Investigations on the properties of atmospheres of isolated neutron stars (NSs) are being carried out at Penn State in collaboration with the Max-Planck-Institut fuer Extraterrsestrische Physik (Garching, Germany and Ioffe Institute of Physics and Technology (St. Petersburg, Russia). Some of the recent results are listed below.
NS are born very hot, and cool down to surface temperatures of about 1,000,000 K during the first thousands years of their life, and to about 100,000 K in the next million years. Thermal-like radiation from such objects can be studied with the X-ray and EUV observatories (ROSAT, ASCA, EUVE). The investigation of this radiation is of crucial importance for understanding the thermal evolution of NSs which, in turn, depends on still poorly known properties of the super-high-dense matter in their interiors. To interprete the existing and future observations, one needs theoretical models of the NS atmospheres. The modeling requires, first of all, knowledge on the physical processes occuring in relatively dense (up to 10 - 100 grams per c.c.) plasma in Teragauss magnetic fields. In these huge magnetic fields, even hydrogen is not fully ionized at the temperatures of interest, and a small fraction of the nonionized atoms gives the main contribution to the opacity.
Making use of the opacities of strongly magnetized plasmas (Potekhin and Pavlov 1993; Pavlov and Potekhin 1995; Bulik and Pavlov 1996; Potekhin, Pavlov and Ventura 1996), one can construct models of the NS atmospheres which are significantly more complicated than the atmospheres of normal stars, mainly because the NS atmospheres are strongly anisotropic and rather dense. The results of the NS atmosphere modeling (Pavlov et al. 1994; Zavlin et al. 1995a; Pavlov et al. 1995) demonstrate that the NS magnetic fields substantially affect the spectra, angular distribution and polarization of the emitted thermal-like radiation. In particular, the spectra deviate substantially from both the blackbody spectrum and from spectra emitted by nonmagnetic atmospheres.
The anisotropy of the NS magnetic field causes a nonuniform distribution of the temperature over the NS surface. In particular, the polar caps of active radio pulsars are much hotter than the rest of the surface, and the temperature outside the polar caps grows with increasing magnetic latitude due to the anisotropy of the heat conducton in the strong magnetic field (Shibanov et al. 1995). To calculate the observed radiation flux by integrating over the NS surface, one should take into account the bending of the photon trajectories in strong gravitational field of the NS (Zavlin, Pavlov and Shibanov 1995b). Since the bending depends on the NS mass and radius, these parameters can be inferred from an analysis of the periodic X-ray light curves which have been observed from several pulsars.
The first results of the NS atmosphere modelling have been appplied to interpret the ROSAT observations of the soft X-ray spectra of the radio pulsar PSR B0656+14 (Anderson et al.~1993) and the gamma-ray pulsar Geminga (Meyer, Pavlov and Meszaros 1994). Fitting the observed spectra with hydrogen atmosphere models yields NS surface temperatures considerably lower than those obtained from blackbody fits. The temperatures inferred are compatible with fast cooling of the NSs by direct Urca processes and may provide evidence for the presence of exotic matter (pion or kaon condensate, quark-gluon plasma) in the NS interiors.
Thermal-like radiation from NSs can be also observed in the UV-optical range with largest ground-based telescopes and with the Hubble Space Telescope (HST). Observations in this range are especially important to trace the thermal evolution of old NSs whose X-ray radiation is too faint to be detected. The first HST observations of three nearby radio pulsars (PSR B0656+14, PSR B1929+10 and PSR B0950+08) have been carried out, and were interpreted with the use of the NS atmosphere models, by Pavlov, Stringfellow and Cordova (1996). It was shown that the UV-optical radiation from PSR B0656+14 is predominatly of a nonthermal origin, whereas radiation from old PSR B1929+10 and PSR B0950+08 is thermal, corresponding to surface temperatures of 200,000 K and 70,000 K, respectively. These temperatures are too high to be explained with the standard theories of NS cooling, which means that some (re)heating mechanisms are operating in old NSs.
Atmosphere models can also be constructed for NSs with superstrong magnetic fields, hundreds and even thousands Teragauss, which are believed to exist in Soft Gamma-ray Repeaters - enigmatic sources of sporadic gamma-ray radiation. Since the luminosities of these sources are much greater than those of cooling NSs (and greater than the standard Eddington luminosity), the proper modelling of such atmospheres must include the radiative force exerted onto the strongly magnetized plasma in outer layers. Bezchastnov et al. (1996) calculated the spectral flux and specific intensity emergent from such an atmosphere and found that the atmosphere is hydrodynamically stable if the magnetic field exceeds 350 Teragauss. The spectrum of the emergent radiation is very close to the blackbody spectrum (with an absorption feature centered at the ion cyclotron frequency), although the radiation is highly anisotropic and linearly polarized.
S. B. Anderson, F. A. Cordova, G. G. Pavlov, C. R. Robinson,
and R. J. Thompson, Jr., 1993
"ROSAT High Resolution Imager Observations of PSR 0656+14",
ApJ, 414, 867-871
V. G. Bezchastnov, G. G. Pavlov, Yu. A. Shibanov, and V. E. Zavlin, 1996,
"Radiative Opacities and Photosphere Models for Soft
Gamma-Ray Repeaters",
in Proc. 3-rd Huntsville Gamma-Ray Burst Symposium ,
AIP Conf. Proc. 384, eds. C. Kouveliotou, M.F. Briggs,
G.J. Fishman (Woodbury: New York) pp.907-91
T. Bulik and G. G. Pavlov, 1996,
"Polarization Modes in a Strongly Magnetized
Hydrogen Gas",
ApJ, 469, 373-387
R. D. Meyer, G. G. Pavlov and P. Meszaros, 1994,
"Soft X-Ray Spectral Fits of Geminga with Model
Neutron Star Atmospheres",
ApJ, 433, 265-275
G. G. Pavlov, Yu. A. Shibanov, J. Ventura, and V. E. Zavlin, 1994,
"Model Atmospheres and Radiation of Magnetic Neutron Stars:
Anisotropic Thermal Emission",
Astron. Astrophys., 289, 837-845
G. G. Pavlov and A. Y. Potekhin, 1995,
"Bound-bound Transitions in Strongly Magnetized Hydrogen Plasma".
ApJ, 450, 883-895
G. G. Pavlov, Yu. A. Shibanov, V. E. Zavlin,
and R. D. Meyer, 1995,
"Neutron Star Atmospheres",
in The Lives of the Neutron Stars , pp. 71-90,
Eds. M.A. Alpar, U. Kiziloglu
and J. van Paradijs, Kluwer
G. G. Pavlov, G. S. Stringfellow, and F. A. Cordova, 1996,
"Hubble Space Telescope Observations of Isolated Pulsars",
ApJ, 467, 370-384
G.G. Pavlov, 1998
"Neutron Star Atmospheres",
in
Atoms and Molecules in Strong External Fields,
eds. P. Schmelcher and W. Schweizer, Plenum: NY, pp.37-48
A. Y. Potekhin and G. G. Pavlov, 1993,
"Photoionization of Hydrogen Atom in Strong Magnetic Field",
ApJ, 407, 330-341
A. Y. Potekhin, G. G. Pavlov, and J. Ventura, 1997,
"Ionization of the Hydrogen Atoms in Strong Magnetic Fields:
Beyond the Adiabatic Approximation",
Astron. Astrophys., 317, 618-629
A.Y. Potekhin and G.G. Pavlov, 1997
"Photoionization of Hydrogen in Atmospheres of Magnetic
Neutron Stars",
ApJ, 483, 414-425
Yu. A. Shibanov, G. G, Pavlov, V. E. Zavlin, L. Qin, and S. Tsuruta, 1995,
"Anisotropic Cooling and Atmospheric Radiation of Neutron Stars
with Strong Magnetic Field",
in Proc. 17-th Texas Symposium on Relativistic Astrophysics ,
Eds. H. Bohringer, G. Morfill and J. Trümper, Ann. New York
Acad. Sci., 759, 291
V. E. Zavlin, G. G. Pavlov, Yu. A. Shibanov and J. Ventura, 1995a,
"Thermal Radiation from Rotating Single Neutron Star:
Effect of the Magnetic Field and Surface Temperature Distribution",
Astron. Astrophys., 297, 441
V. E. Zavlin, Yu. A. Shibanov and G. G. Pavlov, 1995b,
"Effect of the Neutron Star Gravitational Field on
Radiation from the Hot Polar Spots of Radio Pulsars",
Astronomy Letters, 21, 168
V.E. Zavlin, G.G. Pavlov and V.E. Shibanov, 1996,
"Model Neutron Star Atmospheres with Low Magnetic Fields:
I. Atmospheres in Radiative Equilibrium",
Astron. Astrophys,, 315, 141-152
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