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Title: Modeling the X-rays from the central compact object PSR J1852+0040 in Kesteven 79: Evidence for a strongly magnetized neutron star

Abstract

I present modeling of the X-ray pulsations from the central compact object (CCO) PSR J1852+0040 in the Galactic supernova remnant Kesteven 79. In the context of thermal surface radiation from a rotating neutron star (NS), a conventional polar cap model can reproduce the broad, large-amplitude X-ray pulse only with a 'pencil plus fan' beam emission pattern, which is characteristic of ≳ 10{sup 12} G NS atmospheres, much greater than the ∼10{sup 10} G external dipole field inferred from the pulsar spin-down rate. This discrepancy can be explained by an axially displaced dipole. For other beaming patterns, it is necessary to invoke high-aspect-ratio emitting regions that are greatly longitudinally elongated, possibly due to an extremely offset dipole. For all assumed emission models, the existence of strong internal magnetic fields (≳ 10{sup 14} G) that preferentially channel internal heat to only a portion of the exterior is required to account for the implied high-temperature contrast across the stellar surface. This lends further observational evidence in support of the 'hidden' strong magnetic field scenario, in which CCOs possess submerged magnetic fields that are substantially stronger than the external dipole field, presumably due to burial by fallback of supernova ejecta. I also conduct phase-resolvedmore » X-ray spectroscopy and find no evidence for prominent spin-phase-dependent absorption features that could be produced by cyclotron absorption/scattering.« less

Authors:
 [1]
  1. Columbia Astrophysics Laboratory, Columbia University, 550 West 120th Street, New York, NY 10027 (United States)
Publication Date:
OSTI Identifier:
22365524
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 790; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ABSORPTION; AMPLITUDES; ASPECT RATIO; CYCLOTRONS; DIPOLES; EMISSION; MAGNETIC FIELDS; NEUTRON STARS; NEUTRONS; PULSARS; PULSATIONS; SCATTERING; SIMULATION; SPIN; SUPERNOVA REMNANTS; X RADIATION; X-RAY SPECTROSCOPY

Citation Formats

Bogdanov, Slavko, E-mail: slavko@astro.columbia.edu. Modeling the X-rays from the central compact object PSR J1852+0040 in Kesteven 79: Evidence for a strongly magnetized neutron star. United States: N. p., 2014. Web. doi:10.1088/0004-637X/790/2/94.
Bogdanov, Slavko, E-mail: slavko@astro.columbia.edu. Modeling the X-rays from the central compact object PSR J1852+0040 in Kesteven 79: Evidence for a strongly magnetized neutron star. United States. doi:10.1088/0004-637X/790/2/94.
Bogdanov, Slavko, E-mail: slavko@astro.columbia.edu. Fri . "Modeling the X-rays from the central compact object PSR J1852+0040 in Kesteven 79: Evidence for a strongly magnetized neutron star". United States. doi:10.1088/0004-637X/790/2/94.
@article{osti_22365524,
title = {Modeling the X-rays from the central compact object PSR J1852+0040 in Kesteven 79: Evidence for a strongly magnetized neutron star},
author = {Bogdanov, Slavko, E-mail: slavko@astro.columbia.edu},
abstractNote = {I present modeling of the X-ray pulsations from the central compact object (CCO) PSR J1852+0040 in the Galactic supernova remnant Kesteven 79. In the context of thermal surface radiation from a rotating neutron star (NS), a conventional polar cap model can reproduce the broad, large-amplitude X-ray pulse only with a 'pencil plus fan' beam emission pattern, which is characteristic of ≳ 10{sup 12} G NS atmospheres, much greater than the ∼10{sup 10} G external dipole field inferred from the pulsar spin-down rate. This discrepancy can be explained by an axially displaced dipole. For other beaming patterns, it is necessary to invoke high-aspect-ratio emitting regions that are greatly longitudinally elongated, possibly due to an extremely offset dipole. For all assumed emission models, the existence of strong internal magnetic fields (≳ 10{sup 14} G) that preferentially channel internal heat to only a portion of the exterior is required to account for the implied high-temperature contrast across the stellar surface. This lends further observational evidence in support of the 'hidden' strong magnetic field scenario, in which CCOs possess submerged magnetic fields that are substantially stronger than the external dipole field, presumably due to burial by fallback of supernova ejecta. I also conduct phase-resolved X-ray spectroscopy and find no evidence for prominent spin-phase-dependent absorption features that could be produced by cyclotron absorption/scattering.},
doi = {10.1088/0004-637X/790/2/94},
journal = {Astrophysical Journal},
number = 2,
volume = 790,
place = {United States},
year = {Fri Aug 01 00:00:00 EDT 2014},
month = {Fri Aug 01 00:00:00 EDT 2014}
}
  • Using XMM-Newton and Chandra, we achieved phase-connected timing of the 105 ms X-ray pulsar PSR J1852+0040 that provides the first measurement of the spin-down rate of a member of the class of central compact objects (CCOs) in supernova remnants. We measure P-dot=(8.68+-0.09) x 10{sup -18}, and find no evidence for timing noise or variations in X-ray flux over 4.8 year. In the dipole spin-down formalism, this implies a surface magnetic field strength B{sub s} = 3.1 x 10{sup 10} G, the smallest ever measured for a young neutron star, and consistent with being a fossil field. In combination with uppermore » limits on B{sub s} from other CCO pulsars, this is strong evidence in favor of the 'anti-magnetar' explanation for their low luminosity and lack of magnetospheric activity or synchrotron nebulae. While this dipole field is small, it can prevent accretion of sufficient fall-back material so that the observed X-ray luminosity of L{sub x} = 5.3 x 10{sup 33}(d/7.1 kpc){sup 2} erg s{sup -1} must instead be residual cooling. The spin-down luminosity of PSR J1852+0040, E-dot=3.0 x 10{sup 32} erg s{sup -1}, is an order of magnitude smaller than L{sub x} . Fitting of the X-ray spectrum to two blackbodies finds small emitting radii, R{sub 1} = 1.9 km and R{sub 2} = 0.45 km, for components of kT{sub 1} = 0.30 keV and kT{sub 2} = 0.52 keV, respectively. Such small, hot regions are ubiquitous among CCOs, and are not yet understood in the context of the anti-magnetar picture because anisotropic surface temperature is usually attributed to the effects of strong magnetic fields.« less
  • Recent observations of the central compact object in the Kesteven 79 supernova remnant show that this neutron star (NS) has a weak dipole magnetic field (a few Multiplication-Sign 10{sup 10} G) but an anomalously large ({approx}64%) pulse fraction in its surface X-ray emission. We explore the idea that a substantial sub-surface magnetic field exists in the NS crust, which produces diffuse hot spots on the stellar surface due to anisotropic heat conduction, and gives rise to the observed X-ray pulsation. We develop a general-purpose method, termed 'Temperature Template with Full Transport' (TTFT), that computes the synthetic pulse profile of surfacemore » X-ray emission from NSs with arbitrary magnetic field and surface temperature distributions, taking into account magnetic atmosphere opacities, beam pattern, vacuum polarization, and gravitational light bending. We show that a crustal toroidal magnetic field of order a few Multiplication-Sign 10{sup 14} G or higher, varying smoothly across the crust, can produce sufficiently distinct surface hot spots to generate the observed pulse fraction in the Kes 79 NS. This result suggests that substantial sub-surface magnetic fields, much stronger than the 'visible' dipole fields, may be buried in the crusts of some young NSs, and such hidden magnetic fields can play an important role in their observational manifestations. The general TTFT tool we have developed can also be used for studying radiation from other magnetic NSs.« less
  • We discuss the direction-dependent transfer of X-rays in a plane-parallel atmosphere with a strong magnetic field perpendicular to the surface. We present a transfer formalism incorporating the full angular and polarization dependence of the cross sections, including vacuum polarization, for frequencies not too close to the cylcotron resonance. We treat the problem of a slab illuminated from below and of a semi-infinite medium at constant temperature and density and present numerical results for parameters typical of the hot polar caps of accreting magnetized neutron stars. Theoretical beam and X-ray pulse shapes are obtained for various models of X-ray pulsars, andmore » the frequency and phase dependence of the pulse structure is briefly compared with observations.« less
  • We develop a Monte Carlo Comptonization model for the X-ray spectrum of accretion-powered pulsars. Simple, spherical, thermal Comptonization models give harder spectra for higher optical depth, while the observational data from Vela X-1 show that the spectra are harder at higher luminosity. This suggests a physical interpretation where the optical depth of the accreting plasma increases with the mass accretion rate. We develop a detailed Monte Carlo model of the accretion flow, including the effects of the strong magnetic field (∼10{sup 12} G), both in geometrically constraining the flow into an accretion column and in reducing the cross section. Wemore » treat bulk-motion Comptonization of the infalling material as well as thermal Comptonization. These model spectra can match the observed broadband Suzaku data from Vela X-1 over a wide range of mass accretion rates. The model can also explain the so-called 'low state' in which the luminosity decreases by an order of magnitude. Here, thermal Comptonization should be negligible, so the spectrum is instead dominated by bulk-motion Comptonization.« less
  • By performing 2.5-dimensional general relativistic radiation magnetohydrodynamic simulations, we demonstrate supercritical accretion onto a non-rotating, magnetized neutron star, where the magnetic field strength of dipole fields is 10{sup 10} G on the star surface. We found the supercritical accretion flow consists of two parts: the accretion columns and the truncated accretion disk. The supercritical accretion disk, which appears far from the neutron star, is truncated at around ≃3 R {sub *} ( R {sub *} = 10{sup 6} cm is the neutron star radius), where the magnetic pressure via the dipole magnetic fields balances with the radiation pressure of themore » disks. The angular momentum of the disk around the truncation radius is effectively transported inward through magnetic torque by dipole fields, inducing the spin up of a neutron star. The evaluated spin-up rate, ∼−10{sup −11} s s{sup −1}, is consistent with the recent observations of the ultraluminous X-ray pulsars. Within the truncation radius, the gas falls onto a neutron star along the dipole fields, which results in a formation of accretion columns onto the northern and southern hemispheres. The net accretion rate and the luminosity of the column are ≃66 L {sub Edd}/ c {sup 2} and ≲10 L {sub Edd}, where L {sub Edd} is the Eddington luminosity and c is the light speed. Our simulations support a hypothesis whereby the ultraluminous X-ray pulsars are powered by the supercritical accretion onto the magnetized neutron stars.« less