Thermonuclear superburst of MAXI J1752$-$457 observed with NinjaSat and MAXI

Abstract

An uncatalogued bright X-ray transient was detected with MAXI on November 9, 2024, named MAXI J1752$-$457. The NinjaSat X-ray observatory promptly observed the source from November 10 to 18 while the small angular separation from the Sun hampered follow-up campaigns by other X-ray observatories. The MAXI and NinjaSat light curves in the 2-10 keV band showed first and second decaying components at the early and late phases, approximated by exponential functions with e-folding constants of 1.2 $\pm$ 0.2 and 14.9 $\pm$ 0.9 hours (1$\sigma$ errors), respectively. A single blackbody model reproduces the X-ray spectrum with a softening trend of its temperature decreasing from 1.8 $\pm$ 0.1 keV to 0.59 $\pm$ 0.06 keV. Assuming the unknown source distance at 8 kpc, at which the initial X-ray luminosity roughly corresponds to the Eddington limit, the shrinking blackbody radius was estimated at 5-11 km. This X-ray brightening is interpreted as a superburst in a Galactic low-mass X-ray binary, which is powered by thermonuclear burning triggered presumably by carbon ignition close to the outer crust of the neutron star. The transition between two decaying components occurred at 5.5-7.7 hours, corresponding to the thermal time scale of the burning layer. The ignition column density is estimated to be (2.8-5.1)$\times 10^{12}$ g cm${}^{-2}$.

11 pages, 4 figures, accepted for publication in ApJL

References in corpus (7)

• HI4PI: A full-sky HI survey based on EBHIS and GASS
• An Empirical Background Model for the NICER X-ray Timing Instrument
• A catalogue of unusually long thermonuclear bursts on neutron stars
• Maxi observations of long X-ray bursts
• NinjaSat monitoring of Type-I X-ray bursts from the clocked burster SRGA J144459.2$-$604207
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• NinjaSat: Astronomical X-ray CubeSat Observatory

Cited by in corpus (1)

• Return of the Clocked Burster: Exceptionally Short Recurrence Time in GS 1826-238