Entire Neutron Star Surface Research Articles

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 ($L_X \approx 1.6\times 10^{33}\, \mathrm{erg\, s^{-1}}$). Intriguingly, we find a two-component spectrum, with a hard power-law (Γ ≈ 1.5) and a soft blackbody-like excess below ≈1 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 ≈10 km, consistent with emission from the entire neutron star (NS) surface. There is also mild evidence for an absorption line at $\approx 1$ and/or $\approx 1.4\, \mathrm{keV}$. The hard component shows pulsations at P ≈ 31.908 s (pulsed fraction 0.84 ± 0.10), agreeing with the pulse period seen previously in outbursts, but no pulsations were found in the soft excess (pulsed fraction $\lesssim \!31\, {\rm per\, cent}$). We conclude that the pulsed hard component suggests low-level accretion on to the NS 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 cut-off to estimate the magnetic field of the system to be $\lesssim\! 8.4\times 10^{11}\, \mathrm{G}$. We compare the quiescent thermal LX predicted by the standard deep crustal heating model to our observations and find that RX J0812.4–3114 has a high thermal LX, 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.

DEEP CHANDRA OBSERVATIONS OF THE PULSAR WIND NEBULA CREATED BY PSR B0355+54

ABSTRACT We report on Chandra X-ray Observatory (CXO) observations of the pulsar wind nebula (PWN) associated with PSR B0355+54 (eight observations with a 395 ks total exposure, performed over an eight month period). We investigated the spatial and spectral properties of the emission coincident with the pulsar, compact nebula (CN), and extended tail. We find that the CN morphology can be interpreted in a way that suggests a small angle between the pulsar spin axis and our line of sight, as inferred from the radio data. On larger scales, emission from the ( pc) tail is clearly seen. We also found hints of two faint extensions nearly orthogonal to the direction of the pulsar’s proper motion. The spectrum extracted at the pulsar position can be described with an absorbed power-law + blackbody model. The nonthermal component can be attributed to magnetospheric emission, while the thermal component can be attributed to emission from either a hot spot (e.g., a polar cap) or the entire neutron star surface. Surprisingly, the spectrum of the tail shows only a slight hint of cooling with increasing distance from the pulsar. This implies either a low magnetic field with fast flow speed, or particle reacceleration within the tail. We estimate physical properties of the PWN and compare the morphologies of the CN and the extended tail with those of other bow shock PWNe observed with long CXO exposures.

EVOLUTION OF THE X-RAY PROPERTIES OF THE TRANSIENT MAGNETAR XTE J1810–197

ABSTRACT We report on X-ray observations of the 5.54 s transient magnetar XTE J1810–197 using the XMM-Newton and Chandra observatories, analyzing new data from 2008 through 2014, and re-analyzing data from 2003 through 2007 with the benefit of these six years of new data. From the discovery of XTE J1810–197 during its 2003 outburst to the most recent 2014 observations, its 0.3–10 keV X-ray flux has declined by a factor of about 50 from 4.1 × 10−11 to 8.1 × 10−13 erg cm−2 s−1. Its X-ray spectrum has now reached a steady state. Pulsations continue to be detected from a 0.3 keV thermal hot spot that remains on the neutron star (NS) surface. The luminosity of this hot spot exceeds XTE J1810–197’s spin-down luminosity, indicating continuing magnetar activity. We find that XTE J1810–197’s X-ray spectrum is best described by a multiple component blackbody model in which the coldest 0.14 keV component likely originates from the entire NS surface, and the thermal hot-spot is, at different epochs, well described by an either one- or two-component blackbody model. A 1.2 keV absorption line, possibly due to resonant proton scattering, is detected at all epochs. The X-ray flux of the hot spot decreased by ≈ 20 % ?> between 2008 and 2009 March, the same period during which XTE J1810–197 became radio quiet.

Constraining the parameters of the pulsar wind nebula DA 495 and its pulsar withChandraandXMM–Newton

We present spectral and timing analyses of the X-ray emission from the pulsar wind nebula DA 495 and its central object, J1952.2+2925, suggested to be the pulsar, using archival Chandra and XMM-Newton data. J1952.2+2925 has a pure thermal spectrum which is equally well fitted either by the blackbody model with a temperature of $\approx 215$ eV and an emitting area radius of $\approx 0.6$ km or by magnetized neutron star atmosphere models with temperatures of 80-90 eV. In the latter case the thermal emission can come from the entire neutron star surface which temperature is consistent with standard neutron star cooling scenarios. We place also an upper limit on the J1952.2+2925 nonthermal flux. The derived spectral parameters are generally compatible with published ones based only on the Chandra data, but they are much more accurate due to the inclusion of XMM-Newton data. No pulsations were found and we placed an upper limit for the J1952.2+2925 pulsed emission fraction of 40 per cent. Utilizing the interstellar absorption-distance relation, we estimated the distance to DA 495, which can be as large as 5 kpc if J1952.2+2925 emission is described by the atmosphere models. We compiled possible multi-wavelength spectra of the nebula including radio data; they depend on the spectral model of the central object. Comparing the results with other pulsar plus wind nebula systems we set reasonable constraints on the J1952.2+2925 spin-down luminosity and age. We suggest that the Fermi source 3FGL J1951.6+2926 is the likely $\gamma$-ray counterpart of J1952.2+2925.

A non-pulsating neutron star in the supernova remnant HESS J1731−347/G353.6−0.7 with a carbon atmosphere

Context: The CCO candidate in the center of the supernova remnant shell HESS J1731-347 / G353.6-0.7 shows no pulsations and exhibits a blackbody-like X-ray spectrum. If the absence of pulsations is interpreted as evidence for the emitting surface area being the entire neutron star surface, the assumption of the measured flux being due to a blackbody emission translates into a source distance that is inconsistent with current estimates of the remnant's distance. Aims: With the best available observational data, we extended the pulse period search down to a sub-millisecond time scale and used a carbon atmosphere model to describe the X-ray spectrum of the CCO and to estimate geometrical parameters of the neutron star. Methods: To search for pulsations we used data of an observation of the source with XMM-Newton performed in timing mode. For the spectral analysis, we used earlier XMM-Newton observations performed in imaging mode, which permits a more accurate treatment of the background. The carbon atmosphere models used to fit the CCO spectrum are computed assuming hydrostatic and radiative equilibria and take into account pressure ionization and the presence of spectral lines. Results: Our timing analysis did not reveal any pulsations with a pulsed fraction above ~8% down to 0.2 ms. This finding further supports the hypothesis that the emitting surface area is the entire neutron star surface. The carbon atmosphere model provides a good fit to the CCO spectrum and leads to a normalization consistent with the available distance estimates of the remnant. The derived constraints on the mass and radius of the source are consistent with reasonable values of the neutron star mass and radius. After the CCO in Cas A, the CCO in HESS J1731-347 / G353.6-0.7 is the second object of this class for which a carbon atmosphere model provides a consistent description of X-ray emission.

First X-Ray Observations of the Young Pulsar J1357-6429

The first short Chandra and XMM-Newton observations of the young and energetic pulsar J1357-6429 provided strong indications of a tail-like pulsar-wind nebula associated with this object, as well as pulsations of its X-ray flux with a pulsed fraction above 50% and a thermal component dominating at lower photon energies (below 2 keV). The elongated nebula is very compact in size and may be interpreted as evidence for a pulsar jet. The thermal radiation is most plausibly emitted from the entire neutron star surface of a 10 km radius and a 1.0+/-0.1 K temperature, covered with a magnetized hydrogen atmosphere. At higher energies the pulsar's emission is of a nonthermal (magnetospheric) origin, with a power-law spectrum of a photon index of 1.1-1.3. This makes the X-ray properties of PSR J1357-6429 very similar to those of the youngest pulsars J1119-6127 and Vela with a detected thermal radiation.

Revealing the X‐Ray Emission Processes of Old Rotation‐powered Pulsars:XMM‐NewtonObservations of PSR B0950+08, PSR B0823+26, and PSR J2043+2740

We have completed part of a program to study the X-ray emission properties of old rotation-powered pulsars with XMM-Newton in order to probe and identify the origin of their X-radiation. The X-ray emission from these old pulsars is largely dominated by nonthermal processes. None of the observed spectra required adding a thermal component consisting of either a hot polar cap or surface cooling emission to model the data. The X-ray spectrum of PSR B0950+08 is best described by a single power law of photon index α = 1.93. Taking optical data from the VLT FORS1 into account, a broken power-law model with the break point Ebreak = 0.67 keV and the photon indices α1 = 1.27 and α2 = 1.88 for E < Ebreak and E > Ebreak, respectively, is found to describe the pulsar's broadband spectrum from the optical to the X-ray band. Three σ temperature upper limits for possible contributions from a heated polar cap or the whole neutron star surface are T < 0.87 × 106 and T < 0.48 × 106 K, respectively. We also find that the X-ray emission from PSR B0950+08 is pulsed with two peaks per rotation period. The phase separation between the two X-ray peaks is ~144° (maximum to maximum), which is similar to the pulse peak separation observed in the radio band at 1.4 GHz. The main radio peak and the trailing X-ray peak are almost phase-aligned. The fraction of X-ray pulsed photons is ~30%. A phase-resolved spectral analysis confirms the nonthermal nature of the pulsed emission and finds no spectral variations as a function of pulse phase. Detailed pulse profile simulations using the polar gap, the outer gap, and the two-pole caustic model constrain the pulsar's emission geometry to be that of an almost orthogonal rotator, for which the two-pole caustic model can reproduce the observed doubly peaked X-ray pulse profile. The spectral emission properties observed for PSR B0823+26 are similar to those of PSR B0950+08. Its energy spectrum is very well described by a single power law with photon index α = 2.5. Three σ temperature upper limits for thermal contributions from a hot polar cap or from the entire neutron star surface are T < 1.17 × 106 and T < 0.5 × 106 K, respectively. There is evidence for pulsed X-ray emission at the ~97% confidence level with a pulsed fraction of 49% ± 22%. For PSR J2043+2740, which is located ~1° outside the boundary of the Cygnus Loop, we report the first detection of X-ray emission. A power-law spectrum or a combination of a thermal and a power-law spectrum both yield acceptable descriptions of its X-ray spectrum. No X-ray pulses are detected from PSR J2043+2740 and the sensitivity is low—the 2 σ pulsed fraction upper limit is 57% assuming a sinusoidal pulse profile.

X‐Ray, Radio, and Optical Observations of the Putative Pulsar in the Supernova Remnant CTA 1

A Chandra image of the central X-ray source RX J0007.0+7303 in the supernova remnant CTA 1 reveals a point source, a compact nebula, and a bent jet, all of which are characteristic of energetic, rotation-powered pulsars. Using the MDM 2.4 m telescope we obtain upper limits in the optical at the position of the point source, (J2000.0) 00h07m0156, +73°03'081, determined to an accuracy of 01, of B > 25.4, V > 24.8, R > 25.1, and I > 23.7; these correspond to an X-ray-to-optical flux ratio ≳100. Neither a Very Large Array image at 1425 MHz nor a deep pulsar search at 820 MHz using the NRAO Green Bank Telescope reveal a radio pulsar counterpart to an equivalent luminosity limit at 1400 MHz of 0.02 mJy kpc2, which is equal to the lowest luminosity known for any radio pulsar. The Chandra point source accounts for ≈30% of the flux of RX J0007.0+7303, while its compact nebula plus jet comprise ≈70%. The X-ray spectrum of the point source is fitted with a power law-plus-blackbody model with Γ = 1.6 ± 0.6, kT∞ = 0.13 ± 0.05 keV, and R∞ = 0.37 km, values typical of a young pulsar. An upper limit of T < 6.6 × 105 K to the effective temperature of the entire neutron star surface is derived, which is one of the most constraining data points on cooling models. The 0.5-10 keV luminosity of RX J0007.0+7303 is ≈4 × 1031 (d/1.4 kpc)2 ergs s-1, but the larger (~18' diameter) synchrotron nebula in which it is embedded is 2 orders of magnitude more luminous. These properties allow us to estimate, albeit crudely, that the spin-down luminosity of the underlying pulsar is in the range 1036-1037 ergs s-1, and support the identification of the high-energy γ-ray source 3EG J0010+7309 as a pulsar even though its spin parameters have not yet been determined.

Helium Detonations on Neutron Stars

We present the results of a numerical study of helium detonations on the surfaces of neutron stars. We describe two-dimensional simulations of the evolution of a detonation as it breaks through the accreted envelope of the neutron star and propagates laterally through the accreted material. The detonation front propagates laterally at nearly the Chapman-Jouguet velocity, v = 1.3 × 109 cm s-1. A series of surface waves propagate across the pool of hot ash behind the detonation front with the same speed, matching the speed expected from shallow water wave theory. The entire envelope oscillates in the gravitational potential well of the neutron star with a period of ~50 μs. The photosphere reaches an estimated height of 10 km above the surface of the neutron star. Our study confirms that such a detonation can insure the spread of burning over the entire neutron star surface on a timescale consistent with burst rise times. We analyze the sensitivity of the results to the spatial resolution and the assumed initial conditions. We conclude by presenting a comparison of this model to type I X-ray bursts.

The Effect of Deep Hydrogen Burning in the Accreted Envelope of a Neutron Star on the Properties of X-Ray Bursts

The thermal and compositional evolution of a neutron star has been numerically followed to determine the long-term properties of X-ray bursts produced by thermonuclear shell flashes in its accreted hydrogen-rich envelope. Uniform burning over the entire neutron star surface is assumed and mass accretion rates greater than {approximately}0.1 {dot {ital M}}{sub Edd} (where {dot {ital M}}{sub Edd} is the critical mass accretion rate for which the accretion luminosity is equal to the Eddington luminosity) are considered. Specific attention is focused on the consequences of electron capture initiated burning of hydrogen at high densities ({approx_gt}10{sup 7} gcm{sup {minus}3}). The degree of heating associated with the burning of the residual hydrogen (i.e., the matter which is not completely processed in the outburst) is a function of the mass accretion rate and the composition of the accreted matter. Heating of the neutron star envelope is found to be more important for greater mass accretion rates and for greater residual hydrogen abundances. Because of the higher envelope temperatures, the resulting bursts are weaker and recur more frequently, for a given mass accretion rate, than in situations where the deep hydrogen burning does not occur. The mass accretion rate, which delineates strong X-ray bursts (wheremore » the ratio of the peak burst luminosity to the quiescent level of emission is greater than {approximately}3) from weak X-ray bursts, lies in the range of 0.1{endash}0.2 times the Eddington value. Weak burst activity is found for accretion rates extending to about the Eddington limit provided that the helium content of the accreted matter is greater than {approximately}0.23. The implications of our results with regard to the absence of regular, periodic X-ray bursting activity in the bright low-mass X-ray binary sources are briefly discussed. {copyright} {ital 1996 The American Astronomical Society.}« less

X-ray emission from PSR 0355+54

We have obtained a 20 ks observation of PSR 0355+54 using the ROSAT position sensitive proportional counter (PSPC). The pulsar is detected with a count rate of 4.2(+/- 1.3) x 10(exp -3)/s above the background. While the approximately 70 source counts are insufficient for spectral fitting, we have derived source parameters for specific cases of power law as well as blackbody spectra. For a Crab-like spectrum (photon index alpha = 2) we find L(sub x)(0.1-2.4 keV) = 1.0 x 10(exp 32) ergs/s, somewhat higher than upper limits reported from Einstein observations but consistent with typical L(sub x) versus E-dot values for other pulsars. For blackbody emission, we derive a temperature upper limit of approximately 9.5 x 10(exp 5) K for emission from the entire neutron star surface, which is consistent with standard models for cooling of the neutron star interior given a characteristic age 10(exp 5.75) yr. No evidence is present for modulation at the 156 ms pulsar period, setting a weak upper limit of approximately 75% for the pulsed fraction of the X-ray signal.