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The X-ray thermal isolated neutron star (XTINS) RX J0806.4--4123 shows interesting multiwavelength properties that seemingly deviate from those of similar neutron stars. An accurate determination of the spin frequency change over time can assist in interpreting RX J0806.4-4123's properties in comparison to those of other XTINSs and the wider pulsar population. From 2019 to 2023 we carried out a tailored X-ray timing campaign of RX J0806.4-4123 with the NICER instrument. We used statistical properties of the Fourier coefficients and the $Z_K^2$ test for phase-connecting separate observations and finding a timing solution for the entire dataset. We also developed a simple and universal method for estimating the uncertainties of frequency $\nu$ and its derivative $\dot{\nu}$ from the empirical dependencies of $Z_K^2$ on trial values of these parameters, with account of all significant harmonics of the frequency. Applying this method, we determined a spin-down rate $\dot{\nu} = -7.3(1.2)\times 10^{-17}\,{\rm Hz\, s}^{-1}$. The resulting spin-down power $\dot{E}=2.6\times 10^{29}$ erg s$^{-1}$ is the lowest among the XTINSs, and it is a factor of 60 lower than the X-ray luminosity of this neutron star. RX J0806.4-4123 is also among the pulsars with the lowest measured $\dot{E}$ in general., Comment: accepted by ApJ; 16 pages of which 7 pages are Appendix, describing details of optimal planning phase-connected observations and estimating timing uncertainties using the Fourier coefficients and the $Z_K^2$ test; v2: a few surplus words removed in section A5

We present JWST NIRCam (F356W and F444W filters) and MIRI (F770W) images and NIRSpec- IFU spectroscopy of the young supernova remnant Cassiopeia A (Cas A). We obtained the data as part of a JWST survey of Cas A. The NIRCam and MIRI images map the spatial distributions of synchrotron radiation, Ar-rich ejecta, and CO on both large and small scales, revealing remarkably complex structures. The CO emission is stronger at the outer layers than the Ar ejecta, which indicates the reformation of CO molecules behind the reverse shock. NIRSpec-IFU spectra (3 - 5.5 microns) were obtained toward two representative knots in the NE and S fields. Both regions are dominated by the bright fundamental rovibrational band of CO in the two R and P branches, with strong [Ar VI] and relatively weaker, variable strength ejecta lines of [Si IX], [Ca IV], [Ca V] and [Mg IV]. The NIRSpec-IFU data resolve individual ejecta knots and filaments spatially and in velocity space. The fundamental CO band in the JWST spectra reveals unique shapes of CO, showing a few tens of sinusoidal patterns of rovibrational lines with pseudo-continuum underneath, which is attributed to the high-velocity widths of CO lines. The CO also shows high J lines at different vibrational transitions. Our results with LTE modeling of CO emission indicate a temperature of 1080 K and provide unique insight into the correlations between dust, molecules, and highly ionized ejecta in supernovae, and have strong ramifications for modeling dust formation that is led by CO cooling in the early Universe., Comment: accepted for the ApJ letter (17 pages and 10 figures)

We used the Imaging X-ray Polarimetry Explorer (IXPE) satellite to measure, for the first time, the 2-8 keV polarization of NGC 1068. We pointed IXPE for a net exposure time of 1.15 Ms on the target, in addition to two ~ 10 ks each Chandra snapshots in order to account for the potential impact of several ultraluminous X-ray source (ULXs) within IXPE's field-of-view. We measured a 2 - 8 keV polarization degree of 12.4% +/- 3.6% and an electric vector polarization angle of 101{\deg} +/- 8{\deg} at 68% confidence level. If we exclude the spectral region containing the bright Fe K lines and other soft X-ray lines where depolarization occurs, the polarization fraction rises up to 21.3% +/- 6.7% in the 3.5 - 6.0 keV band, with a similar polarization angle. The observed polarization angle is found to be perpendicular to the parsec scale radio jet. Using a combined Chandra and IXPE analysis plus multi-wavelength constraints, we estimated that the circumnuclear "torus" may sustain a half-opening angle of 50{\deg} - 55{\deg} (from the vertical axis of the system). Thanks to IXPE, we have measured the X-ray polarization of NGC 1068 and found comparable results, both in terms of polarization angle orientation with respect to the radio-jet and torus half-opening angle, to the X-ray polarimetric measurement achieved for the other archetypal Compton-thick AGN : the Circinus galaxy. Probing the geometric arrangement of parsec-scale matter in extragalactic object is now feasible thanks to X-ray polarimetry., Comment: 9 pages, 5 figures, 2 tables, submited to A&A

We present the results of the first X-ray polarimetric observation of the low-mass X-ray binary 4U 1957+115, performed with the Imaging X-ray Polarimetry Explorer in May 2023. The binary system has been in a high-soft spectral state since its discovery and is thought to host a black hole. The $\sim$571 ks observation reveals a linear polarisation degree of $1.9\% \pm 0.6\%$ and a polarisation angle of $-41^\circ.8 \pm 7^\circ.9$ in the 2-8 keV energy range. Spectral modelling is consistent with the dominant contribution coming from the standard accretion disc, while polarimetric data suggest a significant role of returning radiation: photons that are bent by strong gravity effects and forced to return to the disc surface, where they can be reflected before eventually reaching the observer. In this setting, we find that models with a black hole spin lower than 0.96 and an inclination lower than $50^\circ$ are disfavoured., Comment: 12 pages, 10 figures, 2 tables, accepted for publication in A&A

We report on a comprehensive analysis of simultaneous X-ray polarimetric and spectral data of the bright atoll source GX 9+9 with the Imaging X-ray Polarimetry Explorer (IXPE) and NuSTAR. The source is significantly polarized in the 4--8 keV band, with a degree of $2.2\% \pm 0.5\%$ (uncertainty at the 68% confidence level). The NuSTAR broad-band spectrum clearly shows an iron line, and is well described by a model including thermal disk emission, a Comptonized component, and reflection. From a spectro-polarimetric fit, we obtain an upper limit to the polarization degree of the disk of 4% (at 99% confidence level), while the contribution of Comptonized and reflected radiation cannot be conclusively separated. However, the polarization is consistent with resulting from a combination of Comptonization in a boundary or spreading layer, plus reflection off the disc, which gives a significant contribution in any realistic scenario., Comment: 10 pages, accepted for publication in A&A

Previous observations of the middle-aged $\gamma$-ray, X-ray, and radio pulsar B1055-52 indicated some peculiarities, such as a suspected changing of the X-ray flux and spectral parameters, a large excess of the alleged thermal component of the ultraviolet (UV) spectrum over the Rayleigh-Jeans extension of the X-ray thermal spectrum, and a possible double break in the nonthermal spectral component between the optical and X-ray bands. We observed PSR B1055-52 with the XMM-Newton observatory in X-rays and the Hubble Space Telescope in near-infrared (NIR). The analysis of the XMM-Newton observations does not support the notion of long-term changes in the X-ray flux and broad-band X-ray spectrum of the pulsar. Using an observing mode less affected by background noise than the previous XMM-Newton observations, we constrain the power-law (PL) spectral index as $\alpha_X=-0.57^{+0.26}_{-0.25} $ ($F_{\nu} \propto \nu^{\alpha}$) in the energy band 3-10 keV. From the NIR-optical data we obtain a PL slope $\alpha_O= -0.24 \pm 0.10$ for the color index $E(B-V)=0.03$ mag. The slopes and fluxes of the NIR-optical and X-ray nonthermal spectra suggest that the NIR through X-ray emission can be described by the same PL and is generated by the same mechanism, unlike the pulsar's $\gamma$-ray emission. The excess of the UV thermal component over the extension of the X-ray thermal component became smaller but did not disappear, indicating a non-uniformity of the bulk surface temperature. The NIR data also enable us to accurately measure the proper motion with values $\mu_\alpha =47.5\pm 0.7\,{\rm mas\,yr}^{-1}$ and $\mu_\delta = -8.7 \pm 0.7 \,{\rm mas\,yr}^{-1}$., Comment: ApJ accepted; 19 pages, 10 Figures, 5 Tables

We present an X-ray spectro-polarimetric analysis of the bright Seyfert galaxy IC 4329A. The Imaging X-ray Polarimetry Explorer (IXPE) observed the source for ~500 ks, supported by XMM-Newton (~60 ks) and NuSTAR (~80 ks) exposures. We detect polarisation in the 2-8 keV band with 2.97 sigma confidence. We report a polarisation degree of $3.3\pm1.1$ per cent and a polarisation angle of $78\pm10$ degrees (errors are 1 sigma confidence). The X-ray polarisation is consistent with being aligned with the radio jet, albeit partially due to large uncertainties on the radio position angle. We jointly fit the spectra from the three observatories to constrain the presence of a relativistic reflection component. From this, we obtain constraints on the inclination angle to the inner disc (< 39 degrees at 99 per cent confidence) and the disc inner radius (< 11 gravitational radii at 99 per cent confidence), although we note that modelling systematics in practice add to the quoted statistical error. Our spectro-polarimetric modelling indicates that the 2-8 keV polarisation is consistent with being dominated by emission directly observed from the X-ray corona, but the polarisation of the reflection component is completely unconstrained. Our constraints on viewer inclination and polarisation degree tentatively favour more asymmetric, possibly out-flowing, coronal geometries that produce more highly polarised emission, but the coronal geometry is unconstrained at the 3 sigma level., Comment: Accepted for publication in MNRAS, 13 pages, 9 figures

We report on the second observation of the radio-quiet active galactic nucleus (AGN) MCG-05-23-16 performed with the Imaging X-ray Polarimetry Explorer (IXPE). The observation started on 2022 November 6 for a net observing time of 640 ks, and was partly simultaneous with NuSTAR (86 ks). After combining these data with those obtained in the first IXPE pointing on May 2022 (simultaneous with XMM-Newton and NuSTAR) we find a 2-8 keV polarization degree $\Pi$ = 1.6 $\pm$ 0.7 (at 68 per cent confidence level), which corresponds to an upper limit $\Pi$ = 3.2 per cent (at 99 per cent confidence level). We then compare the polarization results with Monte Carlo simulations obtained with the MONK code, with which different coronal geometries have been explored (spherical lamppost, conical, slab and wedge). Furthermore, the allowed range of inclination angles is found for each geometry. If the best fit inclination value from a spectroscopic analysis is considered, a cone-shaped corona along the disc axis is disfavoured., Comment: 9 pages, 8 figures, 3 table. Submitted to MNRAS

X Persei is a persistent low-luminosity X-ray pulsar of period of $\sim$835 s in a Be binary system. The field strength at the neutron star surface is not known precisely, but indirect signs indicate a magnetic field above $10^{13}$ G, which makes the object one of the most magnetized known X-ray pulsars. Here we present the results of observations X Persei performed with the Imaging X-ray Polarimetry Explorer (IXPE). The X-ray polarization signal was found to be strongly dependent on the spin phase of the pulsar. The energy-averaged polarization degree in 3-8 keV band varied from several to $\sim$20 per cent over the pulse with a positive correlation with the pulsed X-ray flux. The polarization angle shows significant variation and makes two complete revolutions during the pulse period resulting in nearly nil pulse-phase averaged polarization. Applying the rotating vector model to the IXPE data we obtain the estimates for the rotation axis inclination and its position angle on the sky as well as for the magnetic obliquity. The derived inclination is close to the orbital inclination reported earlier for X Persei. The polarimetric data imply a large angle between the rotation and magnetic dipole axes, which is similar to the result reported recently for the X-ray pulsar GRO J1008$-$57. After eliminating the effect of polarization angle rotation over the pulsar phase using the best-fitting rotating vector model, the strong dependence of the polarization degree with energy was discovered with its value increasing from 0% at $\sim$2 keV to 30% at 8 keV., Comment: 11 pages, 7 figures, submitted to MNRAS

We present an X-ray spectro-polarimetric analysis of the bright Seyfert galaxy NGC4151. The source has been observed with the Imaging X-ray Polarimetry Explorer (IXPE) for 700 ks, complemented with simultaneous XMM-Newton (50 ks) and NuSTAR (100 ks) pointings. A polarization degree ${\Pi} = 4.9 {\pm} 1.1 \%$ and angle ${\Psi}= 86{\deg} {\pm} 7{\deg}$ east of north ($68\%$ confidence level) are measured in the 2-8 keV energy range. The spectro-polarimetric analysis shows that the polarization could be entirely due to reflection. Given the low reflection flux in the IXPE band, this requires however a reflection with a very large ($> 38 \%$) polarization degree. Assuming more reasonable values, a polarization degree of the hot corona ranging from ${\sim}4$ to ${\sim}8\%$ is found. The observed polarization degree excludes a spherical lamppost geometry for the corona, suggesting instead a slab-like geometry, possibly a wedge, as determined via Monte Carlo simulations. This is further confirmed by the X-ray polarization angle, which coincides with the direction of the extended radio emission in this source, supposed to match the disc axis. NGC4151 is the first AGN with an X-ray polarization measure for the corona, illustrating the capabilities of X-ray polarimetry and IXPE in unveiling its geometry., Comment: 9 pages, 4 figures

We report spectro-polarimetric results of an observational campaign of the bright neutron star low-mass X-ray binary Cyg X-2 simultaneously observed by IXPE, NICER and INTEGRAL. Consistently with previous results, the broad-band spectrum is characterized by a lower-energy component, attributed to the accretion disc with $kT_{\rm in} \approx$ 1 keV, plus unsaturated Comptonization in thermal plasma with temperature $kT_{\rm e} = 3$ keV and optical depth $\tau \approx 4$, assuming a slab geometry. We measure the polarization degree in the 2-8 keV band $P=1.8 \pm 0.3$ per cent and polarization angle $\phi = 140^{\circ} \pm 4^{\circ}$, consistent with the previous X-ray polarimetric measurements by OSO-8 as well as with the direction of the radio jet which was earlier observed from the source. While polarization of the disc spectral component is poorly constrained with the IXPE data, the Comptonized emission has a polarization degree $P =4.0 \pm 0.7$ per cent and a polarization angle aligned with the radio jet. Our results strongly favour a spreading layer at the neutron star surface as the main source of the polarization signal. However, we cannot exclude a significant contribution from reflection off the accretion disc, as indicated by the presence of the iron fluorescence line., Comment: 10 pages, 7 figures, accepted for publication in MNRAS

We report on the Imaging X-ray Polarimetry Explorer (IXPE) observation of the closest and X-ray brightest Compton-thick active galactic nucleus (AGN), the Circinus galaxy. We find the source to be significantly polarized in the 2--6 keV band. From previous studies, the X-ray spectrum is known to be dominated by reflection components, both neutral (torus) and ionized (ionization cones). Our analysis indicates that the polarization degree is $28 \pm 7$ per cent (at 68 per cent confidence level) for the neutral reflector, with a polarization angle of $18{\deg} \pm 5{\deg}$, roughly perpendicular to the radio jet. The polarization of the ionized reflection is unconstrained. A comparison with Monte Carlo simulations of the polarization expected from the torus shows that the neutral reflector is consistent with being an equatorial torus with a half-opening angle of 45{\deg}-55{\deg}. This is the first X-ray polarization detection in a Seyfert galaxy, demonstrating the power of X-ray polarimetry in probing the geometry of the circumnuclear regions of AGNs, and confirming the basic predictions of standard Unification Models., Comment: 9 pages, accepted for publication in MNRAS, authors list updated

We report on the first observation of a radio-quiet Active Galactic Nucleus (AGN) using polarized X-rays: the Seyfert 1.9 galaxy MCG-05-23-16. This source was pointed with the Imaging X-ray Polarimetry Explorer (IXPE) starting on May 14, 2022 for a net observing time of 486 ks, simultaneously with XMM-Newton (58 ks) and NuSTAR (83 ks). A polarization degree smaller than $\Pi<4.7\%$ (at the 99% c.l.) is derived in the 2-8 keV energy range, where emission is dominated by the primary component ascribed to the hot corona. The broad-band spectrum, inferred from a simultaneous fit to the IXPE, NuSTAR, and XMM-Newton data, is well reproduced by a power law with photon index $\Gamma=1.85\pm0.01$ and a high-energy cutoff $E_{\rm C}=120\pm15$ keV. A comparison with Monte Carlo simulations shows that a lamp-post and a conical geometry of the corona are consistent with the observed upper limit, a slab geometry is allowed only if the inclination angle of the system is less than 50$^{\circ}$., Comment: 7 pages, 3 figures, 1 table. Submitted to MNRAS Letters

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 ~ 14 years. If a non-magnetic carbon atmosphere is assumed for this youngest known CCO, then the temperature change is constrained to be $\dot{T}=-2900\pm 600$ K yr$^{-1}$ or $\dot{T}=-4500\pm 800 $ K yr$^{-1}$ ($1\sigma$ uncertainties) for constant or varying absorbing hydrogen column density. These values correspond to cooling rates of $-1.5 \pm 0.3$% per 10yr and $-2.3 \pm 0.4$% per 10yr, 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., Comment: Accepted for publication in ApJ, 6 pages, 2 figures, 2 tables

We report on a NuSTAR observation of the young, energetic pulsar PSR J1617-5055. Parkes Observatory 3 GHz radio observations of the pulsar (taken about 7 years before the NuSTAR observations) are also reported here. NuSTAR detected pulsations at a frequency of $f\approx14.4$ Hz ($P\approx69.44$ ms) and, in addition, the observation was long enough to measure the source's frequency derivative, $\dot{f}\approx-2.8\times10^{-11}$ Hz s$^{-1}$. We find that the pulsar shows one peak per period at both hard X-ray and radio wavelengths, but that the hard X-ray pulse is broader (having a duty cycle of $\sim 0.7$), than the radio pulse (having a duty cycle of $\sim 0.08$). Additionally, the radio pulse is strongly linearly polarized. J1617's phase-integrated hard X-ray spectrum is well fit by an absorbed power-law model, with a photon index $\Gamma=1.59\pm 0.02$. The hard X-ray pulsations are well described by three Fourier harmonics, and have a pulsed fraction that increases with energy. We also fit the phase-resolved NuSTAR spectra with an absorbed power-law model in five phase bins and find that the photon index varies with phase from $\Gamma = 1.52\pm 0.03$ at phases around the flux maximum to $\Gamma=1.79\pm 0.06$ around the flux minimum. Lastly, we compare our results with other pulsars whose magnetospheric emission is detected at hard X-ray energies and find that, similar to previous studies, J1617's hard X-ray properties are more similar to the MeV pulsars than the GeV pulsars., Comment: Accepted for publication in ApJ

We report on the results of Chandra X-ray observations of the $\gamma$-ray radio-quiet pulsar J1932+1916. We confirm the previous detection of the pulsar counterpart and its pulsar wind nebula in X-rays from low spatial resolution data obtained by Suzaku and Swift. The Chandra data with much better spatial resolution resolved the fine structure of the nebula in the pulsar vicinity, which can be interpreted as jets bent by the ram pressure. The size of this compact part is about 30'', and it is surrounded by a weak asymmetric diffusive emission extended up to $\approx$ 2.8'. The X-ray spectra of the pulsar, the compact and extended parts of the nebula can be described by the power-law models with photon indexes of 0.44, 2.34 and 2.14, respectively. The actual pulsar's X-ray flux is several times weaker than it was obtained with Suzaku and Swift where it was dominated by the unresolved nebula. We discuss possible associations of J1932+1916 with the nearby supernova remnant G54.4-0.3.

We consider acceleration of leptons up to GeV-TeV energies in the bow shock wind nebula of PSR J0437-4715 and their subsequent diffusion through the interstellar magnetic fields. The leptons accelerated at the pulsar wind termination surface are injected into re-acceleration in colliding shock flows. Modelled spectra of synchrotron emission from the accelerated electrons and positrons are consistent with the far-ultraviolet and X-ray observations of the nebula carried out with the Hubble Space Telescope and Chandra X-ray Observatory. These observations are employed to constrain the absolute fluxes of relativistic leptons, which are escaping from the nebula and eventually reaching the Solar System after energy-dependent diffusion through the local interstellar medium accompanied by synchrotron and Compton losses. It is shown that accelerated leptons from the nebula of PSR J0437-4715 can be responsible both for the enhancement of the positron fraction above a few GeV detected by PAMELA and AMS-02 spectrometers and for the TeV range lepton fluxes observed with H.E.S.S., VERITAS, Fermi, CALET, and DAMPE., Comment: 16 pages, 3 figures, Astrophysical Journal Letters, in print

We report non-detections of the $\sim 3\times 10^8$ yr old, slow, isolated, rotation-powered pulsar PSR J2144$-$3933 in observations with the Hubble Space Telescope in one optical band (F475X) and two far-ultraviolet bands (F125LP and F140LP), yielding upper bounds $F_{\rm F475X}< 22.7$ nJy, $F_{\rm F125LP}< 5.9$ nJy, $F_{\rm F140LP}< 19.5$ nJy, at the pivot wavelengths 4940 \AA, 1438 \AA\ and 1528 \AA, respectively. Assuming a blackbody spectrum, we deduce a conservative upper bound on the surface (unredshifted) temperature of the pulsar of $T<42,000$ K. This makes PSR~J2144--3933 the coldest known neutron star, allowing us to study thermal evolution models of old neutron stars. This temperature is consistent with models with either direct or modified Urca reactions including rotochemical heating, and, considering frictional heating from the motion of neutron vortex lines, it puts an upper bound on the excess angular momentum in the neutron superfluid, $J<10^{44}\,\mathrm{erg\,s}$., Comment: 5 Figure. 3 Tables. Accepted for publication in ApJ

Following up on a faint detection of a near-infrared (NIR) source at the position of the X-ray thermal isolated neutron star RX J0806.4-4123, we present new Hubble Space Telescope observations in the H-band. The NIR source is unambiguously detected with a Vega magnitude of 23.7 +/- 0.2 (flux density of 0.40 +/- 0.06 microJy at lambda =1.54 microm. The source position is coincident with the neutron star position, and the implied NIR flux is strongly in excess of what one would expect from an extrapolation of the optical-UV spectrum of RX J0806.4-4123. The NIR source is extended with a size of at least 0.8arcsec and shows some asymmetry. The conservative upper limit on the flux contribution of a point source is 50%. Emission from gas and dust in the ambient diffuse interstellar medium can be excluded as cause for the extended emission. The source parameters are consistent with an interpretation as either the first NIR-only detected pulsar wind nebula or the first resolved disk around an isolated neutron star., Comment: 17 pages, 8 figures

The Central Compact Object (CCO) in the Cassiopeia A supernova remnant is most likely a very young ($\approx 300$ yr) neutron star. If a previously reported decrease of its surface temperature by 4% in 10 years could be confirmed, it would have profound theoretical implications for neutron star physics. However, the temperature decrease was inferred from Chandra ACIS data affected by instrumental effects which could cause time-dependent spectral distortions. Employing a different instrument setup which minimizes spectral distortions, our 2006 and 2012 Chandra spectra of the CCO did not show a statistically significant temperature decrease. Here, we present additional observations from 2015 taken in the same instrument mode. During the time span of 8.5 years, we detect no significant temperature decrease, using either carbon or hydrogen atmosphere models in the X-ray spectral fits. Our conservative $3\sigma$ upper limits correspond to $<3.3$% and $<2.4$% temperature decrease in 10 years for carbon atmosphere model fits with varying or constant values of the absorbing hydrogen column density, respectively. The recently revised model for the ACIS filter contaminant has a strong effect on the fit results, reducing the significance of the previously reported temperature and flux changes. We expect that a further improved contaminant model and longer time coverage can significantly lower the upper limits in the future., Comment: 9 pages, 9 figures, 3 tables; accepted for publication in ApJ

We report on a Hubble Space Telescope detection of the nearby, old pulsar B0950+08 ($d\simeq 262$ pc, spin-down age 17.5 Myr) in two far-ultraviolet (FUV) bands. We measured the mean flux densities $\bar{f}_\nu = 109\pm 6$ nJy and $83\pm 14$ nJy in the F125LP and F140LP filters (pivot wavelengths 1438 and 1528 \AA). Using the FUV data together with previously obtained optical-UV data, we conclude that the optical-FUV spectrum consists of two components -- a nonthermal (presumably magnetospheric) power-law spectrum ($f_\nu\propto \nu^\alpha$) with slope $\alpha\sim -1.2$ and a thermal spectrum emitted from the bulk of the neutron star surface with a temperature in the range of $(1-3)\times 10^5$ K, depending on interstellar extinction and neutron star radius. These temperatures are much higher than predicted by neutron star cooling models for such an old pulsar, which means that some heating mechanisms operate in neutron stars. A plausible mechanism responsible for the high temperature of PSR B0950+08 is the interaction of vortex lines of the faster rotating neutron superfluid with the slower rotating normal matter in the inner neutron star crust (vortex creep heating)., Comment: 9 pages, 5 figures. Accepted by the Astrophysical Journal

We present a detailed analysis of the morphological properties of the Crab Nebula across the electromagnetic spectrum based on new and previous high-quality data from radio to X-rays. In the radio range we obtained an image of the entire nebula at 3 GHz with subarcsecond angular resolution using the VLA (NRAO) and an image at 100 GHz of the central region using the ALMA array. Simultaneously with the VLA observations we performed HST WFPC3 near infrared and Chandra X-ray observations of the central region of the nebula. In addition we produced a new UV image of the Crab nebula at 291 nm by co-adding 75 individual exposures of the Optical-UV Monitor on board XMM-Newton. The high-angular resolution and high-dynamic range radio image at 3 GHz allowed us to improve the detection and characterization of peculiar morphological features including arches with foot brightening and intercrossed loop-like structures, likely originating in plasma confined to magnetic field lines. Based on the new radio image, we carried out a detailed multiwavelength correlation. In the central area, the comparison of the almost simultaneous images confirms that the wisps in the three spectral ranges do not generally coincide in location, the radio emission being the most discordant, which is suggestive of the existence of two different synchrotron components. The X-ray pulsar jet does not have a radio counterpart. Instead, another jet-like feature is seen in radio, though with different curvature and starting point., Comment: 29 pages, 10 figures, accepted for publication in ApJ

We observed a nearby millisecond pulsar J2124-3358 with the Hubble Space Telescope in broad far-UV (FUV) and optical filters. The pulsar is detected in both bands with fluxes F(1250-2000 A)= (2.5+/-0.3)x10^-16 erg/s/cm^2 and F(3800-6000 A)=(6.4+/-0.4)x10^-17 erg/s/cm^2, which correspond to luminosities of ~5.8x10^27 and 1.4x10^27 erg/s, for d=410 pc and E(B-V)=0.03. The optical-FUV spectrum can be described by a power-law model, f_nu~nu^alpha, with slope alpha=0.18-0.48 for a conservative range of color excess, E(B-V)=0.01-0.08. Since a spectral flux rising with frequency is unusual for pulsar magnetospheric emission in this frequency range, it is possible that the spectrum is predominantly magnetospheric (power law with alpha<0) in the optical while it is dominated by thermal emission from the neutron star surface in the FUV. For a neutron star radius of 12 km, the surface temperature would be between 0.5x10^5 and 2.1x10^5 K, for alpha ranging from -1 to 0, E(B-V)=0.01-0.08, and d=340-500 pc. In addition to the pulsar, the FUV images reveal extended emission spatially coincident with the known Halpha bow shock, making PSR J2124-3358 the second pulsar (after PSR J0437-4715) with a bow shock detected in FUV., Comment: Accepted to ApJ

We report on six new Chandra observations of the Geminga pulsar wind nebula (PWN). The PWN consists of three distinct elongated structures - two $\approx 0.2 d_{250}$ pc long lateral tails and a segmented axial tail of $\approx 0.05 d_{250}$ pc length, where $d_{250}=d/(250 {\rm pc})$. The photon indices of the power law spectra of the lateral tails, $\Gamma \approx 1$, are significantly harder than those of the pulsar ($\Gamma \approx 1.5$) and the axial tail ($\Gamma \approx 1.6$). There is no significant diffuse X-ray emission between the lateral tails -- the ratio of the X-ray surface brightness between the south tail and this sky area is at least 12. The lateral tails apparently connect directly to the pulsar and show indication of moving footpoints. The axial tail comprises time-variable emission blobs. However, there is no evidence for constant or decelerated outward motion of these blobs. Different physical models are consistent with the observed morphology and spectra of the Geminga PWN. In one scenario, the lateral tails could represent an azimuthally asymmetric shell whose hard emission is caused by the Fermi acceleration mechanism of colliding winds. In another scenario, the lateral tails could be luminous, bent polar outflows, while the blobs in the axial tail could represent a crushed torus. In a resemblance to planetary magnetotails, the blobs of the axial tail might also represent short-lived plasmoids which are formed by magnetic field reconnection in the relativistic plasma of the pulsar wind tail., Comment: 21 pages, 10 figures, 5 tables, accepted by ApJ

We report on the analysis of two deep XMM-Newton observations of the magnetar Swift J1834.9-0846 and its surrounding extended emission taken in March 2014 and October 2014, 2.5 and 3.1 years after the source went into outburst. The magnetar is only weakly detected in the first observation with an absorption corrected flux $F_{\rm 0.5-10 keV}\approx4\times10^{-14}$ erg s$^{-1}$ cm$^{-2}$, and a $3\sigma$ upper limit during the second observation of about $3\times10^{-14}$ erg s$^{-1}$ cm$^{-2}$. This flux level is more than 3 orders of magnitude lower than the flux measured at the outburst onset on September 2011. The extended emission, centered at the magnetar position and elongated towards the south-west, is clearly seen in both observations; it is best fit by a highly absorbed power-law (PL), with a hydrogen column density of $N_{\rm H}=8.0\times10^{22}$ cm$^{-2}$ and PL photon index $\Gamma=2.2\pm0.2$. Its flux is constant between the two observations at $F_{\rm 0.5-10 keV}=1.3\times10^{-12}$ erg s$^{-1}$ cm$^{-2}$. We find no statistically significant changes in the spectral shape or the flux of this extended emission over a period of 9 years from 2005 to 2014. These new results strongly support the extended emission nature as a wind nebula and firmly establish Swift J1834.9-0846 as the first magnetar to show a surrounding wind nebula. Further, our results imply that such nebulae are no longer exclusive to rotation-powered pulsars and narrow the gap between these two sub-populations of isolated neutron stars. The size and spectrum of the nebula are compatible with those of pulsar-wind nebulae but its radiative efficiency $\eta_{\rm X}=L_{\rm X}/\dot{E}\approx0.1$ is markedly high, possibly pointing to an additional wind component in Swift J1834.9-0846., Comment: 12 Pages, 7 Figures, accpeted for publication in ApJ

PSR B0656+14 is a middle-aged pulsar with a characteristic age $\tau_c=110$ kyr and spin-down power $\dot{E}= 3.8\times 10^{34}$ erg s$^{-1}$. Using Chandra data, we searched for a pulsar wind nebula (PWN) and found evidence of extended emission in a 3.5-15 arcsec annulus around the pulsar, with a luminosity $L_{\rm 0.5-8\,keV}^{\rm ext} \sim 8\times 10^{28}$ erg s$^{-1}$ (at the distance of 288 pc), which is a fraction of $\sim 0.05$ of the non-thermal pulsar luminosity. If the extended emission is mostly due to a PWN, its X-ray efficiency, $\eta_{\rm pwn} = L_{\rm 0.5-8\,keV}^{\rm ext}/\dot{E} \sim 2\times 10^{-6}$, is lower than those of most other known PWNe but similar to that of the middle-aged Geminga pulsar. The small radial extent and nearly round shape of the putative PWN can be explained if the pulsar is receding (or approaching) in the direction close to the line of sight. The very soft spectrum of the extended emission ($\Gamma\sim 8$), much softer than those of typical PWNe, could be explained by a contribution from a faint dust scattering halo, which may dominate in the outer part of the extended emission., Comment: 13 pages, 9 figures, 4 tables; Accepted by ApJ

The nearby, middle-aged PSR B1055-52 has many properties in common with the Geminga pulsar. Motivated by the Geminga's enigmatic and prominent pulsar wind nebula (PWN), we searched for extended emission around PSR B1055-52 with Chandra ACIS. For an energy range 0.3-1 keV, we found a 4 sigma flux enhancement in a 4.9-20 arcsec annulus around the pulsar. There is a slight asymmetry in the emission close, 1.5-4 arcsec, to the pulsar. The excess emission has a luminosity of about 10^{29} erg s^{-1} in an energy range 0.3-8 keV for a distance of 350 pc. Overall, the faint extended emission around PSR B1055-52 is consistent with a PWN of an aligned rotator moving away from us along the line of sight with supersonic velocity, but a contribution from a dust scattering halo cannot be excluded. Comparing the properties of other nearby, middle-aged pulsars, we suggest that the geometry -- the orientations of rotation axis, magnetic field axis, and the sight-line -- is the deciding factor for a pulsar to show a prominent PWN. For PSR B1055-52, we also report on a flux decrease of at least 30% between the 2000 XMM-Newton and our 2012 Chandra observation. We tentatively attribute this flux decrease to a cross-calibration problem, but further investigations of the pulsar are required to exclude actual intrinsic flux changes., Comment: 12 pages, 6 figures, 4 tables, accepted for publication in ApJ

Supernova fallback disks around neutron stars have been discussed to influence the evolution of the diverse neutron star populations. Slowly rotating neutron stars are most promising to find such disks. Searching for the cold and warm debris of old fallback disks, we carried out Herschel PACS (70 $\mu$m, 160 $\mu$m) and Spitzer IRAC (3.6 $\mu$m, 4.5 $\mu$m) observations of eight slowly rotating ($P\approx 3 - 11$ s) nearby ($<1$ kpc) isolated neutron stars. Herschel detected 160 $\mu$m emission ($>5\sigma$) at locations consistent with the positions of the neutron stars RX J0806.4-4123 and RX J2143.0+0654. No other significant infrared emission was detected from the eight neutron stars. We estimate probabilities of 63%, 33% and 3% that, respectively, none, one, or both Herschel PACS 160 $\mu$m detections are unrelated excess sources due to background source confusion or an interstellar cirrus. If the 160 $\mu$m emission is indeed related to cold (10 K to 22 K) dust around the neutron stars, this dust is absorbing and re-emitting $\sim 10$% to $\sim 20$% of the neutron stars' X-rays. Such high efficiencies would be at least three orders of magnitude larger than the efficiencies of debris disks around nondegenerate stars. While thin dusty disks around the neutron stars can be excluded as counterparts of the 160 $\mu$m emission, dusty asteroid belts constitute a viable option., Comment: 22 pages, 26 Figures, 5 tables; accepted for publication in ApJS

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 $P\approx 48.6$ ms, and its derivative $\dot{P}\approx 8.6\times 10^{-14}$, twice larger than the previously reported values. The pulsar exhibits two very narrow (FWHM $\sim 1.2$ ms) X-ray pulses each rotation, separated by $\approx 0.48$ of the period, with a pulsed fraction of $\approx 0.8$. Using the correct values of $P$ and $\dot{P}$, we calculate the pulsar's spin-down power $\dot{E}=3.0 \times 10^{37}$ erg s$^{-1}$ and magnetic field $B=2.1\times 10^{12}$ G. The pulsar spectrum is well modeled with a hard power-law (PL) model (photon index $\Gamma = 0.9\pm0.1 $, hydrogen column density $n_H = (2.3\pm0.3) \times 10^{22}\,{\rm cm}^{-2}$). We detect a weak off-pulse emission which can be modeled with a softer PL ($\Gamma \approx 1.7\pm0.7$), 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, $\eta_{\rm PSR}= L_{\rm PSR}/\dot{E} = 2 \times 10^{-4} (D/10\,{\rm kpc})^2$, is similar to those of other pulsars. The $XMM-Newton$ observation did not detect extended emission around the pulsar. Our re-analysis of $Chandra$ X-ray observatory archival data shows a hard, $\Gamma \approx 0.9 \pm 0.5$, spectrum and a low efficiency, $\eta_{\rm PWN}\sim 2\times 10^{-5} (D/10\,{\rm kpc})^2$, for the compact pulsar wind nebula, unresolved in the $XMM-Newton$ images., Comment: 10 pages, 13 figures; accepted by ApJ

The accuracy of measuring the basic parameters of neutron stars is limited in particular by uncertainties in chemical composition of their atmospheres. For example, atmospheres of thermally - emitting neutron stars in supernova remnants might have exotic chemical compositions, and for one of them, the neutron star in CasA, a pure carbon atmosphere has recently been suggested by Ho & Heinke (2009). To test such a composition for other similar sources, a publicly available detailed grid of carbon model atmosphere spectra is needed. We have computed such a grid using the standard LTE approximation and assuming that the magnetic field does not exceed 10^8 G. The opacities and pressure ionization effects are calculated using the Opacity Project approach. We describe the properties of our models and investigate the impact of the adopted assumptions and approximations on the emergent spectra., Comment: 8 pages, 11 figures; accepted for publication in ApJS

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 years 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, R_Em, the effective temperature remains essentially the same. If R_Em is held constant, the best-fit temperature change is negative, but its statistical significance ranges from 0.8sigma to 2.5sigma, 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 0.8sigma to 3.1sigma, for the carbon atmospheres with constant R_Em. 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% (1.9-2.9sigma significance) for the absorbed flux in the energy range 0.6-6keV 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., Comment: 20 pages, 19 figures, 3 tables; accepted for publication in ApJ

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 Gamma~3 or by a combination of thermal and non-thermal components, e.g., a power-law component with fixed photon index Gamma~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. The non-thermal X-ray efficiency in the single power-law model is eta^PL (1-10 keV) = L^PL (1-10 keV) / Edot ~ 0.003, 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, eta^PL (1-10 keV) ~ eta^bb ~ 0.006. We detected X-ray pulsations at the radio period of P=0.808s with significance of 7sigma. The pulse shape in the folded X-ray lightcurve (0.15-2 keV) is asymmetric, with statistically significant contributions from up to 5 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 sinusodial shape. The radio pulse peak leads the 0.15-2 keV X-ray pulse peak by delta phi = 0.06 +/- 0.03., Comment: 11 pages, 11 figures, accepted by ApJ; Figures 2 and 3 are in addition to the ApJ paper version

We report on radio and X-ray observations of PSR 1832+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 spindown 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 time-scale, PSR 1832+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 spindown rates is $1.77 \pm 0.03$. 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., Comment: 16 pages, 3 figures, 1 table, submitted to ApJ

Recent X-ray observations of the supernova remnant IC443 interacting with molecular clouds have shown the presence of a new population of hard X-ray sources related to the remnant itself, which has been interpreted in terms of fast ejecta fragment propagating inside the dense environment. Prompted by these studies, we have obtained a deep {\sl XMM-Newton} observation of the supernova remnant (SNR) Kes 69, which also shows signs of shock-cloud interaction. We report on the detection of 18 hard X-ray sources in the field of Kes 69, a significant excess of the expected galactic source population in the field, 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 index <2 plus line emission associated to K-shell transitions. We discuss the two most promising scenarios for the interpretation of the sources, namely fast ejecta fragments (as in IC443) and cataclysmic variables. While most of the observational evidences are consistent with the former interpretation, we cannot rule out the latter., Comment: 9 pages, 5 figures, A&A in press