Kavli Affiliate: Deepto Chakrabarty
| First 5 Authors: Funda Bostanci, Tugba Boztepe, Tolga Guver, Tod E. Strohmayer, Yuri Cavecchi
| Summary:
We present temporal and time-resolved spectral analyses of all the
thermonuclear X-ray bursts observed from the neutron star low-mass X-ray binary
(LMXB) 4U 1728-34 with NICER from June 2017 to September 2019. In total, we
detected 11 X-ray bursts from the source and performed time-resolved
spectroscopy. Unlike some of the earlier results for other bursting sources
from NICER, our spectral results indicate that the use of a scaling factor for
the persistent emission is not statistically necessary. This is primarily a
result of the strong interstellar absorption in the line of sight towards 4U
1728-34, which causes the count rates to be significantly lower at low
energies. We also searched for burst oscillations and detected modulations in
six different bursts at around the previously known burst oscillation frequency
of 363 Hz. Finally, we report the detection of oscillations prior to two bursts
at 356 and 359 Hz, respectively. This is the first time in the literature where
burst oscillations are detected before the rapid rise in X-ray flux, from any
known burster. These oscillations disappear as soon as the burst rise starts
and occur at a somewhat lower frequency than the oscillations we detect during
the bursts.
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