Soft excess in the quiescent Be/X-ray pulsar RX J0812.4-3114

Abstract

We report a 72 ks XMM-Newton observation of the Be/X-ray pulsar (BeXRP) RX J0812.4-3114 in quiescence ($L_X\approx 1.6\times 10^{33}\mathrm{erg}{\mathrm{s}}^{-1}$). Intriguingly, we find a two component spectrum, with a hard power-law ($\Gamma \approx 1.5$) and a soft blackbody-like excess below $\approx 1\mathrm{keV}$. The blackbody component is consistent in $kT$ with a prior quiescent Chandra observation reported by Tsygankov et al. and has an inferred blackbody radius of $\approx 10\mathrm{km}$, consistent with emission from the entire neutron star (NS) surface. There is also mild evidence for an absorption line at $\approx 1\mathrm{keV}$ and/or $\approx 1.4\mathrm{keV}$. The hard component shows pulsations at $P\approx 31.908\mathrm{s}$ (pulsed fraction $0.84\pm 0.10$), agreeing with the pulse period seen previously in outbursts, but no pulsations were found in the soft excess (pulsed fraction $\lesssim 31\%$). We conclude that the pulsed hard component suggests low-level accretion onto the neutron star poles, while the soft excess seems to originate from the entire NS surface. We speculate that, in quiescence, the source switches between a soft thermal-dominated state (when the propeller effect is at work) and a relatively hard state with low-level accretion, and use the propeller cutoff to estimate the magnetic field of the system to be $\lesssim 8.4\times 10^{11}\mathrm{G}$. We compare the quiescent thermal $L_X$ predicted by the standard deep crustal heating model to our observations and find that RX J0812.4-3114 has a high thermal $L_X$, at or above the prediction for minimum cooling mechanisms. This suggests that RX J0812.4-3114 either contains a relatively low-mass NS with minimum cooling, or that the system may be young enough that the NS has not fully cooled from the supernova explosion.

arXiv comment13 pages, 10 figures, accepted for publication in MNRAS

References in corpus (53)

• Gaia Data Release 2. Summary of the contents and survey properties
• On the Absorption of X-rays in the Interstellar Medium
• The Leiden/Argentine/Bonn (LAB) Survey of Galactic HI: Final data release of the combined LDS and IAR surveys with improved stray-radiation corrections
• Estimating distances from parallaxes IV: Distances to 1.33 billion stars in Gaia Data Release 2
• Be/X-ray binaries
• Classical Be Stars: Rapidly Rotating B Stars with Viscous Keplerian Decretion Disks
• Minimal Cooling of Neutron Stars: A New Paradigm
• Crustal Heating and Quiescent Emission from Transiently Accreting Neutron Stars
• Neutron stars - cooling and transport
• Mapping crustal heating with the cooling lightcurves of quasi-persistent transients
• Models of crustal heating in accreting neutron stars
• Origin of the Soft Excess in X-ray Pulsars
• Crustal Emission and the Quiescent Spectrum of the Neutron Star in KS 1731-260
• The Propeller Regime of Disk Accretion to a Rapidly Rotating Magnetized Star
• Neutron star cooling after deep crustal heating in the X-ray transient KS 1731-260
• Temperature distribution in magnetized neutron star crusts
• Thermal structure and cooling of neutron stars with magnetized envelopes
• Tests of the nuclear equation of state and superfluid and superconducting gaps using the Cassiopeia A neutron star
• Global X-ray properties of the O and B stars in Carina
• The quiescent X-ray emission of three transient X-ray pulsars
• Low-level accretion in neutron-star X-ray binaries
• Spin period change and the magnetic fields of neutron stars in Be X-ray binaries in the Small Magellanic Cloud
• Model X-ray Spectra of Magnetic Neutron Stars with Hydrogen Atmospheres
• Propeller effect in two brightest transient X-ray pulsars: 4U 0115+63 and V 0332+53
• Cooling of Accretion-Heated Neutron Stars
• Discovery of a 112 ms X-ray Pulsar in Puppis A: Further Evidence of Neutron Stars Weakly Magnetized at Birth
• Pairing in high-density neutron matter including short- and long-range correlations
• Pairing in high-density neutron matter including short- and long-range correlations
• The transient X-ray pulsar 4U 0115+63 from quiescence to outburst through the centrifugal transition
• Distribution of High-Mass X-ray Binaries in the Milky Way
• Stable accretion from a cold disc in highly magnetized neutron stars
• A strong shallow heat source in the accreting neutron star MAXI J0556-332
• The X-ray properties of Be/X-ray pulsars in quiescence
• Witnessing the magnetospheric boundary at work in Vela X-1
• A universal relation for the propeller mechanisms in magnetic rotating stars at different scales
• XMM-Newton observation of the Be/neutron star system RX J0146.9+6121: a soft X-ray excess in a low luminosity accreting pulsar
• X-ray properties of the transient pulsar 3A 0535+262 in quiescence
• XMM-Newton observation of the persistent Be/neutron-star system X Persei at a high-luminosity level
• In-medium enhancement of the modified Urca neutrino reaction rates
• XMM-Newton observations of 1A 0535+262 in quiescence
• A Chandra Look at the X-ray Faint Millisecond Pulsars in the Globular Cluster NGC 6752
• The Be/X-ray binary LS 992/RX J0812.4-3114: physical parameters and long-term variability
• Accretion column eclipses in the X-ray pulsars GX 1+4 and RX J0812.4-3114
• On the cyclotron line in Cep X-4
• XMM-Newton observation of the persistent Be/NS X-ray binary pulsar RX J1037.5-5647 in a low luminosity state
• Stellar wind accretion and accretion disk formation: applications to neutron star high mass X-ray binaries
• Discovery of X-ray pulsations in the Be/X-ray binary LS992/ RX J0812.4-3114
• The impact of surface temperature inhomogeneities on quiescent neutron star radius measurements
• The quiescent state of the accreting X-ray pulsar SAX J2103.5+4545
• Opacities and spectra of hydrogen atmospheres of moderately magnetized neutron stars
• The Orbital Period of the Be/Neutron Star Binary RX J0812.4-3114
• Discovery of accretion-driven pulsations in the prolonged low X-ray luminosity state of the Be/X-ray transient GX 304-1
• Swift monitoring of the massive X-ray binary SAX J0635.2+0533

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• A catalogue of high-mass X-ray binaries in the Galaxy: from the INTEGRAL to the Gaia era
• Thermodynamically consistent equation of state for an accreted neutron star crust
• Catalog of the Galactic population of X-ray pulsars in High-mass X-ray binary systems
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