Magnetized Iron Atmospheres for Neutron Stars

Using a Hartree-Fock formalism, we estimate energy levels and photon cross sections for atomic iron in magnetic fields B ~ 10^13 G. Computing ionization equilibrium and normal mode opacities with these data, we construct LTE neutron star model atmospheres at 5.5 < Log(T_eff) < 6.5 and compute emergent spectra. We examine the dependence of the emergent spectra on T_eff and B. We also show the spectral variation with the angle between the magnetic field and the atmosphere normal and describe the significant limb darkening in the X-ray band. These results are compared with recent detailed computations of neutron star H model atmospheres in high fields and with low field Fe and H model atmospheres constructed from detailed opacities. The large spectral differences for different surface compositions may be discernible with present X-ray data; we also note improvements needed to allow comparison of Fe models with high quality spectra.Comment: 18 pages with 5 eps figures, accepted for publication in ApJ Replaced due to clerical error only: one more author, no new conten

Optical identification of the 3C 58 pulsar wind nebula

We have performed a deep optical imaging of 3C 58 SNR with the NOT in the B and V bands to detect the optical counterpart of the associated pulsar J0295+6449 and its torus-like wind nebula visible in X-rays. We analyzed our data together with the archival data obtained with the Chandra in X-rays and with the Spitzer in the mid-IR. We detect a faint extended elliptical object with B=24.06 and V=23.11 whose peak brightness and center position are consistent at the sub-arcsecond level with the position of the pulsar. Its morphology and orientation are in excellent agreement with the torus-like pulsar nebula, seen almost edge on in X-rays although its extension is only about a half of that in X-rays. In the optical we likely see only the brightest central part of the torus with the pulsar. The object is identical to the counterpart of the torus recently detected in the mid-IR. The estimated pulsar contribution to the optical flux is less than 10%. Combinig the optical/mid-IR fluxes and X-ray power-law spectrum extracted from the spatial region constrained by the optical/IR source extent we compile a tentative multi-wavelength spectrum of the central part of the nebula. Within uncertainties of the interstellar extinction it is reminiscent of either the Crab or B0540-69 pulsar wind nebula spectra. The properties of the object strongly suggest it to be the optical counterpart of the 3C 58 pulsar + its wind nebula system, making 3C 58 the third member of such a class of the torus-like systems identified in the optical and mid-IR.Comment: 12 pages including 7 figures, submitted for publication in A&A. For high resolution images, see http://www.ioffe.ru/astro/NSG/obs/3C58

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

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

X-ray observations of the high magnetic field radio pulsar PSR J1814-1744

PSR J1814-1744 is a 4 s radio pulsar with surface dipole magnetic field strength 5.5*10^13 G, inferred assuming simple magnetic dipole braking. This pulsar's spin parameters are very similar to those of anomalous X-ray pulsars (AXPs), suggesting that this may be a transition object between the radio pulsar and AXP population, if AXPs are isolated, high magnetic field neutron stars as has recently been hypothesized. We present archival X-ray observations of PSR J1814-1744 made with ROSAT and ASCA. X-ray emission is not detected from the position of the radio pulsar. The derived upper flux limit implies an X-ray luminosity significantly smaller than those of all known AXPs. This conclusion is insensitive to the possibility that X-ray emission from PSR J1814-1744 is beamed or that it undergoes modest variability. When interpreted in the context of the magnetar mechanism, these results argue that X-ray emission from AXPs must depend on more than merely the inferred surface magnetic field strength. This suggests distinct evolutionary paths for radio pulsars and AXP, despite their proximity in period--period derivative phase space.Comment: 11 pages, including 2 embedded figures. Accepted by Ap

Detection of Giant Radio Pulses from the Pulsar PSR B0656+14

Giant pulses (GPs) have been detected from the pulsar PSR B0656+14. A pulse that is more intense than the average pulse by a factor of 120 is encountered approximately once in 3000 observed periods of the pulsar. The peak flux density of the strongest pulse, 120 Jy, is a factor of 630 higher than that of the average pulse. The GP energy exceeds the energy of the average pulse by up to a factor of 110, which is comparable to that for other known pulsars with GPs, including the Crab pulsar and the millisecond pulsar PSR B1937+21. The giant pulses are a factor of 6 narrower than the average pulse and are clustered at the head of the average pulse. PSR B0656+14 along with PSR B0031-07, PSR B1112+50, and PSR J1752+2359 belong to a group of pulsars that differ from previously known ones in which GPs have been detected without any extremely strong magnetic field on the light cylinder.Comment: 10 pages, 3 figures, 1 table; originally published in Russian in Pis'ma Astron. Zh., 2006, v.32, 650; translated by George Rudnitskii; the English version will be appear in Astronomy Letter

Surface Emission Properties of Strongly Magnetic Neutron Stars

We construct radiative equilibrium models for strongly magnetized (B > 10^13 G) neutron-star atmospheres taking into account magnetic free-free absorption and scattering processes computed for two polarization modes. We include the effects of vacuum polarization in our calculations. We present temperature profiles and the angle-, photon energy-, and polarization-dependent emerging intensity for a range of magnetic field strengths and effective temperatures of the atmospheres. We find that for B < 10^14 G, the emerging spectra are bluer than the blackbody corresponding to the effective temperature, T_eff, with modified Planckian shapes due to the photon-energy dependence of the magnetic opacities. However, vacuum polarization significantly modifies the spectra for B~10^15 G, giving rise to power-law tails at high photon energies. The angle-dependence (beaming) of the emerging intensity has two maxima: a narrow (pencil) peak at small angles (<5 degrees) with respect to the normal and a broad maximum (fan beam) at intermediate angles (~20-60 degrees). The relative importance and the opening angle of the radial beam decreases strongly with increasing magnetic field strength and decreasing photon energy. We finally compute a T_eff-T_c relation for our models, where T_c is the local color temperature of the spectrum emerging from the neutron star surface, and find that T_c/T_eff ranges between 1.1-1.8. We discuss the implications of our results for various thermally emitting neutron star models.Comment: 30 pages, 10 color figures, ApJ in pres