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

Abstract

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 (fa 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 fa values are clustered around 1–3, but can reach up to 11 in a burst where photospheric radius expansion is observed. Such fa 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 fa 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%.

Burst–Disk Interaction in 4U 1636–536 as Observed by NICER

“…This increase is attributed to the increased mass accretion rate onto the neutron star, due to Poynting-Robertson (PR) drag caused by the burst emission, and supported by simulations of accretion disks illuminated by thermonuclear X-ray bursts (Fragile et al 2018(Fragile et al , 2020. Güver et al (2022) showed that the X-ray spectra can also be fit with models taking into account the reflection of the burst emission off the accretion disk.…”

“…The soft X-ray sensitivity combined with the larger effective area of NICER allow for an effective time-resolved spectral study of thermonuclear X-ray bursts. Following previous studies (see, e.g., Güver et al 2022), we extracted timeresolved X-ray spectra for each burst, with varying exposure times, which depend on the total observed count rate from the source. To be able to catch the spectral evolution during the rise of the bursts, we used 0.5 s as a fixed exposure time up to the peak moment, and then allowed the exposure time to be longer.…”

“…However, apart from a few exceptions (Ballantyne & Strohmayer 2004;Worpel et al 2013Worpel et al , 2015, the effects of the bursts on the accretion disk and surrounding material have remained mostly unstudied, due to the lack of low-energy sensitivity of RXTE/PCA. Unlike RXTE, the Neutron Star Interior Composition Explorer (NICER) allows for the systematic study of thermonuclear X-ray bursts in the soft X-rays and allows the probing of the effects of the bursts on the surrounding material (see Keek et al 2018aKeek et al , 2018bBult et al 2019;Buisson et al 2020;Güver et al 2022). Recently, we studied all the X-ray bursts from Aql X-1 prior to the end of 2021, using NICER observations, and showed that in approximately 68% of the spectra there is statistical evidence for an excess emission that can be characterized by the application of a scaling factor to the persistent emission of the system (Güver et al 2022).…”

Discovery of a 584.65 Hz Burst Oscillation in the Low-mass X-Ray Binary 4U 1730–22

“…Recently, the burst oscillation candidate from XTE J1739-285 has been observed at 386.5 Hz (Bult et al 2021a), rather than 1122 Hz reported by Kaaret et al (2007). However, most of the sources have not reported the detection of burst oscillations (see, e.g., Aql X-1 and 4U 1636-536; Li et al 2021;Güver et al 2022;Zhao et al 2022).…”

Discovery of a 584.65 Hz Burst Oscillation in the Low Mass X-Ray Binary 4U 1730$-$22

“…Recently, the burst oscillation candidate from XTE J1739-285 has been observed at 386.5 Hz (Bult et al 2021a), rather than 1122 Hz reported by Kaaret et al (2007). However, most of sources have not been reported the detection of burst oscillations (see e.g., Aql X-1 and 4U 1636-536, Li et al 2021;Güver et al 2022;Zhao et al 2022), or the type-I X-ray bursts.…”