NICER Follow-up of the UCXB MAXI J0911-655
ATel #14767; M. Ng (MIT), P. M. Bult, T. E. Strohmayer (NASA/GSFC), A. Sanna (Univ. of Cagliari), K. C. Gendreau (NASA/GSFC), W. C.G. Ho (Haverford), Z. Arzoumanian (NASA/GSFC), on behalf of the NICER team
on 8 Jul 2021; 22:37 UT
Credential Certification: Mason Ng (masonng@mit.edu)
Subjects: X-ray, Neutron Star, Transient, Pulsar
Referred to by ATel #: 16358
On 2021 July 07, weak hard X-ray emission was reported by INTEGRAL from MAXI J0911-655, a known ultracompact X-ray binary hosting a 340 Hz millisecond pulsar in the globular cluster NGC 2808 (ATel #14761). NICER has since conducted follow-up observations and obtained 4.2 ks of accumulated exposure between 2021 July 07 13:43 UTC and 2021 July 08 08:30 UTC. The 0.3-12 keV count rate is approximately 61 c/s (including a background rate of < 1 c/s), on par with archival NICER observations of this source (ATel #12869).
We searched the observations for the presence of pulsations at the known 340 Hz spin frequency by folding the data on the orbital ephemeris (Sanna et al. 2017, A&A, 598, 34), while optimizing for the uncertainty in the binary phase. We did not detect any pulsations from the source. Considering the power density spectrum, we find that the characteristic noise frequencies are a factor 5 higher than previously reported (Bult 2017, ApJ, 837, 61), so it is likely that some evolution in the accretion state has occurred relative to past observations.
The time-averaged 0.3-10 keV energy spectrum is well-described by an absorbed power law and blackbody model (chisq/d.o.f. = 205.58/224), with nH = 0.118(8) × 1022 cm-2, kT = 0.656(12) keV, and photon index Gamma = 1.27(4). This implies a 0.3-10 keV absorbed flux of 2.11 × 10-10 erg/s/cm2, and a 0.3-10 keV unabsorbed flux of 2.31 × 10-10 erg/s/cm2. This continuum shape is similar to the spectra of archival NICER observations collected in 2017 and 2019.
Long-term monitoring of MAXI J0911-655 suggests that the source never returned to quiescence since it was first discovered in 2016 (although it briefly dipped in flux around June 2019; ATel #12831, #12846). While the change in the noise frequencies points to a (modest) evolution in the source accretion state, this change is not obviously reflected in the X-ray spectrum. Hence, it is at present unclear if this is associated with gradual long-term evolution of the source, or if it relates to a sudden event associated with the INTEGRAL detection. We note, however, that MAXI J0911-655 is a member of the globular cluster NGC 2808, hence, it is conceivable that a secondary X-ray source was responsible for the hard X-ray fluctuation reported by INTEGRAL. Subsequent imaging observations may help resolve this uncertainty in the source association.
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.
