NICER detects 528 Hz pulsations and a thermonuclear X-ray burst from MAXI J1816-195

ATel #15425; P. M. Bult (NASA/GSFC), M. Ng (MIT), W. Iwakiri (Chuo U.) D. Altamirano (University of Southampton), D. Chakrabarty (MIT), K. C. Gendreau, T. E. Strohmayer, Z. Arzoumanian (NASA/GSFC), A. Sanna (University of Cagliari), S. Guillot (IRAP/CNRS), P. S. Ray (NRL), T. Mihara, T. Enoto (RIKEN), J. Homan (Eureka Scientific), E. C. Ferrara (UMCP, NASA/GSFC)
on 8 Jun 2022; 23:19 UT
Credential Certification: Peter Bult (p.m.bult@nasa.gov)

Tweet

Following the MAXI/GSC detection of a new X-ray transient, MAXI J1816-195 (ATel #15418), NICER performed multiple pointings to localize the source beginning at 2022-06-07 14:32 (UTC). Starting from the nominal coordinates reported by MAXI, NICER performed a series of offset pointings with 60 s exposure for each. A preliminary source position was obtained at R.A. = 274.2205 deg, Dec = -19.627 deg (J2000), with 3 arcmin radius uncertainty set by the NICER field of view. This position is consistent with the localization of the object as determined with Swift/XRT (ATel #15421).

After performing our raster scan, we obtained a 1.2 ks pointed observation on the best NICER position starting at 7:35 UTC on June 8. The average source count-rate was 470 ct/s in the 0.5-10 keV energy band. We searched this observation for the presence of coherent pulsations, finding a significant (6 sigma, single trial) signal at 528.6 Hz (using photons in the 1-10 keV range). This detection shows that MAXI J1816-195 is a neutron star and a new accreting millisecond X-ray pulsar.

We collected another 1.4 ks exposure starting on 18:26 UTC June 8 using the Swift/XRT coordinates. At this time the source intensity had increased to 560 ct/s, and pulsations could again be observed at >6 sigma significance. Additionally, we observed a thermonuclear X-ray burst, with a peak bolometric blackbody flux of ~4 × 10^-8 erg/s/cm². We did not observe evidence of photospheric radius expansion. Assuming the empirical Eddington luminosity of 3.8 × 10³⁸ erg/s (Kuulkers et al. 2003, A&A 399, 663), our flux measurement places an upper limit on the source distance of 9 kpc. No burst oscillations were detected at the reported pulse frequency.

A full pulsar timing analysis is currently underway and will be circulated as more data become available. Additional multiwavelength follow-up 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.