Search

Taking advantage of the high sensitivity of the Chandra Advanced CCD Imaging Spectrometer, we have conducted a snap-shot survey of pulsars previously undetected in X-rays. We detected 12 pulsars and established deep flux limits for 11 pulsars. Using these new results, we revisit the relationship between the X-ray luminosity, L_psr_x, and spin-down power, Edot. We find that the obtained limits further increase the extremely large spread in the non-thermal X-ray efficiencies, eta_psr_x=L_psr_x/Edot, with some of them being now below 1e-5. Such a spread cannot be explained by poorly known distances or by beaming of pulsar radiation. We also find evidence of a break in the dependence of L_psr_x on Edot, such that pulsars become more X-ray efficient at Edot, Comment: Submitted to ApJ

Radiation of most of the known pulsars is powered by the loss of their rotational energy and angular momentum. Thanks to the Chandra and XMM-Newton observatories, X-ray emission has been detected from over 100 rotation-powered pulsars in the last 12 years. I will overview the current results of X-ray observations of rotation-powered pulsars, including the evolution of the X-ray properties, thermal and magnetospheric components of X-ray emission, and connection between the X-ray emission and emission at other wavelengths.

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 2MASS 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 Gamma=2.4 (+0.4/-0.3) and a unrealistically high absorbing column density N(H)= (2.5 (+1.2/-1.3)) * 10^(21) cm^-2, or by a combination of a kT=0.35 (+0.12/-0.07) keV blackbody and a Gamma = 1.4 +/- 0.5 power-law component for N(H)[DM]= 2.6 * 10^(20) cm^-2, estimated from the pulsar dispersion measure. At the parallactic, Lutz-Kelker bias corrected distance of 480 pc, the non-thermal X-ray luminosities in the 0.3-8 keV energy band are either Lx(nonth)= (11.3 +/- 1.7) * 10^(29) erg/s or Lx(nonth)= (5.9 (+4.9/-5.0)) * 10^(29) erg/s, respectively. This corresponds to non-thermal X-ray efficiencies of either eta(nonth)= Lx(nonth) / (dE/dt) ~ 0.005 or 0.003, respectively., Comment: 19 pages, 9 figures, 2 tables, accepted by ApJ

We present X-ray observations of the new transient magnetar Swift J1834.9-0846, discovered with Swift BAT on 2011 August 7. The data were obtained with Swift, RXTE, CXO, and XMM-Newton both before and after the outburst. Timing analysis reveals singe peak pulsations with a period of 2.4823 s and an unusually high pulsed fraction, 85+/-10%. Using the RXTE and CXO data, we estimated the period derivative, dot{P}=8\times 10^{-12} s/s, and confirmed the high magnetic field of the source, B=1.4\times 10^{14} G. The decay of the persistent X-ray flux, spanning 48 days, is consistent with a power law, t^{-0.5}. In the CXO/ACIS image, we find that the highly absorbed point source is surrounded by extended emission, which most likely is a dust scattering halo. Swift J1834.9-0846 is located near the center of the radio supernova remnant W41 and TeV source HESS J1834-087. An association with W41 would imply a source distance of about 4 kpc; however, any relation to the HESS source remains unclear, given the presence of several other candidate counterparts for the latter source in the field. Our search for an IR counterpart of Swift J1834.9-0846 revealed no source down to K_s=19.5 within the 0.6' CXO error circle., Comment: 32 pages, 13 figures, 3 tables; submitted to ApJ

We observed the young pulsar J1357--6429 with the {\it Chandra} and {\it XMM-Newton} observatories. The pulsar spectrum fits well a combination of absorbed power-law model ($\Gamma=1.7\pm0.6$) and blackbody model ($kT=140^{+60}_{-40}$ eV, $R\sim2$ km at the distance of 2.5 kpc). Strong pulsations with pulsed fraction of $42%\pm5%$, apparently associated with the thermal component, were detected in 0.3--1.1 keV. Surprisingly, pulsed fraction at higher energies, 1.1--10 keV, appears to be smaller, $23%\pm4%$. The small emitting area of the thermal component either corresponds to a hotter fraction of the neutron star (NS) surface or indicates inapplicability of the simplistic blackbody description. The X-ray images also reveal a pulsar-wind nebula (PWN) with complex, asymmetric morphology comprised of a brighter, compact PWN surrounded by the fainter, much more extended PWN whose spectral slopes are $\Gamma=1.3\pm0.3$ and $\Gamma=1.7\pm0.2$, respectively. The extended PWN with the observed flux of $\sim7.5\times10^{-13}$ erg s$^{-1}$ cm$^{-2}$ is a factor of 10 more luminous then the compact PWN. The pulsar and its PWN are located close to the center of the extended TeV source HESS J1356--645, which strongly suggests that the VHE emission is powered by electrons injected by the pulsar long ago. The X-ray to TeV flux ratio, $\sim0.1$, is similar to those of other relic PWNe. We found no other viable candidates to power the TeV source. A region of diffuse radio emission, offset from the pulsar toward the center of the TeV source, could be synchrotron emission from the same relic PWN rather than from the supernova remnant.

We report broad-band Hubble Space Telescope imaging of the field of soft gamma-ray repeater SGR 0418+5729 with ACS/WFC and WFC3/IR. Observing in two wide filters F606W and F110W, we find no counterpart within the positional error circle derived from Chandra observations, to limiting magnitudes mF606W>28.6, mF110W>27.4 (Vega system), equivalent to reddening-corrected luminosity limits LF606W, Accepted for publication in ApJ

Context. Optical-ultraviolet (UV) observations of > 100 Myr pulsars are crucial to understand the long-term evolution of neutron stars, including the late stages of the neutron star cooling. The 166 Myr old pulsar PSR J0108-1431 is one of the best targets since it is the oldest non-recycled pulsar with a candidate counterpart, detected with the Very Large Telescope (VLT). Aims. Aim of our observations is to obtain a firm detection of its candidate counterpart, only detected with marginal significance and to measure anew its flux in the U and B bands, for which we obtained only uncertain values. Methods. We observed the PSR J0108-1431 field with the FOcal Reducer/low dispersion Spectrograph (FORS2) at the VLT, exploiting the updated pulsar radio coordinates obtained from recent VLBI observations. Results. Due to non-optimal seeing conditions, we only reached 3σ detection limits of U ∼ 26.5 and B ∼ 27.2, i.e. not incompatible with the fluxes of the candidate counterpart (U = 26.4 ± 0.3; B = 27.9 ± 0.5) that we measured in our previous VLT observations. Conclusions. We can not rule out that the proposed counterpart, detected at the edge of an elliptical galaxy, is real and that we could not detect it just because its flux is close to the detection limit of our new VLT observations. Due to its blue spectrum and proximity to the galaxy, UV observations with the Hubble Space Telescope are more suited to confirm the pulsar identification.

PSR B1259-63 is a middle-aged radio pulsar (P=48 ms, tau=330 kyr, Edot=8.3*10^{35} erg/s) in an eccentric binary (P_orb =3.4 yr, e=0.87) with a high-mass Be companion, SS 2883. We observed the binary near apastron with the Chandra ACIS detector on 2009 May 14 for 28 ks. In addition to the previously studied pointlike source at the pulsar's position, we detected extended emission on the south-southwest side of this source. The pointlike source spectrum can be described by the absorbed power-law model with the hydrogen column density N_H = (2.5+/-0.6)*10^{21} cm^{-2}, photon index Gamma = 1.6+/-0.1, and luminosity L_{0.5-8 keV} = 1.3*10^{33} d_3^2 erg/s, where d_3 is the distance scaled to 3 kpc. This emission likely includes an unresolved part of the pulsar wind nebula (PWN) created by the colliding winds from the pulsar and the Be companion, and a contribution from the pulsar magnetosphere. The extended emission apparently consists of two components. The highly significant compact component looks like a southward extension of the pointlike source image, seen up to about 4 arcsec from the pulsar position. Its spectrum has about the same slope as the pointlike source spectrum, while its luminosity is a factor of 10 lower. We also detected an elongated feature extended ~15 arcsec southwest of the pulsar, but significance of this detection is marginal. We tentatively interpret the resolved compact PWN component as a shocked pulsar wind blown out of the binary by the wind of the Be component, while the elongated component could be a pulsar jet., 13 pages, 7 figures. Submitted to ApJ

Even old (106 to 107 yr) pulsars within a few hundred parsecs of the Sun should give UV and optical fluxes via thermal radiation or radiation from relativistic particles. The surface temperature of a neutron star depends on its mass, radius, magnetic field, and internal composition (existence of pion condensate, superfluidity of nucléons, etc.). If the temperature exceeds ~2x104 K, the thermal radiation can be detected by the Hubble Space Telescope. An analysis of the results will allow one to study the thermal evolution and inner structure of neutron stars in order to obtain additional constraints on pulsar models.

We investigate the properties of the normal modes in a strongly magnetized hydrogen gas for conditions expected in atmospheres of isolated neutron stars. We derive and compute the polarizations and absorption coefficients of the normal modes from the polarizabily tensor using both analytical approximations and numerical calculations. We find that the spectral features and anisotropy associated with the bound-bound and bound-free transitions in the magnetized hydrogen are manifested in the polarization characteristics, which affects substantially the spectral and angular dependences of the absorption coefficients of the normal modes and the transfer of radiation in neutron star atmospheres., Comment: LaTeX uses AAS macros, 30 pages, 8 postscript figures in a separate file

The spectrum of the soft X-ray pulsar Geminga consists of two components, a softer one which can be interpreted as thermal-like radiation from the surface of the neutron star, and a harder one interpreted as radiation from a polar cap heated by relativistic particles. We have fitted the soft spectrum using a detailed magnetized hydrogen atmosphere model. The fitting parameters are the hydrogen column density, the effective temperature T(sub eff), the gravitational redshift z, and the distance to radius ratio, for different values of the magnetic field B. The best fits for this model are obtained when B less than or approximately 1 x 10(exp 12) G and z lies on the upper boundary of the explored range (z = 0.45). The values of T(sub eff) approximately = (2-3) x 10(exp 5) K are a factor of 2-3 times lower than the value of T(sub eff) obtained for blackbody fits with the same z. The lower T(sub eff) increases the compatibility with some proposed schemes for fast neutrino cooling of neutron stars (NSs) by the direct Urca process or by exotic matter, but conventional cooling cannot be excluded. The hydrogen atmosphere fits also imply a smaller distance to Geminga than that inferred from a blackbody fit. An accurate evaluation of the distance would require a better knowledge of the ROSAT Position Sensitive Proportional Counter (PSPC) response to the low-energy region of the incident spectrum. Our modeling of the soft component with a cooler magnetized atmosphere also implies that the hard-component fit requires a characteristic temperature which is higher (by a factor of approximately 2-3) and a surface area which is smaller (by a factor of 10(exp 3), compared to previous blackbody fits.

The photoionization of the hydrogen atom in magnetic fields B about 10 exp 11 - 10 exp 13 G typical of the surface layers of neutron stars is investigated analytically and numerically. We consider the photoionization from various tightly bound and hydrogen-like states of the atom for photons with arbitrary polarizations and wave-vector directions. It is shown that the length form of the interaction matrix elements is more appropriate in the adiabatic approximation than the velocity form, at least in the most important frequency range omega much less than omega(B), where omega(B) is the electron cyclotron frequency. Use of the length form yields nonzero cross sections for photon polarizations perpendicular to the magnetic field at omega less than omega(B); these cross sections are the ones that most strongly affect the properties of the radiation escaping from an optically thick medium, e.g., from the atmosphere of a neutron star. The results of the numerical calculations are fitted by simple analytical formulas.

The radiative line widths of the Landau levels in a superstrong magnetic field are calculated and simple analytic expressions and fits are given for these which are valid over a wide range of the principal quantum number and the magnetic field strength. If QED corrections are not taken into account, all levels but the first are doubly degenerate, corresponding to the two possible spin projections. However, the interaction with the QED vacuum removes this degeneracy, leading to an energy splitting of each level which exceeds the radiative linewidth for low Landau levels if the magnetic field is not too large (B is less than approximately 10 exp 13 G). Estimations are presented of the splitting in various limits as a function of field strength and Landau number. The possibility of observing this splitting in accreting X-ray pulsars and in gamma-ray bursters is discussed.

We present a unified treatment of the continuum radiative transfer in optically thick hot plasmas where incoherent Compton scattering is important, either in the presence or absence of strong magnetic fields, as occurs in X-ray emitting neutron stars. Numerical solutions of the transfer equations are obtained and compared to a simplified analytical treatment, including the effect of stimulated scattering in an approximate manner. The latter effect is essential in order to obtain the correct spectrum at frequencies ω ⪡ kT. In field-free plasmas the Comptonization drastically changes the outgoing spectrum if ( N e 10 22 cm −3 ) 1 2 (kT/10 keV ) − 9 4 ⪡ 10 , where Ne and T are the electron density and temperature of the plasma. In strongly magnetized plasmas (ωB≫kT, where ωB is the cyclotron frequency) the Comptonization effects are less significant than the magnetic effects at frequencies ω ⪡ ωB, for parameters typical of accreting pulsars. The Comptonization is significant only for the ordinary polarization mode, which is much less intense than the extraordinary polarization mode. On the contrary, the Comptonization changes essentially all the properties of the radiation escaping at frequencies close to the cyclotron resonance, ω ⋍ ωB. These results are useful in the optically thick regions of neutron star models of gamma ray bursters, and are of major relevance for an appropriate treatment of models of accreting pulsars and X-ray bursters.

Spectral evolution is investigated and interpretation of the spectra are considered for a burst from the X-ray burster MXB 1728-34 detected by ASTRON. Interpretation of the spectra around the burst maximum in terms of radiation from an isothermal neutron star (NS) photosphere with dominating role of the Thomson scattering allows one to obtain reasonable constraints on NS mass M, radius R and distance D to the burster. In particular, M=(1.4−2)M ⊗ , R=(6.5−12) km for D=6 kpc and helium photosphere. This interpretation leads to a conclusion that the photosphere expands up to several tens of kilometers in the burst maximum. The luminosity remains close to the Eddington limit during expansion and contraction, maxima of the observed flux corresponding to minima of the luminosity due to general relativity effects.

The spectral flux is calculated for x-ray emission from a rotating neutron star uniformly heated over its surface (T = 10 /sup 80/K) and having a strong dipolar magnetic field (B = 5 x 10/sup 12/ G at the pole). It is shown that because of the strong anisotropy of the local intensity and its dependence on the magnetic field, the flux in the photon energy interval E approx. 2-70 keV varies over time with the period of rotation, that is, a star such as this manifests itself as an x-ray pulsar. The amplitude of the variations and the shape of the brightness curves are strongly dependent on the energy. The spectrum of the flux contains two emission cyclotron features. Their position, like the shape of the continuum, depends on the phase of the rotation. The results obtained demonstrate the possibility of models or x-ray pulsars in which the radiating (hot) regions are not localized around the magnetic poles, but occupy a large part of the surface of a neutron star.

Two approximate procedures for taking into account stimulated scattering in radiative transfer calculations, which effectively linearize the problem are discussed. One of these methods is useful in the diffusion approximation, while the other can be used also in treatments which calculate explicitly the angle dependence of the radiation field. These considerations are particularly useful in models of X-ray sources which are not too far from thermal equilibrium, but where scattering is the main opacity, such as in steady or bursting X-ray sources and accreting pulsars. 17 references.