The textbook picture of a core collapse supernova remnant includes an expanding blast wave sweeping up and shock heating ambient material and a central, rapidly spinning pulsar whose strong magnetic field interacts with its surroundings to produce a pulsar wind nebula (PWN). During the Einstein era, the most vexing mystery about SNRs was the dearth of such "composite" remnants. Even the closest Galactic match, the Vela remnant, deviated from the model in having no detected X-ray pulsations. The Crab Nebula, considered the prototype pulsar wind nebula, fails to satisfy the model because it lacks a thermally emitting shell. One of the great successes of the X-ray imaging missions, starting with Einstein, but brought to fruition by more recent missions, has been revealing how naive this preconception was.
While a few pulsars with Crab-like properties were detected using Einstein, including fast pulsars in MSH 15-52  and the LMC remnant 0540-69.3 , most newly discovered central objects do not fit the paradigm. 1E 2259+586 in the center of the remnant CTB 109 was found to have an 8 s pulsation period . The stellar remnants 1E 1613-509 in RCW 103 and 1E 1207.4-5209 in PKS 120952 were found to have modest luminosity, and no evidence for either pulsations or counterparts in other bands [51,162]. None of these objects have an associated PWN. Of the roughly 50 Galactic remnants observed by Einstein, stellar remnants were known or found in 11 , leaving open the question of where (and what) the central stellar objects are.
The rich solution to this mystery came with ROSAT, ASCA, Chandra and XMM-Newton. ROSAT revealed a central stellar object in the —4 000-year-old Puppis A remnant with a low X-ray luminosity, no radio or optical counterpart or PWN, and no pulsations3 . A discrete source was found in the LMC remnant N49, and associated with the bright y-ray burst from that direction . The Vela pulsar was found to pulse, but in soft X-rays only, suggesting that the pulsations arise from surface hot spots at the poles . ROSAT discovered a shell around the pulsar wind nebula in the LMC remnant 0540-69.3 ; this remnant is now the closest known to the textbook picture. ASCA detection of variability in 1E 1613-509 in RCW 103 refuted the hypothesis that it is a quiescent neutron star emitting thermal X-rays from its surface . The extended bandpass of ASCA allowed discovery of spectrally hard PWNe embedded in the soft thermal remnants W44  and G11.2-0.3 . Subsequently a 65ms pulsar was found within G11.2-0.3 . ROSAT and ASCA found stellar remnants were identified in the synchrotron-dominated shell-like remnants RX J1713.7-3946 and RX J0852.0-4622 [5,150], indicating that environment is the fundamental determinant in whether a remnant is synchrotron dominated. The zoology of central stellar objects now includes pulsars, anomalous X-ray pulsars, soft gamma-ray repeaters, objects defying classification (such as SS 433), and neutron stars without a PWN, which are simply called "central compact objects."
XMM-Newton and Chandra have added to the wealth of observed phenomena. Chandra's high resolution revealed point sources previously hidden by their low luminosity, the most dramatic of which is the source in Cas A, revealed in the very first Chandra image. It is similar to the source in Puppis A, with no apparent pulsations, no counterparts, and no PWN . Pulsations with period 0.42 s were discovered from 1E 1207.4-5209, the source in PKS 1209-52 . Additionally, Chandra has been used to detect low luminosity pulsars hidden in PWNe. For instance, the location and detection of X-ray pulsations from the X-ray pulsar J0205+6449 in 3C 58 led to the subsequent discovery of radio pulsations . Chandra and XMM-Newton have also revealed pulsar wind nebulae and bow shock nebulae embedded within thermal emission. Chandra revealed the presence of a PWN within the young, O-rich SNR G292.0+1.8 . A region of hard emission detected using ASCA in the evolved remnant IC 443 was resolved into a compact source embedded in a bow shock nebula . XMM-Newton observations show that the X-ray bow shock nebula is substantially larger than the radio nebula, a fact inconsistent with the expected inverse correlation between an electron's synchrotron lifetime and its energy .
On the other hand, the central stars in other SNRs remain elusive. A central hot spot in 3C 397 thought to house a compact stellar remnant showed no source to a limiting luminosity of 7x 1033 erg s^1 . Neither of the oxygen-rich Magellanic Cloud remnants, N132D or 1E 0102.2-72.2, shows evidence for a central source. A systematic optical/IR identification program of the unresolved Chandra
3 Pulsations were reported based on ROSAT data, but not confirmed in Chandra data 
sources embedded in four SNRs revealed no stellar remnant candidates above an X-ray luminosity of 1031 erg s^1 .
Chandra and XMM-Newton have also found shells around Crab-like remnants. A soft thermal shell was found around 3C 58 . A limb-brightened shell has also been found around the periphery of G21.5-0.9. This shell shows no evidence for thermal emission, and while dust scattering could contribute to its total flux, it is possible that the X-ray emission in the shell arises from shock accelerated electrons as in SN 1006 . Ironically, all searches for thermal emission from the Crab have failed to find any, leaving the Crab as an exception rather than a true prototype.
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