X-RAY OBSERVATIONS OF ISOLATED NEUTRON STARS

Neutron stars were first observed in 1967 as radio pulsars. In 1971 first binary X-ray pulsars were discovered with Uhuru. First X-rays from isolated neutron stars (INSs) were detected with Einstein in 1982. A real breakthrough in X-ray observations of INSs is due to ROSAT (1991-98) and ASCA (1993-99). X-ray emitting INSs can be divided in two separate groups -- active radio and/or gamma-ray pulsars, and radio-quiet (or radio-silent) INSs.

X-rays from Radio Pulsars.

Most X-ray emitting INSs are radio pulsars. Their X-ray radiation may be generated by relativistic particles in the pulsar magnetosphere (nonthermal radiation) or it may be emitted from the NS surface if its temperature is high enough. So far, more than 30 radio pulsars have been detected in X-rays. We believe that at least 4 of them (PSR B0656+14, PSR B1055-52, the Vela pulsar PSR 0833-45, and Geminga, whose radio pulsations have been discovered only recently) are thermal emitters. Since their radiation comes from the NS surface layers (atmospheres), studying this radiation enables one to investigate temperature, magnetic field, gravitational acceleration, chemical composition, and other important properties of neutron stars. That is why these objects are of special interest for X-ray observations, and we have been studying them since 1993 (Anderson et al. 1993; Meyer, Pavlov and Meszaros 1994). The main objective of these investigations is to interpret the soft X-ray spectra of these pulsars with the aid of INS atmosphere models, developed by our group.

The four above-mentioned pulsars are in their adolescent age -- the yongest, Vela pulsar, is only 10,000 years old, and the oldest PSR B1055-52 is 500,000 years old. Most radio pulsars are much older and colder objects, with temperatures less than 100,000 Kelvins, so that their thermal radiation is extremely faint in the X-ray range (although they can be observed in the UV/optical range --- see Observations of Neutron Stars with the Hubble Space Telescope). However, a small fraction of their surfaces near the magnetic poles, so-called polar caps, may be hot enough to emit detectable X-rays. Even very old millisecond pulsars, whose typical ages are of a few billion years, may have hot polar caps. Studying their X-ray radiation is of particular interest because the shape of the X-ray pulse depends on the NS mass-to-radius ratio (Pavlov and Zavlin 1997), due to the gravitational bending of photon trajectories, one of the effects of General Relativity. Therefore, studying the pulse shape allows one not only to measure the polar cap temperature, but also to evaluate the mass-to-radius ratio, which is the critical parameter to understand still poorly known equation of state of the superdense matter in NS interior. The nearby PSR J0437-4715 is the brightest of the X-ray emitting millisecond pulsars, and we have investigated its spectrum and pulse shape applying our NS atmosphere models to the spectral and timing analysis of the ROSAT and EUVE data (Zavlin, Pavlov and Trümper 1998).

We plan to study the publicly available archival data on these and other pulsars where the observed radiation may be, at least, partly, of the thermal origin. This is work is supported by the NASA ADP grant "Soft X-ray Radiation from Isolated Neutron Stars" (PI: G. Pavlov, Co-I: V. Zavlin). According to our proposal (PI: G. Pavlov, Co-Is: J. Halpern, V. Zavlin), the most interesting of these objects, PSR B0656+14, was observed with ASCA in October 1998. This very long observation (160 ksec useful exposure obtained during 4 days) has provided us with the most accurate spectrum and pulse shape in the energy range 0.5-5 keV. The work on the data, also supported by an ADP grant, is in progress. Finally, two our propsals (PI: G. Pavlov, Co-Is: V. Zavlin, J. Halpern, W. Becker) to observe the radio pulsars PSR 1055-52 and PSR J0437-4715 with the X-ray observatory AXAF (to be launched in 1999, hopefully) have been accepted. We expect that these observations will enable us to obtain qualitatively new results which will solve most important problems of NS physics.

Radio-quiet Isolated Neutron Stars in Supernova Remnants

Majority of NSs should be radio-quiet, as follows, for instance, from the comparison of the number of detected radio pulsars (slightly above 1,000) and theoretically estimated number of NSs in our Galaxy (about 100,000,000). It is extremely difficult to detect old, radio-quiet INSs because they are very cold. However, very young NSs, with ages about 1,000 - 10,000 years can be detected much easier because they are hot enough (surface temperatures are about 1 Megakelvins), and we know where to search for them. Neutron stars are born in extremely powerful explosions of massive progenitors which have exhausted their thermonuclear fuel. In the course of these Supernova explosions several solar masses are ejected in the interstellar medium. These Supernova Remnants (SNRs) are bright extended sources of radio, optical and X-ray radiation, and it is natural to search for radio-quiet NSs within SNRs which do not have radio pulsars.

Indeed, several X-ray bright, compact objects with thermal-like spectra have been found in SNRs. For instance, here is an image of the shell-type Supernova Remnant PKS 1209-58 obtained with the ROSAT Position Sensitive Proportional Counter (PSPC) in July 1993. The bright source near the center of the SNR is the radio-quiet isolated neutron star 1E1207.4-5209, discovered with Einsten in 1984. We have analyzed the spectra of this source obtained with the ROSAT and ASCA observatories (Zavlin, Pavlov and Trümper 1998) and showed that most realistic parameters of the NS can obtained with hydrogen NS atmosphere models. In particular, the NS surface temperature, 1.4-1.9 million Kelvins, is consistent with standard NS cooling models. ....

Another interesting example of a radio-quiet INS is RX J0822-4300 in the SNR Puppis A. We analyzed the ROSAT and ASCA observations of this bright X-ray source and found that its spectrum can be explained if the NS is covered with a hydrogen or helium atmosphere with a strong magnetic field, B > 5 Teragauss, and a tempearture 1.6-1.9 Megakelvins. A new method for the period search enabled us to find pulsations with a period of 75 millisecond, typical for a young pulsar (Pavlov, Zavlin and Trümper 1999).

The Lone Neutron Star

Among several detected radio-quiet INSs, there is one particularly interesting - RX J1856-3754. This NS is a bright soft-X-ray source, but it is not in a supernova remnant. Most likely, it is nearby star, at a distance of less than 120 pc. Its X-ray radiation has a thermal-like spectrum, with a tempearture of 400,000-600,000 Kelvins. Why this apparently old NS is so hot? What is the true distance to this source? What is the chemical composition of its surface? How large is its magnetic field? How old it is? Why does not it show any pulsations? We still do not know correct answers to these questions. Pavlov et al. (1996) have shown that the answers can be found if X-ray observations of this object are supplemented by optical/UV observations -- the optical flux, measurable with the Hubble Space Telescope, is very sensitive to the chemical composition and magnetic field. ...

Neutron Stars in Transient X-ray Binaries

There exist a class of NSs which are not truly isolated but may display themselves as INSs during long time intervals. These NSs are components of binary systems which show transient behavior -- long periods of quiescence are interrupted by powerful X-ray bursts caused by accretion of stellar material from the secondary companion. The energy released during these accretion episodes heats the NS crust, and this heat is further radiated from NS surface during the quiescence. Analyzing the quescent radiation of transient X-ray binaries (e.g., Aql X-1, Cen X-4, etc) enables one to understand the accretion mechanisms, chemical composition of the accreting matter, estimate the NS radius. First attempt to interpret the quiescent emission from transient binaries in the framework of this model has been undertaken by Rutledge et al. (1999). They showed that .....

References

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"
Astrophys. J., 416, 752

R.D. Meyer, G.G. Pavlov and P. Meszaros 1994
"Soft X-Ray Spectral Fits of Geminga with Model Neutron Star Atmospheres"
Astrophys. J., 433, 265

G.G. Pavlov, V.E. Zavlin, J. Trümper and R. Neuhäuser 1996
"Multiwavelength Observations of Isolated Neutron Stars as a Tool to Probe the Properties of Their Surfaces"
Astrophys. J., 472, L33

G.G. Pavlov and V.E. Zavlin 1997
"Mass-to-Radius Ratio for the Millisecond Pulsar J0437--4715"
Astrophys. J., 490, L91

G.G. Pavlov, V.E. Zavlin and J. Trümper 1999
"X-ray Pulsations from the Central Source in Puppis A"
Astrophys. J., 511, L45

R. Rutledge, L. Bildsten, E. Brown, G.G. Pavlov and V.E. Zavlin 1998
"The Thermal X-ray Spectra of Transient Neutron Stars in Quiescence"
Bull. Amer. Astron. Soc., 30, 1420

V.E. Zavlin and G.G. Pavlov 1998
"Soft X-rays from Polar Caps of the Millisecond Pulsar J0437--4715"
Astron. Astrophys., 329, 583

V.E. Zavlin, G.G. Pavlov and J. Trümper 1998
"The Neutron Star in the Supernova Remnant PKS 1209--52"
Astron. Astrophys., 331, 821

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