Different Accretion Heating of the Neutron Star Crust during Multiple Outbursts in MAXI J0556–332

Abstract

The transient neutron star (NS) low-mass X-ray binary MAXI J0556−332 provides a rare opportunity to study NS crust heating and subsequent cooling for multiple outbursts of the same source. We examine MAXI, Swift, Chandra, and XMM-Newton data of MAXI J0556−332 obtained during and after three accretion outbursts of different durations and brightnesses. We report on new data obtained after outburst III. The source has been tracked up to ∼1800 days after the end of outburst I. Outburst I heated the crust strongly, but no significant reheating was observed during outburst II. Cooling from ∼333 eV to ∼146 eV was observed during the first ∼1200 days. Outburst III reheated the crust up to ∼167 eV, after which the crust cooled again to ∼131 eV in ∼350 days. We model the thermal evolution of the crust and find that this source required a different strength and depth of shallow heating during each of the three outbursts. The shallow heating released during outburst I was ∼17 MeV nucleon −1 and outburst III required ∼0.3 MeV nucleon −1 . These cooling observations could not be explained without shallow heating. The shallow heating for outburst II was not well constrained and could vary from ∼0 to 2.2 MeV nucleon −1 , i.e., this outburst could in principle be explained without invoking shallow heating. We discuss the nature of the shallow heating and why it may occur at different strengths and depths during different outbursts.

Crust cooling of the neutron star in Aql X-1: different depth and magnitude of shallow heating during similar accretion outbursts

“…mass, spin, magnetic field strength; Waterhouse et al 2016). There are two other neutron stars for which crust cooling has been studied after different outbursts; MAXI J0556-332 (Homan et al 2014;Parikh et al 2017a) and MXB 1659-298 (e.g. Wijnands et al 2004;Cackett et al 2013b;Parikh et al 2019).…”

“…For MAXI J0556-332, which exhibited 3 outbursts of different duration and peak intensity, there is clear evidence that the depth and magnitude of shallow heating varies between the different outbursts. This rules out that basic neutron star parameters play an important role in regulating shallow heating (Parikh et al 2017a). MXB 1659-298, on the other hand, showed remarkably consistent heating parameters for 2 outbursts that were of similar brightness but different duration (2.5 and 1.5 yr; Parikh et al 2019).…”

“…Degenaar et al 2011Degenaar et al , 2014Degenaar et al , 2015Page & Reddy 2013;Parikh et al 2017bParikh et al , 2019Ootes et al 2018), although one extreme case requires ∼15-17 MeV nucleon −1 (e.g. Homan et al 2014;Deibel et al 2015;Parikh et al 2017a). It is currently unclear what is causing this shallow heating.…”

“…It remains to be established to what extent shallow heating depends on neutron-star specific properties (e.g., spin, magnetic field strength, mass, radius, superfluid properties, age), and on the detailed properties of the accretion outburst (e.g., brightness, duration, accretion geometry). Studying multiple cooling curves of a single source may be a promising way to understand the origin of shallow heating; because the fundamental properties of a neutron star remain virtually unchanged, the effect of the outburst parameters on the crust heating and cooling can potentially be isolated (Waterhouse et al 2016;Parikh et al 2017aParikh et al , 2019.…”

The effect of diffusive nuclear burning in neutron star envelopes on cooling in accreting systems

“…For several crust cooling sources, the high observed surface temperatures 100 days after the accretion outburst can not be explained using the standard deep crustal heating model but require the presence of an additional shallow heating source (at densities ρ < 10 11 g cm −3 ) during outburst. Typically, during the outbursts, an amount of ∼1-2 MeV per accreted nucleon of shallow heating is needed to explain the observed cooling curves after the outbursts are over (e.g., Brown & Cumming 2009;Degenaar et al 2014;Merritt et al 2016), but for one source (MAXI J0556-332) as much as ∼17 MeV nucleon −1 was needed (Homan et al 2014;Deibel et al 2015;Parikh et al 2017). This means that the amount of shallow heating can be larger than that due to deep crustal heating, which releases ∼2 MeV nucleon −1 .…”