X-ray Spectrum and Pulsations of the Vela Pulsar

We report the results of the spectral and timing analysis of observations of the Vela pulsar with the Chandra X-ray Observatory. The spectrum shows no statistically significant spectral lines in the observed 0.25--8.0 keV band. It consists of two distinct continuum components. The softer component can be modeled as either a magnetic hydrogen atmosphere spectrum with kT = 59 +- 3 eV, R = 15.5 +- 1.5 km, or a standard blackbody with kT = 129 +- 4 eV, R = 2.1 +- 0.2 km (the radii are for a distance of 250 pc). The harder component, modeled as a power-law spectrum, gives photon indices depending on the model adopted for the soft component: gamma = 1.5 +- 0.3 for the magnetic atmosphere soft component, and gamma = 2.7 +- 0.4 for the blackbody soft component. Timing analysis shows three peaks in the pulse profile, separated by about 0.3 in phase. Energy-resolved timing provides evidence for pulse profile variation with energy. The higher energy (E > 1.8 keV) profile shows significantly higher pulsed fraction.Comment: 4 pages, 2 figures, To appear in "Neutron Stars in Supernova Remnants" (ASP Conference Proceedings), eds P. O. Slane and B. M. Gaensler Corrected TYPO

Possible optical detection of a fast, nearby radio pulsar PSR B1133+16

Aims: We performed deep optical observations of the field of an old, fast-moving radio pulsar PSR B1133+16 in an attempt to detect its optical counterpart and a bow shock nebula. Methods: The observations were carried out using the direct imaging mode of FORS1 at the ESO VLT/UT1 telescope in the B, R, and H_alpha bands. We also used archival images of the same field obtained with the VLT in the B band and with the Chandra/ACIS in X-rays. Results: In the B band we detected a faint (B=28.1+/-0.3) source that may be the optical counterpart of PSR B1133+16, as it is positionally consistent with the radio pulsar and with the X-ray counterpart candidate published earlier. Its upper limit in the R band implies a color index B-R <0.5, which is compatible with the index values for most pulsars identified in the optical range. The derived optical luminosity and its ratio to the X-ray luminosity of the candidate are consistent with expected values derived from a sample of pulsars detected in both spectral domains. No Balmer bow shock was detected, implying a low density of ambient matter around the pulsar. However, in the X-ray and H_alpha images we found the signature of a trail extending ~4"-5" behind the pulsar and coinciding with the direction of its proper motion. If confirmed by deeper studies, this is the first time such a trail has been seen in the optical and X-ray wavelengths. Conclusions: Further observations at later epochs are necessary to confirm the identification of the pulsar by the candidate's proper motion measurements.Comment: 11 pages, 6 figures, A&A, accepte

New constraints on the cooling of the central compact object in CAS a

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, 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 0.8σ 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 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% (1.9σ-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. © 2013. The American Astronomical Society. All rights reserved

The Compact Central Object in the Supernova Remnant G266.2-1.2

We observed the compact central object CXOU J085201.4--461753 in the supernova remnant G266.2--1.2 (RX J0852.0--4622) with the Chandra ACIS detector in timing mode. The spectrum of this object can be described by a blackbody model with the temperature kT=404 eV and radius of the emitting region R=0.28 km, at a distance of 1 kpc. Power-law and thermal plasma models do not fit the source spectrum. The spectrum shows a marginally significant feature at 1.68 keV. Search for periodicity yields two candidate periods, about 301 ms and 33 ms, both significant at a 2.1 sigma level; the corresponding pulsed fractions are 13% and 9%, respectively. We find no evidence for long-term variability of the source flux, nor do we find extended emission around the central object. We suggest that CXOU J085201.4--461753 is similar to CXOU J232327.9+584842, the central source of the supernova remnant Cas A. It could be either a neutron star with a low or regular magnetic field, slowly accreting from a fossil disk, or, more likely, an isolated neutron star with a superstrong magnetic field. In either case, a conservative upper limit on surface temperature of a 10 km radius neutron star is about 90 eV, which suggests accelerated cooling for a reasonable age of a few thousand years.Comment: Accepted to ApJ, 13 pages, 1 figur

Multi-wavelength observations of 2HWC J1928+177: dark accelerator or new TeV gamma-ray binary?

2HWC J1928+177 is a Galactic TeV gamma-ray source detected by the High Altitude Water Cherenkov (HAWC) Observatory up to ~ 56 TeV. The HAWC source, later confirmed by H.E.S.S., still remains unidentified as a dark accelerator since there is no apparent supernova remnant or pulsar wind nebula detected in the lower energy bands. The radio pulsar PSR J1928+1746, coinciding with the HAWC source position, has no X-ray counterpart. Our SED modeling shows that inverse Compton scattering in the putative pulsar wind nebula can account for the TeV emission only if the unseen nebula is extended beyond r ~ 4 [arcmin]. Alternatively, TeV gamma rays may be produced by hadronic interactions between relativistic protons from an undetected supernova remnant associated with the radio pulsar and a nearby molecular cloud G52.9+0.1. NuSTAR and Chandra observations detected a variable X-ray point source within the HAWC error circle, potentially associated with a bright IR source. The X-ray spectra can be fitted with an absorbed power-law model with $N_{\rm H} = (9\pm3)\times10^{22}$ cm$^{-2}$ and $\Gamma_X = 1.6\pm0.3$ and exhibit long-term X-ray flux variability over the last decade. If the X-ray source, possibly associated with the IR source (likely an O star), is the counterpart of the HAWC source, it may be a new TeV gamma-ray binary powered by collisions between the pulsar wind and stellar wind. Follow-up X-ray observations are warranted to search for diffuse X-ray emission and determine the nature of the HAWC source.Comment: accepted to ApJ, 8 pages, 7 figure

On the evolution of the Gamma- and X-ray luminosities of Pulsar Wind Nebulae

Pulsar wind nebulae are a prominent class of very high energy (E > 0.1 TeV) Galactic sources. Their Gamma-ray spectra are interpreted as due to inverse Compton scattering of ultrarelativistic electrons on the ambient photons, whereas the X-ray spectra are due to synchrotron emission. We investigate the relation between the Gamma- and-X-ray emission and the pulsars' spin-down luminosity and characteristic age. We find that the distance-independent Gamma- to X-ray flux ratio of the nebulae is inversely proportional to the spin-down luminosity, (\propto \dot{E}^-1.9), while it appears proportional to the characteristic age, (\propto tau_c^2.2), of the parent pulsar. We interpret these results as due to the evolution of the electron energy distribution and the nebular dynamics, supporting the idea of so-called relic pulsar wind nebulae. These empirical relations provide a new tool to classify unidentified diffuse Gamma-ray sources and to estimate the spin-down luminosity and characteristic age of rotation powered pulsars with no detected pulsation from the X- and Gamma-ray properties of the associated pulsar wind nebulae. We apply these relations to predict the spin-down luminosity and characteristic age of four (so far unpulsing) candidate pulsars associated to wind nebulae.Comment: Accepted for publication in ApJ (6 pages, 2 figures

Protoneutron star dynamos and pulsar magnetism

We have investigated the turbulent mean-field dynamo action in protoneutron stars that are subject to convective and neutron finger instabilities during the early evolutionary phase. While the first one develops mostly in the inner regions of the star, the second one is favored in the outer regions, where the Rossby number is much smaller and a mean-field dynamo action is more efficient. By solving the mean-field induction equation we have computed the critical spin period below which no dynamo action is possible and found it to be $\sim 1$ s for a wide range of stellar models and for both axisymmetric and non-axisymmetric magnetic fields. Because this critical period is substantially longer than the characteristic spin period of very young pulsars, we expect that a mean-field dynamo will be effective for most protoneutron stars. The saturation dipole field estimated by making use of the model of ``global'' quenching fits well the pulsar magnetic fields inferred from the spin-down data. Apart from the large scale magnetic field, our model predicts also a generation of small scale fields which are typically stronger than the poloidal field and can survive during the lifetime of pulsars. Extremely rapidly rotating protoneutron stars ($P \sim 1$ ms) may have the dipole field $\sim (3-6) \times 10^{14}$ G.Comment: 7 pages, 6 figures, to appear on A&

The Variable Jet of the Vela Pulsar

Observations of the Vela pulsar-wind nebula (PWN) with the Chandra X-ray Observatory have revealed a complex, variable PWN structure, including inner and outer arcs, a jet in the direction of the pulsar's proper motion, and a counter-jet in the opposite direction, embedded in diffuse nebular emission. 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. 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 faint, strongly bent, extension of the outer jet. The more extreme bends closer to the pulsar, as well as the apparent side motions of the outer jet, can be associated with kink instabilities of a magnetically confined, pinched jet flow. Another feature found in the summed image is a dim, 2'-long outer counter-jet, which also shows a power-law spectrum with photon index of 1.2-1.5. Southwest of the jet/counter-jet (i.e., approximately perpendicular to the direction of pulsar's proper motion), an extended region of diffuse emission is seen. Relativistic particles responsible for this radiation are apparently supplied by the outer jet.Comment: 13 pages, including 10 figures and 2 tables, accepted for publication in ApJ. The Vela Pulsar Jet movie and full resolution images are avaliable at http://www.astro.psu.edu/users/pavlov/vela_jet_movie.htm