NICER Discovers Millisecond Pulsations and a Type I X-ray Burst from SRGA J144459.2-604207

ATel #16474; M. Ng (MIT), A. Sanna (University of Cagliari), T. E. Strohmayer, Z. Arzoumanian, K. C. Gendreau (NASA GSFC), P. S. Ray (NRL), J. B. Coley (Howard University, NASA GSFC, CRESST II), D. Chakrabarty (MIT), S. Guillot (IRAP/CNRS), S. Bogdanov (Columbia), D. Altamirano (University of Southampton), J. Chenevez (DTU Space), J. Hare (CUA, NASA GSFC, CRESST II), M. T. Wolff (NRL), T. Guver (Istanbul Univ.), G. K. Jaisawal (DTU Space), Z. Wadiasingh, E. C. Ferrara (Univ. of Maryland College Park, GSFC, CRESST II) on behalf of the NICER team
on 23 Feb 2024; 00:17 UT
Credential Certification: Mason Ng (masonng@mit.edu)

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SRG/ART-XC reported the discovery of a new, bright Galactic X-ray transient on February 21, 2024 (ATel #16464). MAXI/GSC observations reported the brightening nature of the source, reaching ~100 mCrab on February 21, 2024 (ATel #16469), which was confirmed by Swift/XRT follow-up observations (ATel #16471). Comparison with the 2MASS catalog suggested a possible near-infrared counterpart, the Gaia EDR3 distance of which, 1.9 kpc, implied an X-ray luminosity of about 8×10³⁵ erg/s, suggesting SRGA J144459.2-604207 is a Galactic low-mass X-ray binary (ATel #16470, Bailer-Jones et al. 2021). NICER observed SRGA J144459.2-604207 for a total exposure time of 2.7 ks integrated over several snapshots starting from 19:58 UTC on February 21. NICER detected a type I X-ray burst from the source at 08:24 UTC on February 22, securing the identification of SRGA J144459.2-604207 as a new neutron star low-mass X-ray binary.

In the 1-10 keV energy band, the type I X-ray burst displayed a slow rise of about 6 seconds to the peak count rate of around 950 c/s (1-10 keV, normalized to 52 detectors), and exhibited a plateau lasting around 10 seconds, before decaying down back to the persistent level of around 300 c/s (1-10 keV, normalized to 52 detectors) over 50 seconds. The decay was well described by a cooling thermal blackbody component, which confirms the thermonuclear nature of the X-ray burst.

To characterize the pre-burst emission, we extracted a spectrum in the time interval between 40 and 5 seconds before the burst onset. The spectrum was well described by an absorbed thermal blackbody and power law with nH = 2.7×10²² cm^-2, blackbody temperature kT = 0.71 (-0.09, +0.13) keV, blackbody normalization 207 (-110, +184) (R_km/D₁₀)² (R_km is source radius in km; D₁₀ is distance to source in units of 10 kpc), photon index 1.63 (-0.11, +0.10), and power law normalization 0.50 (-0.09, +0.08) photons/s/keV/cm². The 2-10 keV unabsorbed flux is 2.66×10^-9 erg/s/cm² (~110 mCrab). Over the course of the NICER observations that ended on 11:38 UTC on February 22, the 1-10 keV count rate (normalized to 52 detectors) decreased from 316.1 +/- 0.7 c/s to 281.8 +/- 0.7 c/s.

We also conducted a periodicity search and discovered pulsations at a frequency of around 447.8 Hz at a trials-adjusted significance of about 5σ, further identifying the source as a new accreting millisecond X-ray pulsar. The signal, detected in multiple independent time intervals, is clearly accelerated, suggesting an orbital period of less than one day. Additional observations are needed to constrain the orbital parameters.

NICER will continue to observe SRGA J144459.2-604207 at least through February 26. The latest schedule is available at https://heasarc.gsfc.nasa.gov/docs/nicer/schedule/nicer_sts_current.html. We encourage further multiwavelength observations with other facilities.

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.