Thermal Radiation from Neutron Stars: Chandra Results

The outstanding capabilities of the Chandra X-ray observatory have greatly increased our potential to observe and analyze thermal radiation from the surfaces of neutron stars (NSs). Such observations allow one to measure the surface temperatures and confront them with the predictions of the NS cooling models. Detection of gravitationally redshifted spectral lines can yield the NS mass-to-radius ratio. In rare cases when the distance is known, one can measure the NS radius, which is particularly important to constrain the equation of state of the superdense matter in the NS interiors. Finally, one can infer the chemical composition of the NS surface layers, which provides information about formation of NSs and their interaction with the environment. We overview the recent Chandra results on the thermal radiation from various types of NSs -- active pulsars, young radio-quiet neutron stars in supernova remnants, old radio-silent ``dim'' neutron stars -- and discuss their implications.Comment: URL changed for Figures 1, 12 and 18: ftp://ftp.xray.mpe.mpg.de/people/zavli

Variations in the spin period of the radio-quiet pulsar 1E 1207.4-5209

The X-ray source 1E 1207.4-5209 is a compact central object in the G296.5+10.0 supernova remnant. Its spin period of 424 ms, discovered with the Chandra X-ray Observatory, suggests that it is a neutron star. The X-ray spectrum of this radio-quiet pulsar shows at least two absorption lines, first spectral features discovered in radiation from an isolated neutron star. Here we report the results of timing analysis of Chandra and XMM-Newton observations of this source showing a non-monotonous behavior of its period. We discuss three hypotheses which may explain the observational result. The first one assumes that 1E 1207.$-5209 is a glitching pulsar, with frequency jumps of \Delta f > 5 \muHz occurring every 1-2 years. The second hypothesis explains the deviations from a steady spin-down as due to accretion, with accretion rate varying from \sim 10^{13} to >10^{16} g s^{-1}, from a disk possibly formed from ejecta produced in the supernova explosion. Finally, the period variations could be explained assuming that the pulsar is in a wide binary system with a long period, P_orb \sim 0.2-6 yr, and a low-mass companion, M_2 < 0.3 M_\odot.Comment: 20 pages, 5 figures, accepted for publications in ApJ. 2004 ApJ, in pres

X-ray emission from PSR J1809-1917 and its pulsar wind nebula, possibly associated with the TeV gamma-ray source HESS J1809-193

We detected X-ray emission from the 50-kyr-old pulsar J1809-1917 and resolved its pulsar wind nebula (PWN) with Chandra. The pulsar spectrum fits PL+BB model with the photon index of 1.2 and the BB temperature of 2 MK for n_{H}=0.7\times 10^{22} cm^{-2}. The luminosities are(4\pm 1)\times 10^{31} ergs s^{-1} for the PL component (in the 0.5-8 keV band) and ~1\times 10^{32} ergs s^{-1} for the BB component (bolometric) at a plausible distance of 3.5 kpc. The bright inner PWN component of a 3''\times12'' size is elongated in the north-south direction, with the pulsar close to its south end. This component is immersed in a larger (20''\times40''), similarly elongated outer PWN component of lower surface brightness. The elongated shape of the compact PWN can be explained by the ram pressure confinement of the pulsar wind due to the supersonic motion of the pulsar. The PWN spectrum fits a PL model with photon index of 1.4\pm0.1 and 0.5-8 keV luminosity of 4\times10^{32} ergs s^{-1}. The compact PWN appears to be inside a large-scale (~4'\times4') emission more extended to the south of the pulsar, i.e. in the direction of the alleged pulsar motion. To explain the extended X-ray emission ahead of the moving pulsar, one has to invoke strong intrinsic anisotropy of the pulsar wind or assume that this emission comes from a relic PWN swept by the asymmetrical reverse SNR shock. The pulsar and its PWN are located within the extent of the unidentified TeV source HESS J1809-193 whose brightest part is offset by ~8' to the south of the pulsar, i.e. in the same direction as the large-scale X-ray emission. Although the association between J1809-1917 and HESS J1809-193 is plausible, an alternative source of relativistic electrons powering HESS J1809-193 might be the serendipitously discovered X-ray source CXOU J180940.7-192544.Comment: 13 pages, 10 figures and 3 tables, submitted to ApJ. Version with the high-resolution figures is available at http://www.astro.psu.edu/users/green/J1809/ms_astroph.pd

The jets of the Vela pulsar

Chandra observations of the Vela pulsar-wind nebula (PWN) have revealed a jet in the direction of the pulsar's proper motion, and a counter-jet in the opposite direction, embedded in diffuse nebular emission. The jet consists of a bright, 8''-long inner jet, between the pulsar and the outer arc, and a dim, curved outer jet that extends up to 100'' in approximately the same direction. From the analysis of thirteen Chandra observations spread over about 2.5 years we found that this outer jet shows particularly strong variability, changing its shape and brightness. We observed bright blobs in the outer jet moving away from the pulsar with apparent speeds (0.3-0.6)c and fading on time-scales of days to weeks. The spectrum of the outer jet fits a power-law model with a photon index of 1.3\pm0.1. The X-ray emission of the outer jet can be interpreted as synchrotron radiation of ultrarelativistic electrons/positrons. This interpretation allows one to estimate the magnetic field, ~100 microGauss, maximum energy of X-ray emitting electrons, ~2\times 10^{14} eV, and energy injection rate, ~8\times 10^{33} erg/s, for the outer jet. In the summed PWN image we see a dim, 2'-long outer counter-jet, which also shows a power-law spectrum with photon ined of 1.2-1.5. Southwest of the jet/counter-jet an extended region of diffuse emission is seen. Relativistic particles responsible for this radiation are apparently supplied by the outer jet.Comment: 4 pages, including 1 figure, accepted for publication in New Astronomy Reviews; proceedings of the conference "The Physics of Relativistic Jets in the CHANDRA and XMM Era", 23-27 September 2002, Bologna. The full resolution versions of the images shown in the fugure are avaliable at http://www.astro.psu.edu/users/green/vela_jet_proc/vela_jet_proc.htm

Fading of the Transient Anomalous X-ray Pulsar XTE J1810-197

Three observations of the 5.54 s Transient Anomalous X-ray Pulsar XTE J1810-197 obtained over 6 months with the Newton X-Ray Multi-Mirror Mission (XMM-Newton) are used to study its spectrum and pulsed light curve as the source fades from outburst. The decay is consistent with an exponential of time constant 300 days, but not a power law as predicted in some models of sudden deep crustal heating events. All spectra are well fitted by a blackbody plus a steep power law, a problematic model that is commonly fitted to anomalous X-ray pulsars (AXPs). A two-temperature blackbody fit is also acceptable, and better motivated physically in view of the faint optical/IR fluxes, the X-ray pulse shapes that weakly depend on energy in XTE J1810-197, and the inferred emitting areas that are less than or equal to the surface area of a neutron star. The fitted temperatures remained the same while the flux declined by 46%, which can be interpreted as a decrease in area of the emitting regions. The pulsar continues to spin down, albeit at a reduced rate of (5.1+/-1.6)x10^{-12} s s^{-1}. The inferred characteristic age Tau_c = P/2Pdot ~17,000 yr, magnetic field strength B_s ~1.7x10^{14} G, and outburst properties are consistent with both the outburst and quiescent X-ray luminosities being powered by magnetic field decay, i.e., XTE J1810-197 is a magnetar.Comment: 10 pages, 5 figures, accepted by Ap.