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We report on the study of six Chandra observations (four epochs) of the Central Compact Object (CCO) in the Cassiopeia A supernova remnant with the ACIS instrument in the subarray mode. This mode minimizes spectrum-distorting instrumental effects such as pileup. The data were taken over a time span of ∼14 yr. If a non-magnetic carbon atmosphere is assumed for this youngest known CCO, then the temperature change is constrained to be T ̇ = âˆ' 2900 ± 600 K yrâˆ'1 or T ̇ = âˆ' 4500 ± 800 K yrâˆ'1 (1 σ uncertainties) for constant or varying absorbing hydrogen column density. These values correspond to cooling rates of âˆ'1.5% ± 0.3% per 10 yr and âˆ'2.3% ± 0.4% per 10 yr, respectively. We discuss an apparent increase in the cooling rate in the last five years and the variations of the inferred absorbing hydrogen column densities between epochs. Considered together, these changes could indicate systematic effects such as caused by, e.g., an imperfect calibration of the increasing contamination of the ACIS filter. [ABSTRACT FROM AUTHOR]

To examine the previously claimed fast cooling of the Central Compact Object (CCO) in the Cas A supernova remnant (SNR), we analyzed two Chandra observations of this CCO, taken in a setup minimizing instrumental spectral distortions. We fit the two CCO X-ray spectra from 2006 and 2012 with hydrogen and carbon neutron star atmosphere models. The temperature and flux changes in the 5.5 yr between the two epochs depend on the adopted constraints on the fitting parameters and the uncertainties of the effective area calibrations. If we allow a change of the equivalent emitting region size, REm, the effective temperature remains essentially the same. If REm is held constant, the best-fit temperature change is negative, but its statistical significance ranges from 0.8σ to 2.5σ, depending on the model. If we assume that the optical depth of the ACIS filter contaminant in 2012 was ±10% different from its default calibration value, the significance of the temperature drop becomes 0.8σ-3.1σ, for the carbon atmospheres with constant REm. Thus, we do not see a statistically significant temperature drop in our data, but the involved uncertainties are too large to firmly exclude the previously reported fast cooling. Our analysis indicate a decrease of 4%-6% (1.9σ-2.9σ significance) for the absorbed flux in the energy range 0.6-6 keV between 2006 and 2012, most prominent in the ≈1.4-1.8 keV energy range. It could be caused by unaccounted changes of the detector response or contributions from unresolved SNR material along the line of sight to the CCO. [ABSTRACT FROM AUTHOR]

Recent X-ray observations of the supernova remnant (SNR) IC 443 interacting with molecular clouds detected a new population of hard X-ray sources related to the remnant itself, which has been proposed to be fast ejecta fragments propagating within the dense environment. Encouraged by these studies, we obtained a deep XMM-Newton observation of the SNR Kes 69, which also shows signs of a shock-cloud interaction.We report on the detection of 18 hard X-ray sources in the field of Kes 69, which is a number sognificantly higher than expected for the Galactic source population in the field. The sources are spatially correlated with CO emission from the cloud in the remnant environment. The spectra of 3 of the 18 sources can be described as hard power-laws with photon indices smaller than two plus line emission associated with K-shell transitions. We discuss the two most promising scenarios for the interpretation of the sources, namely fast ejecta fragments (as in IC 443) and cataclysmic variables. While most of the observational evidence is consistent with the former interpretation, we cannot rule out the latter. [ABSTRACT FROM AUTHOR]

We present 35 ks Chandra ACIS observations of the 42 Myr old radio pulsar PSR B1451-68. A point source is detected 0".32±0".73 from the expected radio pulsar position. It has ~200 counts in the 0.3-8 keV energy range. We identify this point source as the X-ray counterpart of the radio pulsar. PSR B1451-68 is located close to a Two Micron All Sky Survey point source for which we derive 7% as the upper limit on the flux contribution to the measured pulsar X-ray flux. The pulsar spectrum can be described by either a power-law model with photon index Γ = 2.4-0.3+0.4 and a unrealistically high absorbing column density NH = (2.5-1.3+1.2) x 1021 cm-2, or by a combination of a kT = 0.35-0.07+0.12 keV blackbody and a Γ = 1.4±0.5 power-law component for NHDM = 2.6 x 1020 cm-2, estimated from the pulsar dispersion measure. At the parallactic, a Lutz-Kelker bias corrected distance of 480 pc, the non-thermal X-ray luminosities in the 0.3-8 keV energy band are either L0.3-8keVnonth = (11.3±1.7) x 1029 erg s-1 or L0.3-8k eVnonth = (5.9-5.0+4.9 x 1029 erg s-1, respectively. This corresponds to non-thermal X-ray efficiencies of either η0.3-8k eVnonth = L0.3-8k eVnonth/E ~ 5 x 10-3 or 3 x 10-3, respectively. [ABSTRACT FROM AUTHOR]

ABSTRACT Synchrotron radiation of ultrarelativistic particles accelerated in a pulsar wind nebula may dominate its spectrum up to γ-ray energies. Because of the short cooling time of the γ-ray-emitting electrons and positrons, the γ-ray-emission zone is in the immediate vicinity of the acceleration site. Particle acceleration likely occurs at the termination shock of a relativistic striped wind, where multiple forced magnetic field reconnections provide strong magnetic fluctuations facilitating Fermi acceleration processes. The acceleration mechanisms imply the presence of stochastic magnetic fields in the particle acceleration region, which cause stochastic variability of the synchrotron emission. This variability is particularly strong in the steep γ-ray tail of the spectrum, where modest fluctuations of the magnetic field lead to strong flares of spectral flux. In particular, stochastic variations of magnetic field, which may lead to quasi-cyclic γ-ray flares, can be produced by the relativistic cyclotron ion instability at the termination shock. Our model calculations of the spectral and temporal evolution of synchrotron emission in the spectral cut-off regime demonstrate that the intermittent magnetic field concentrations dominate the γ-ray emission from highest energy electrons and provide fast, strong variability even for a quasi-steady distribution of radiating particles. The simulated light curves and spectra can explain the very strong γ-ray flares observed in the Crab Nebula and lack of strong variations at other wavelengths. The model predicts high polarization in the flare phase, which can be tested with future polarimetry observations. [ABSTRACT FROM AUTHOR]

Candidate supernova remnants (SNRs) G23.5+0.1 and G25.5+0.0 were observed by XMM-Newton in the course of a snapshot survey of plerionic and composite SNRs in the Galactic plane. In the field of G23.5+0. I, we detected an extended source, ~3' in diameter, which we tentatively interpret as a pulsar wind nebula (PWN) of the middle-aged radio pulsar B1830-08 (J1833-0827; P = 85.3 ms, τ = 147 kyr, Ė = 5.8 x 1035 erg s-1, d = 5.7 kpc), with the PWN luminosity L0.2-10keV ≈ 5 X 1033 erg s-1 ≈ 8 x 10-3 Ė. The pulsar is not resolved in the EPIC images. Our analysis suggests an association between PSR B 1830-08 and the surrounding diffuse radio emission. If the radio emission is due to the SNR, then the pulsar must be significantly younger than its characteristic age. Alternatively. the radio emission may come from a relic PWN. The field also contains SGR 1833-0832 and another middle-aged pulsar B 1829-08 (J1832-0827; P = 647 ms, τ = 161 kyr, Ė = 9.3 x 1033 erg s-1, d = 4.7 kpc), none of which are detected in our observation. In the field of G25.5+0.0, which contains the extended TeV source HESS J1837-069, we detected the recently discovered young high-energy pulsar J1838-0655 (P = 70.5 ms, τ = 23 kyr, Ė = 5.5 x 1036 erg s-1) embedded in a PWN with extent of 1.'3. The unabsorbed pulsar + PWN luminosity is L2-11 keV ≈ 2 X 1034 erg s-1 ≈ 4 x 10-3/Ė at an assumed distance of 7 kpc. We also detected another PWN candidate (AX J1837.3-0652) with an extent oft and unabsorbed luminosity L2-10keV ≈ 4 x 1033 erg s-1 at d = 7 kpc. The third X-ray source, located within the extent of the HESS J1837-069, has a peculiar extended radio counterpart, possibly a radio galaxy with a double nucleus or a microquasar. We did not find any evidence of the SNR emission in the G25.5+0.0 field. We provide detailed multiwavelength analysis and identifications of other field sources and discuss robustness of the G25.5+0.0 and G23.5+0.1 classifications as SNRs. [ABSTRACT FROM AUTHOR]

High-magnetic-field pulsars represent an important class of objects for studying the relationship between magnetars and radio pulsars. Here we report on four Chandra observations of the high-magnetic-field pulsar J 1718-3718 (B = 7.4 × 1013 G) taken in 2009 as well as a reanalysis of 2002 Chandra observations of the region. We also report an improved radio position for this pulsar based on ATCA observations. We detect X-ray pulsations at the pulsar's period in the 2009 data, with a pulsed fraction of 52% ± 13% in the 0.8-2.0 keV band. We find that the X-ray pulse is aligned with the radio pulse. The data from 2002 and 2009 show consistent spectra and fluxes: a merged overall spectrum is well fit by a blackbody of temperature 186-18+19 eV, slightly higher than predicted by standard cooling models; however, the best-fit neutron star atmosphere model is consistent with standard cooling. We find the bolometric luminosity Lbb∞ = 4-2+5 × 1032 erg s-1 ~ 0.3 E for a distance of 4.5 kpc. We compile measurements of the temperatures of all X-ray-detected high-B pulsars as well as those of low-B radio pulsars and find evidence for the former being hotter on average than the latter. [ABSTRACT FROM AUTHOR]

We report on XMM-Newton EPIC observations of the young pulsar J2022+3842, with a characteristic age of 8.9 kyr. We detected X-ray pulsations and found the pulsation period P ≈ 48.6 ms, and its derivative , which is two times larger than previously reported values. The pulsar exhibits two very narrow (FWHM ∼1.2 ms) X-ray pulses each rotation, separated by ≈0.48 of the period, with a pulsed fraction of ≈0.8. Using the correct values of P and , we calculate the pulsar's spin-down power erg s–1 and magnetic field B = 2.1 × 1012 G. The pulsar spectrum is well modeled with a hard power-law (PL) model (photon index Γ = 0.9 ± 0.1, hydrogen column density nH = (2.3 ± 0.3) × 1022 cm–2). We detect a weak off-pulse emission, which can be modeled with a softer PL (Γ ≈ 1.7 ± 0.7), that is poorly constrained because of contamination in the EPIC-pn timing mode data. The pulsar's X-ray efficiency in the 0.5-8 keV energy band, , is similar to those of other pulsars. The XMM-Newton observation did not detect extended emission around the pulsar. Our re-analysis of ChandraX-ray Observatory archival data shows a hard, Γ ≈ 0.9 ± 0.5, spectrum and a low efficiency, ηPWN ∼ 2 × 10–5(D/10 kpc)2, for the compact pulsar wind nebula, unresolved in the XMM-Newton images. [ABSTRACT FROM AUTHOR]

We report on an X-ray observation of the 166 Myr old radio pulsar J0108–1431 with XMM-Newton. The X-ray spectrum can be described by a power-law model with a relatively steep photon index Γ ≈ 3 or by a combination of thermal and non-thermal components, e.g., a power-law component with fixed photon index Γ = 2 plus a blackbody component with a temperature of kT = 0.11 keV. The two-component model appears more reasonable considering different estimates for the hydrogen column density NH. The non-thermal X-ray efficiency in the single power-law model is , higher than in most other X-ray-detected pulsars. In the case of the combined model, the non-thermal and thermal X-ray efficiencies are even higher, . We detected X-ray pulsations at the radio period of P ≈ 0.808 s with significance of ≈7σ. The pulse shape in the folded X-ray light curve (0.15-2 keV) is asymmetric, with statistically significant contributions from up to five leading harmonics. Pulse profiles at two different energy ranges differ slightly: the profile is asymmetric at low energies, 0.15-1 keV, while at higher energies, 1-2 keV, it has a nearly sinusoidal shape. The radio pulse peak leads the 0.15-2 keV X-ray pulse peak by ▵ϕ = 0.06 ± 0.03. [ABSTRACT FROM AUTHOR]

We report on radio and X-ray observations of PSR J1832+0029, a 533 ms radio pulsar discovered in the Parkes Multibeam Pulsar Survey. From radio observations taken with the Parkes, Lovell, and Arecibo telescopes, we show that this pulsar exhibits two spin-down states akin to PSRs B1931+24 reported by Kramer et al. and J1841–0500 reported by Camilo et al. Unlike PSR B1931+24, which switches between “on” and “off” states on a 30-40 day timescale, PSR J1832+0029 is similar to PSR J1841–0500 in that it spends a much longer period of time in the off-state. So far, we have fully sampled two off-states. The first one lasted between 560 and 640 days and the second one lasted between 810 and 835 days. From our radio timing observations, the ratio of on/off spin-down rates is 1.77 ± 0.03. Chandra observations carried out during both the on- and off-states of this pulsar failed to detect any emission. Our results challenge but do not rule out models involving accretion onto the neutron star from a low-mass stellar companion. In spite of the small number of intermittent pulsars currently known, difficulties in discovering them and in quantifying their behavior imply that their total population could be substantial. [ABSTRACT FROM AUTHOR]

We report on the analysis of two XMM-Newton observations of the recently discovered soft gamma repeater Swift J1834.9–0846, taken in 2005 September and one month after the source went into outburst on 2011 August 7. We performed timing and spectral analyses on the point source as well as on the extended emission. We find that the source period is consistent with an extrapolation of the Chandra ephemeris reported earlier and the spectral properties remained constant. The source luminosity decreased to a level of 1.6 × 1034 erg s–1 following a decay trend of ∝t–0.5. Our spatial analysis of the source environment revealed the presence of two extended emission regions around the source. The first (region A) is a symmetric ring around the point source, starting at 25″ and extending to ∼50″. We argue that region A is a dust scattering halo. The second (region B) has an asymmetrical shape extending between 50″ and 150″, and is detected both in the pre- and post-outburst data. We argue that this region is a possible magnetar wind nebula (MWN). The X-ray efficiency of the MWN with respect to the rotation energy loss is substantially higher than those of rotation-powered pulsars: . The higher efficiency points to a different energy source for the MWN of Swift J1834.9–0846, most likely bursting activity of the magnetar, powered by its high magnetic field, B = 1.4 × 1014 G. [ABSTRACT FROM AUTHOR]

All the neutron star (NS) atmosphere models published so far have been calculated in the “cold plasma approximation,” which neglects the relativistic effects in the radiative processes, such as cyclotron emission/absorption at harmonics of cyclotron frequency. Here, we present new NS atmosphere models which include such effects. We calculate a set of models for effective temperatures Teff = 1-3 MK and magnetic fields B ∼ 1010-1011 G, typical for the so-called central compact objects (CCOs) in supernova remnants, for which the electron cyclotron energy Ec, e and its first harmonics are in the observable soft X-ray range. Although the relativistic parameters, such as kTeff/mec2 and Ec, e/mec2, are very small for CCOs, the relativistic effects substantially change the emergent spectra at the cyclotron resonances, E ≈ sEc, e (s = 1, 2, …). Although the cyclotron absorption features can form in a cold plasma due to the quantum oscillations of the free-free opacity, the shape and depth of these features change substantially if the relativistic effects are included. In particular, the features acquire deep Doppler cores, in which the angular distribution of the emergent intensity is quite different from that in the cold plasma approximation. The relative contributions of the Doppler cores to the equivalent widths of the features grow with increasing quantization parameter beff ≡ Ec, e/kTeff and harmonic number s. The total equivalent widths of the features can reach ∼150-250 eV; they increase with growing beff and are smaller for higher harmonics. [ABSTRACT FROM AUTHOR]