Very hard states in neutron star low-mass X-ray binaries

Abstract

We report on unusually very hard spectral states in three confirmed neutron-star low-mass X-ray binaries (1RXS J180408.9−342058, EXO 1745−248, and IGR J18245−2452) at a luminosity between ∼ 10 36−37 erg s −1. When fitting the Swift X-ray spectra (0.5 -10 keV) in those states with an absorbed power-law model, we found photon indices of Γ ∼ 1, significantly lower than the Γ = 1.5 -2.0 typically seen when such systems are in their so called hard state. For individual sources very hard spectra were already previously identified but here we show for the first time that likely our sources were in a distinct spectral state (i.e., different from the hard state) when they exhibited such very hard spectra. It is unclear how such very hard spectra can be formed; if the emission mechanism is similar to that operating in their hard states (i.e., up-scattering of soft photons due to hot electrons) then the electrons should have higher temperatures or a higher optical depth in the very hard state compared to those observed in the hard state. By using our obtained Γ as a tracer for the spectral evolution with luminosity, we have compared our results with those obtained by Wijnands et al. (2015). We confirm their general results in that also our sample of sources follow the same track as the other neutron star systems, although we do not find that the accreting millisecond pulsars are systematically harder than the non-pulsating systems.

Rapid X-ray variability properties during the unusual very hard state in neutron-star low-mass X-ray binaries

“…The photon index versus the 0.5-10 keV X-ray luminosity for our five sources. For 1RXS J180408.9-342058, EXO 1745-248, and IGR J18245-2452 we show the same data as presented in Parikh et al (2017, their Figure 1). In addition we have now included IGR J00291+5934 and Swift J0911.9-6452 for the first time.…”

“…Both stay in a quasi-stable state for 1 to 2 months at a luminosity between 10 36 and 10 37 erg s −1 before they exhibited the brightest part of their outbursts (where the outburst luminosity peaked around a factor 10 higher; i.e., the transition into the soft, banana branch state). During this quasi-stable state the spectra for both sources were very hard (as measured in the 0.5-10 keV range) with photon indices between 1 and 1.5 (see also Parikh et al 2017). In addition, for both sources we found that they exhibited very strong variability over a large range of frequencies (see Figure 4, left panel).…”

“…The full log of the observations used in our work is given in Table 1. In Figures A1-A3 we show, for each source in our sample, the outburst light curves and how the photon index (obtained from spectral fits in the 0.5-10 keV energy range) varies in time (see also Parikh et al 2017). In those figures we have also indicated when the XMM-Newton observations were performed; it is clear that those observations were indeed performed during times when very hard spectra were observed for 1RXS J180408.9-342058, EXO 1745-248 and IGR J18245-2452 (bottom panels of Figures A1-A2; photon indices <1.4).…”

A systematic study of the advection-dominated accretion flow for the origin of the X-ray emission in weakly magnetized low-level accreting neutron stars

“…The X-ray spectrum in the range of 0.5 − 10 keV for lowlevel accreting NSs can generally be well fitted with the model with two components, i.e., a thermal soft X-ray component plus a power-law component (e.g. Jonker et al 2004b;Campana et al 2008a,b;Fridriksson et al 2010Fridriksson et al , 2011Armas Padilla et al 2013a;Degenaar et al 2013;Campana et al 2014;Parikh et al 2017;Vats et al 2018). For certain low-level accreting NSs, the temperature of the soft X-ray component decreases with decreasing the X-ray luminosity during the decay after the outburst (Armas Padilla et al 2013b;Degenaar et al 2013;Bahramian et al 2014).…”

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

“…The origin of this shallow heat source is unknown and is important to resolve because the cooling curve can be used to constrain a number of different aspects of crust physics (such as conductivity of the crust and pasta, and the core specific heat; Brown & Cumming 2009;Horowitz et al 2015;Cumming et al 2017). Most systems need ∼1-2 MeV nucleon −1 of shallow heating to explain their cooling curves (e.g., Degenaar et al 2014;Parikh et al 2017;Wijnands et al 2017). The transient NS LMXB MAXI J0556−332 (hereafter J0556) was discovered on 2011 January 11 (Matsumura et al 2011) and exhibited a ∼16 month outburst.…”