NICER observes a secondary peak in the decay of a thermonuclear burst from 4U 1608-52

Abstract

We report for the first time below 1.5 keV, the detection of a secondary peak in an Eddington-limited thermonuclear X-ray burst observed by the Neutron Star Interior Composition Explorer (NICER) from the low-mass X-ray binary 4U 1608-52. Our time-resolved spectroscopy of the burst is consistent with a model consisting of a varying-temperature blackbody, and an evolving persistent flux contribution, likely attributed to the accretion process. The dip in the burst intensity before the secondary peak is also visible in the bolometric flux. Prior to the dip, the blackbody temperature reached a maximum of $\approx 3$ keV. Our analysis suggests that the dip and secondary peak are not related to photospheric expansion, varying circumstellar absorption, or scattering. Instead, we discuss the observation in the context of hydrodynamical instabilities, thermonuclear flame spreading models, and re-burning in the cooling tail of the burst.

arXiv comment8 pages, 6 figures, Accepted for publication in ApJ

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• A NICER look at thermonuclear X-ray bursts from Aql X-1
• Burst-Disk Interaction in 4U 1636-536 as observed by NICER
• A Comprehensive Study of Thermonuclear X-ray Bursts from 4U 1820-30 with NICER: Accretion Disk Interactions and a Candidate Burst Oscillation
• Thermonuclear X-ray Bursts with late secondary peaks observed from 4U 1608-52
• Discovery of Thermonuclear Type-I X-ray Bursts from the X-ray binary MAXI J1807+132
• The study of multi-peaked type-I X-ray bursts in the neutron-star low mass X-ray binary 4U 1636$-$536 with RXTE
• Catalog of outbursts of neutron star LMXBs
• The thermonuclear X-ray bursts of 4U 1730-22
• Insight-HXMT observations on thermonuclear X-ray bursts from 4U~1608--52 in 2022: the accretion rate dependent anisotropy of burst emission
• Insight-HXMT observations on thermonuclear X-ray bursts from 4U~1608--52 in the low/hard state: the energy-dependant hard X-ray deficit and cooling saturation of the corona
• Probing thermonuclear bursts and X-ray reflection features in Aql X-1 during 2024 outburst
• Return of 4U~1730--22 after 49 years silence: the peculiar burst properties of the 2021/2022 outbursts observed by Insight-HXMT
• Radiation Driven Warping of Accretion Discs Due to X-ray Bursts
• NICER observations of type-I X-ray bursts from the ultra-compact X-ray binary M15 X-2
• Tracing the outburst decay of soft X-ray transients Aql X-1 and 4U 1608-52 with XSPECT
• An unusual type-I X-ray burst from the neutron star X-ray binary IGR J17591-2342: a double-photospheric-radius-expansion burst?
• Photospheric Radius Expansion and a double-peaked type-I X-ray burst from GRS 1741.9-2853