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Published January 1, 2021 | Version v1
Journal article Open

Thermonuclear X-Ray Bursts with Late Secondary Peaks Observed from 4U 1608-52

Creators

  • 1. Istanbul Univ, Grad Sch Sci, Dept Astron & Space Sci, TR-34119 Istanbul, Turkey
  • 2. Sabanci Univ, Fac Engn & Nat Sci, TR-34956 Istanbul, Turkey
  • 3. Indian Inst Technol Indore, Dept Astron Astrophys & Space Engn, Khandwa Rd, Indore 453552, India
  • 4. NASA, Astrophys Sci Div & Joint Space Sci Inst, Goddard Space Flight Ctr, Greenbelt, MD 20771 USA
  • 5. Univ Southampton, Sch Phys & Astron, Southampton SO17 1BJ, Hants, England
  • 6. Tech Univ Denmark, Natl Space Inst, Elektrovej 327-328, DK-2800 Lyngby, Denmark
  • 7. NASA, Astrophys Sci Div, Goddard Space Flight Ctr, Greenbelt, MD 20771 USA
  • 8. MIT, Kavli Inst Astrophys & Space Res, 77 Massachusetts Ave, Cambridge, MA 02139 USA

Description

We report the temporal and spectral analysis of three thermonuclear X-ray bursts from 4U 1608-52, observed by the Neutron Star Interior Composition Explorer (NICER) during and just after the outburst observed from the source in 2020. In two of the X-ray bursts, we detect secondary peaks 30 and 18 s after the initial peaks. The secondary peaks show a fast rise exponential decay-like shape resembling a thermonuclear X-ray burst. Time-resolved X-ray spectral analysis reveals that the peak flux, blackbody temperature, and apparent emitting radius values of the initial peaks are in agreement with X-ray bursts previously observed from 4U 1608-52, while the same values for the secondary peaks tend toward the lower end of the distribution of bursts observed from this source. The third X-ray burst, which happened during much lower accretion rates, did not show any evidence for a deviation from an exponential decay and was significantly brighter than the previous bursts. We present the properties of the secondary peaks and discuss the events within the framework of short recurrence time bursts or bursts with secondary peaks. We find that the current observations do not fit in standard scenarios and challenge our understanding of flame spreading.

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