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

Abstract

We present the detection of 51 thermonuclear X-ray bursts observed from 4U 1636–536 by the Neutron Star Interior Composition Explorer (NICER) over the course of a 3 yr monitoring campaign. We perform time-resolved spectroscopy for 40 of these bursts and show the existence of a strong soft excess in all the burst spectra. The excess emission can be characterized by the use of a scaling factor (the f a method) to the persistent emission of the source, which is attributed to the increased mass accretion rate onto the neutron star due to Poynting–Robertson drag. The soft excess emission can also be characterized by the use of a model taking into account the reflection of the burst emission off the accretion disk. We also present time-resolved spectral analysis of five X-ray bursts simultaneously observed by NICER and AstroSat, which confirm the main results with even greater precision. Finally, we present evidence for Compton cooling using seven X-ray bursts observed contemporaneously with NuSTAR, by means of a correlated decrease in the hard X-ray lightcurve of 4U 1636–536 as the bursts start.

The Thermonuclear X-Ray Bursts of 4U 1730–22

“…The weak bursts had peak f a values of about 2-3, while the bright PRE bursts showed peak f a values between 7 and 10. These results are similar to those found with RXTE (Worpel et al 2013(Worpel et al , 2015, as well as more recent studies with NICER (Güver et al 2022a(Güver et al , 2022b. A more subtle effect apparent in our observations is that the X-ray bursts observed at later times appear to have higher peak f a values.…”

“…In this context, one might also ask if other X-ray bursters exhibit behavior similar to what we observe in 4U 1730−22. Indeed, the type of shoulders we find in the bright bursts are also seen in the profiles of prominent bursters such as 4U 1636-536 and Aql X-1 (Galloway et al 2008;Güver et al 2022aGüver et al , 2022b and have been previously attributed to the rp process (in 't Zand et al 2017). Interestingly, these two bursters also show weak bursts that appear to have a similar (if much less pronounced) alignment to the cooling tails of their brighter counterparts (see, e.g., Güver et al 2022a).…”

“…Indeed, the type of shoulders we find in the bright bursts are also seen in the profiles of prominent bursters such as 4U 1636-536 and Aql X-1 (Galloway et al 2008;Güver et al 2022aGüver et al , 2022b and have been previously attributed to the rp process (in 't Zand et al 2017). Interestingly, these two bursters also show weak bursts that appear to have a similar (if much less pronounced) alignment to the cooling tails of their brighter counterparts (see, e.g., Güver et al 2022a). Additional similarities between these sources exist; like 4U 1730−22, both 4U 1636-536 and Aql X-1 have high (>500 Hz) spin frequencies (Strohmayer et al 1998;Zhang et al 1998;Casella et al 2008) and are known to show millihertz QPOs (Revnivtsev et al 2001).…”

“…The detection of excess burst emission has become increasingly common in recent years, either through the increased sensitivity afforded by NICER (Keek et al 2018a(Keek et al , 2018bBult et al 2019;Jaisawal et al 2019;Güver et al 2022aGüver et al , 2022b or through broadband X-ray coverage with AstroSat (Bhattacharyya et al 2018;Roy et al 2021;Kashyap et al 2022) or Insight-HXMT (Chen et al 2019). While the emission of a neutron star atmosphere is expected to deviate from Planck's law (London et al 1986;Madej et al 2004;Suleimanov et al 2011), the observed deviations from a blackbody spectrum are well in excess of what an atmosphere model can explain (in't Zand et al 2017).…”

The Prolific Thermonuclear X-Ray Bursts from the Outburst of the Newly Discovered Millisecond Pulsar MAXI J1816–195 Observed by Insight-HXMT and NICER

“…The shortage during bursts in the hard X-ray of the continuum emission is reported on several bursters, i.e., IGR J17473-2721 (Chen et al 2011(Chen et al , 2012, Aql X-1 (Maccarone & Coppi 2003;Chen et al 2013), 4U 1636-536 (Ji et al 2013Chen et al 2018;Güver et al 2022a), GS 1826-238 (Ji et al 2014aSánchez-Fernández et al 2020), KS 1731-260 (Ji et al 2014b), 4U 1705-44 (Ji et al 2014b), 4U 1728-34 (Kajava et al 2017) and 4U 1724-30 (Kashyap & Chakraborty 2022), based on RXTE, INTEGRAL, Insight-HXMT, AstroSat and NuSTAR observations. MAXI J1816-195, an accretion-powered millisecond pulsar discovered by MAXI in 2022 June (Negoro et al 2022) with peak flux ∼ 100 mCrab, spin frequency 528 Hz ) and binary orbital period 17402.27 s (Bult et al 2022a).…”