ABSTRACT
The paradigm that young (< 10,000 yr-old) neutron stars evolve as rapidly rotating Crab-like pulsars requires re-examination. Recent comprehensive radio surveys suggest that most radio pulsars near supernova remnants (SNRs) can be attributed to chance overlap (e.g. Lorimer et al. 1998; Gaensler & Johnston 1995; see Kaspi et al. 1996 for a review). With the results of these new surveys, traditional arguments for the lack of observed radio pulsars associated with SNRs, such as those invoking beaming and large ``kick'' velocities, are less compelling. It is now clear that this discrepancy is an important and vexing problem in current astrophysics.
Progress in resolving this mystery is suggested by recent X-ray observations of young SNRs. Evidence is accumulating that many young neutron stars (NSs) are slowly rotating (P ~ 10 s) X-ray pulsars, lacking in detectable radio emission. There are currently about a dozen slow X-ray pulsars apparently associated with young SNRs. These include the four known soft Gamma-ray repeaters (SGR), which have recently been confirmed as slow rotators.
In fact, there are now more known slow, radio-quiet X-ray pulsars in the center of identified SNRs than confirmed Crab-like radio pulsars! Here, we consider the observational properties of these radio-quiet NS candidates associated with supernova remnants, which suggests that alternative evolutionary-paths exist for young pulsars. We postulate that such objects account for the apparent paucity of radio pulsars in supernova remnants.
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Where Are All The Young Neutron Stars?
Neutron stars are thought to be born as rapidly rotating (~ 10 ms) radio pulsars created during a Type II/Ib supernova explosion involving a massive star. Their existence was postulated in 1934 by Baade & Zwicky (1934) based on theoretical arguments, but awaited the 1970s for observational support, provided by the remarkable discoveries of the Crab and Vela pulsars in their respective supernova remnants (SNRs).
Most supernovae (non-Type Ia) are expected to produce a NS, whose unpulsed emission should be easily discernible in the radio-band during the lifetime of a typical SNR (> 10,000 yrs) as a radio-loud ``plerion'' (Weiler & Sramek 1988). So it is quite remarkable that, despite detailed radio searches, few of the hundreds of known SNRs have yielded a NS candidate. Most radio pulsars near SNRs are now considered to be consistent with chance superpositions.
Traditional arguments for the lack of observed radio pulsars associated with SNR, such as those invoking beaming and large ``kick'' velocities are less compelling with each new radio survey in which the list of SNRs grows without new NS candidates.
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Is the Typical Young Pulsar Really Typical?
The properties of the Crab and Vela pulsars were found to be uniquely explained in the context of rapidly rotating, magnetized neutron stars emitting beamed non-thermal radiation. Their fast rotation rates and large magnetic fields (~ 10^12 G) are consistent with those of a main-sequence star collapsed to NS dimension and density.
A fast period essentially precluded all but a NS hypothesis and thus provided direct evidence for the reality of NSs (see Shapiro & Teukolsky 1983 for a brief history and intro to NS physics). Furthermore, their inferred age and association with SNRs provided strong evidence that NSs are indeed born in supernova explosions.
With the nature of the Crab and Vela well understood, their properties have been considered typical of all young pulsars. But as we shall see, there is new evidence that suggests that these pulsars are, in fact, the rare, but highly visible, examples of NS evolution.
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The "Missing" Young Neutron Star Problem
--> NSs manifest themselves as rapidly rotating (P ~ 100 ms) radio-loud Crab-like (plerionic) pulsars.
--> Supernova explosions give rise to radio SNRs which can be clearly distinguished.
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ASCA Reveals New NS Candidates in the Light of X-rays
The Advance Satellite for Cosmology and Astrophysics (ASCA) spectro-imaging capability in the 0.5-10 keV energy range allows us to locate X-ray pulsars previously "hidden" to earlier X-ray missions with none or poor imaging above 2 keV. Most of the new slow pulsar candidates where first detected with ASCA. The following is an example of the ASCA detection of the compact source in SNR RCW 103.
The two X-ray pictures shown below are from the same location on the sky, but at different energies. The image at higher energies (Left) reveal a point-like source of X-rays embedded in the center of bright shell-like supernova remnant. A second source was also discovered which was found to be a high energetic rapidly spining pulsar. (Right) In contrast, at lower energies, X-ray emission from these sources are dominated by X-ray's from the supernova remnant.
See adjacent poster (#41.03) on SNR RCW 103 for new results.
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The Revolution Evolution: Slowly Rotating Young X-ray Pulsars
Recent X-ray observations of known SNRs have revealed several X-ray bright, but radio-quiet compact objects at their centers. Some of these sources have been found to be slowly rotating pulsars with unique properties. Their temporal signal is characterized by spin periods in the range of 5 - 12 s, steady spin-down rates, and highly modulated sinusoidal pulse profiles (~ 30%).
These are now about a dozen slow X-ray pulsars apparently associated with young SNRs (see Table I). These include seemingly isolated pulsars currently referred to as the anomalous X-ray pulsars (AXPs; Mereghetti & Stella 1995, Duncan & Thompson 1996; Van Paradijs et al. 1995; Vasisht & Gotthelf 1997) and the the four known soft Gamma-ray repeaters (SGR) (Cline et al. 1982; Kulkarni & Frail 1993).
Nearly half of the AXPs and all of the SGRs are located at the centers of SNRs, suggesting that they are relatively young ( ~< 100,000 yr-old). And so far, no counterparts at other wavelengths have been identified for these X-ray bright objects.
We stress again that there are currently more known slow, radio-quiet X-ray pulsars in the center of identified SNRs than confirmed Crab-like radio pulsars.
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Slow X-ray Pulsars as Ultra-magnetized NSs
If the slow X-ray pulsar are isolated NSs, then they have remarkable properties:
The slow X-ray pulsars likely represent a population of ultra-magnetized neutron stars, or ``magnetars'' (Thompson & Duncan 1993), whose luminosity is powered by magnetic field decay instead of spin down energy. The magnetic field may be the critical parameter which governs the evolution of young NSs. These pulsar were likely born as fast rotators (P ~ 10 ms), but spun down rapidly due to their enormous magnetic fields.
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Comparison of the Crab-like & the Slow X-ray Pulsars in SNRs
Table I summaries the timing properties of the AXPs, SGRs, & Crab-like pulsars. The table is ranked by the inferred magnetic field strength assuming a nominal rotating dipole model.
The three classes of pulsars are distinguished by their periods and derived age and magnetic fields.
The inferred magnetic field for the AXPs and SGRs are well above the quantum critical field B_qed = 4.4 X 10^13 G. AXP 1841-045 in Kes 73 (see adjacent poster) and the recently discovered SGR 1900+14 (and perhaps SGR 1806-20) are likely magnetar candidates, whose luminosity is dominated by magnetic field decay.
There is a natural progression in the X-ray properties of the SGRs and AXPs. The nature of both the SGRs and AXPs may be similar, distinguished by age and magnetic field strength. The SGR may represent a more active phase which manifest Gamma-ray bursts not seen from the AXPs. The SGRs may evolve into AXPs with a decreased period, period derivative, and magnetic field.
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Summary and Conclusions
The main focus of this research is to understand the nature of radio-quiet X-ray pulsars, to explore possible alternative evolutionary paths for NSs, which might help resolve the mystery of the missing NSs and further our understanding of NS physics.
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Author Bibliography of Relavent Papers
All the papers below refer to the missing pulsar problem in some way or another.
Also, see adjacent poster on the central sources in SNRs Kes 73 (#41.02) and RCW 103 (#41.03).
"A New View on Young Pulsars in Supernova Remnants: Slow, Radio-quiet, & X-ray Bright," Gotthelf, E. V., Vasisht, G. 2000, in "IAU Colloquium 177: 2000 and Beyond", ASP Conf. Series; astro-ph/9911344; also ApJ 2000, in prep.
"Radio-quiet X-ray pulsars in Supernova Remnants and the "Missing" Pulsar," Gotthelf, E. V. 1998, in "Workshop on Neutron Star/Supernova Remnant Connection", Memorie della Societa' Astronomica Italiana, also available via astro-ph/9809139
"Discovery of a 7 Second Anomalous X-ray Pulsar in the Distant Milky Way," Gotthelf, E. V. & Vasisht, G. 1998, New Astronomy, 3, 293
" A New Supernova Remnant Coincident with the Slow X-Ray Pulsar AX J1845-0258, " Gaensler, B. M., Gotthelf, E. V., Vasisht, G. 1999, ApJ, 526, L37
"A New Pulsar/SNR Pair: AX J1845-0258 in G29.6+0.1," E.V. Gotthelf, E. V., Vasisht, G., Torii, K., Gaensler, B. M. 1999 Proceedings of the Bologna Conference on "X-ray Astronomy 1999: Stellar Endpoints, AGNs and the Diffuse X-ray Background" (Astroph. Lett. and Comm.)
"The Discovery of an Anomalous X-ray Pulsar in the Supernova Remnant Kes 73," Vasisht, G. & Gotthelf, E. V. 1997, ApJ, 486, L129
"Kes 73: A Young Supernova Remnant with an X-ray Bright, Radio-Quiet Central Source, " Gotthelf, E. V. & Vasisht, G. 1997, ApJ, 486, L133
"On the Spin History of the X-Ray Pulsar in Kes 73: Further Evidence for an Ultramagnetized Neutron Star, " Gotthelf, E. V., Vasisht, G., Dotani, T. 1999, ApJ, 522, L49
"The Nature of the Radio-Quiet Compact X-ray Source in SNR RCW 103, " Gotthelf, E. V., Petre R. & Hwang U. 1997, ApJ, 487, L175
"Discovery of a 69 millisecond X-ray Pulsar: A compact source in the Vicinity of the Supernova Remnant RCW 103," Torii, K, Kinugsa, K., Toneri, T., Asanuma, T., Tsunemi, H., Dotani, T., Mitsuda, K., Gotthelf E. V. & Petre, R. 1998, ApJ, 494, L207
"X-ray Variability from the Compact X-ray Source in SNR RCW 103, " Gotthelf, E. V., Petre, R. & Vasisht, G. 1999, ApJ, 514, L107
"ROSAT AND ASCA Observations of the Crab-like Supernova Remnant N157B in the Large Magellanic Cloud," Wang, D. Q. & Gotthelf, E. V. 1998, ApJ, 494, 623
"Discovery of an Ultra-fast X-ray Pulsar in the Supernova Remnant N157B," F. E. Marshall, E. V. Gotthelf, W. Zhang, J. Middleditch, & Q. D. Wang 1998, ApJ, ApJ, 499, L179
"ROSAT HRI Detection of the 16 ms Pular PSR J0537-6910 Inside SNR N157B," Wang, D. Q. & Gotthelf, E. V. 1998, ApJ, 509, L109
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Baade, W. & Zwicky, F. 1934, Phys. Rev., 45, 138
Cline, T. L. 1982, ApJ, L255, 45
Duncan, R. C. & Thompson, C. 1996, ApJ, 473, 322
Gotthelf, E. V., Petre, R. & Hwang, U. 1997, ApJ, 487, L175
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Vasisht, G. & Gotthelf, E. V. 1997, ApJ, 486, L129
Weiler, K. W. & Sramek, R. A. 1988, ARA&A, 26, 29
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File was last modified on Tue Jan 5 1999