Switches between accretion structures during flares in 4U 1901+03

Abstract

We report on our analysis of the 2019 outburst of the X-ray accreting pulsar 4U 1901+03 observed with Insight-HXMT and NICER. Both spectra and pulse profiles evolve significantly in the decaying phase of the outburst. Dozens of flares are observed throughout the outburst. They are more frequent and brighter at the outburst peak. We find that the flares, which have a duration from tens to hundreds of seconds, are generally brighter than the persistent emission by a factor of ∼1.5. The pulse-profile shape during the flares can be significantly different from that of the persistent emission. In particular, a phase shift is clearly observed in many cases. We interpret these findings as direct evidence of changes of the pulsed beam pattern, due to transitions between the sub- and supercritical accretion regimes on a short time-scale. We also observe that at comparable luminosities the flares’ pulse profiles are rather similar to those of the persistent emission. This indicates that the accretion on the polar cap of the neutron star is mainly determined by the luminosity, i.e. the mass accretion rate.

RX J0529.8-6556: a BeXRB pulsar with an evolving optical period and out of phase X-ray outbursts

“…For example 4U 1901+03 has shown sudden changes between six different pulse profile morphology between L X of 0.7-1.2×10 37 erg s −1 (Ji et al 2020). For 4U 1901+03 an asymmetric pulse profile with a minor peak leading a major peak appeared in the brightest stages (Ji et al 2020). Archival ROSAT observations have shown a similar asymmetric pulse profile at L X of 5×10 35 erg s −1 , however at that stage the major peak was leading in pulse phase (Reig & Milonaki 2016).…”

“…In this regime radiation escapes perpendicular to the magnetic field lines in a fan beam pattern (Basko & Sunyaev 1976). Observationally, changes in the morphology of pulse profiles with luminosity are typical for X-ray pulsars (e.g., Parmar et al 1989;Doroshenko et al 2017;Koliopanos & Vasilopoulos 2018;Ji et al 2020). High luminosity is associated with double-peaked profiles and fan beam emission patterns, while sinusoidal pulse profiles are traditionally attributed to lower accretion rates and pencil beam emission patterns.…”

“…A rapid change in the pulse profile (for small change in L X ) 5 Image Reduction and Analysis Facility: iraf.noao.edu is not uncommon for X-ray pulsars (e.g., Ji et al 2020). For example 4U 1901+03 has shown sudden changes between six different pulse profile morphology between L X of 0.7-1.2×10 37 erg s −1 (Ji et al 2020).…”

“…This trend is also consistent with the change in the pulse profile morphology. In particular, the transition between a profile having two pulses with comparable heights (i.e., epoch 2) and two asymmetric pulses (i.e., epoch 1) is proposed to be a characteristic of the change to the super-critical regime (Ji et al 2020). Thus we can use the luminosity range between epochs 1 and 2 as an approximate limit for L crit .…”

Evolution of timing and spectral characteristics of 4U 1901+03 during its 2019 outburst using the Swift and NuSTAR observatories

“…This was recorded as the fourth outburst from this source in 40 years (Nakajima et al 2019;Mereminskiy et al 2019;Hemphill et al 2019), and lasted for about ∼ 160 days. During this outburst several optical and X-ray observations were made and the peak X-ray luminosity measured during the 2019 outburst is about 1.4 × 10 37 erg s −1 , assuming source distance of 3 kpc (Ji et al 2020). Based on optical spectroscopy of the candidate Be star counterpart (B8/9 IV star), the source distance was proposed to be greater than 12 kpc (Strader et al 2019;McCollum & Laine 2019).…”