A NICER look at thermonuclear X-ray bursts from Aql X-1

Kavli Affiliate: Deepto Chakrabarty

| First 5 Authors: Tolga Guver, Tugba Boztepe, David R. Ballantyne, Z. Funda Bostanci, Peter Bult

| Summary:

We present spectral and temporal properties of all the thermonuclear X-ray
bursts observed from Aql X-1 by the Neutron Star Interior and Composition
Explorer (NICER) between 2017 July and 2021 April. This is the first systematic
investigation of a large sample of type I X-ray bursts from Aql X-1 with
improved sensitivity at low energies. We detect 22 X-ray bursts including two
short recurrence burst events in which the separation was only 451 s and 496 s.
We perform time resolved spectroscopy of the bursts using the fixed and scaled
background (f_a method) approaches. We show that the use of a scaling factor to
the pre-burst emission is the statistically preferred model in about 68% of all
the spectra compared to the fixed background approach. Typically the f_a values
are clustered around 1-3, but can reach up to 11 in a burst where photospheric
radius expansion is observed. Such f_a values indicate a very significant
increase in the pre-burst emission especially at around the peak flux moments
of the bursts. We show that the use of the f_a factor alters the best fit
spectral parameters of the burst emission. Finally, we employed a reflection
model instead of scaling the pre-burst emission. We show that reflection models
also do fit the spectra and improve the goodness of the fits. In all cases we
see that the disc is highly ionized by the burst emission and the fraction of
the reprocessed emission to the incident burst flux is typically clustered
around 20%.

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