC and 23:12 UTC, finding the source had re-brightened to 250 c/s, well above the historic NICER count-rate of ~45-70 c/s (ATel #12869).
During the initial count-rate decline, we observed a significant shift in the shape of the continuum 0.5-10 keV X-ray spectrum. This evolution could be well described with an absorbed power-law plus blackbody model, where the blackbody is solely responsible for the change in spectral shape. At the start of our observations, the blackbody temperature was 1.08(3) keV, and as time progressed, we observed it to cool down to 0.88(3) keV. Meanwhile, the power-law photon index held constant at 2.4(1), and the absorption column density was measured at nH = 2.7(1) x 1021 cm-2. The 1-10 keV flux declines along with the blackbody emission, dropping from about 1.1 x 10-9 erg cm-2 s-1 to 3.5 x 10-10 erg cm-2 s-1.
In contrast to the early observations, the re-brightening measured on the 23rd appears to be due to a change in the non-thermal continuum. The 0.5-10 keV spectrum became dominated by the power law, whose photon index flattened to 2.0(1), while the 1-10 keV flux increased to 7 x 10-10 erg cm-2 s-1.
After applying barycenter corrections to the observed data, we attempted to recover pulsations using the known binary ephemeris and 340 Hz spin frequency of MAXI J0911-655 (Sanna et al. 2017, A&A, 598, 34). No pulsations were detected to an upper limit of 2% fractional sinusoidal amplitude.
The evolution of the X-ray spectrum and exponential decay of the count-rate are consistent with this emission being due to the cooling tail of the thermonuclear X-ray burst reported by Nakajima et al. (ATel #13754), who measured a blackbody temperature of 2.5 (+0.2 -0.2) keV and 0.8 (+0.3 -0.2) keV at 17:04 and 18:37 UTC on May 22, respectively. Given the MAXI/GSC alert trigger of 17:04 UTC, our NICER observations indicate a burst duration of approximately 5 hours. The subsequently observed re-brightening further indicates that MAXI J0911-655 remains active at elevated levels (or possibly that a new source has become active within the field of view).
NICER will continue to monitor this source and any additional observations are 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.
