The absence of a supernova remnant (SNR) shell surrounding the Crab and other plerions (pulsar wind nebulae) has been a mystery for 3 decades. G21.5-0.9 is a particularly intriguing plerionic SNR in which the central powering engine is not yet detected. Early CHAN DRA observations revealed a faint extended X-ray halo which was suggested to be associated with the SNR shell; however its spectrum was nonthermal, unlike what is expected from an SNR shell. On the other hand, a plerionic origin to the halo is problematic since the X-ray plerion would be larger than the radio plerion. We present here our analysis of an integrated 245 ksec of archival CHAN DRA data acquired with the High-Resolution Camera (HRC) and 520 ksec acquired with the Advanced CCD Imaging Spectrometer (ACIS). This study provides the deepest and highest resolution images obtained to date. The resulting images reveal for the first time 1) a limb-brightened morphology in the eastern section of the halo, and 2) a rich structure in the inner (40 ′′ -radius) bright plerion including wisps and a double-lobed morphology with an axis of symmetry running in the northwest-southeast direction. Our spatially resolved spectroscopic study of the ACIS-I data indicates that the photon index steepens with increasing distance from the central point source out to a radius of 40 ′′ then becomes constant at ∼2.4 in the X-ray halo (for a column density N H =2.2×10 22 cm −2 ). No line emission was found from the eastern limb; however marginal evidence for line emission in the halo's northern knots was found. This study illustrates the need for deep CHAN DRA observations to reveal the missing SNR material in Crab-like plerions.

In 1999, the Chandra X-ray Observatory revealed a 150′′ -radius halo surrounding the 40 ′′ -radius pulsar wind nebula (PWN) G21.5−0.9. A 2005 imaging study of G21.5−0.9 showed that the halo is limb-brightened, and suggested this feature is a candidate for the long-sought supernova remnant (SNR) shell. We present a spectral analysis of SNR G21.5−0.9, using the longest effective observation to date (578.6 ks with the Advanced CCD Imaging Spectrometer (ACIS), 278.4 ks with the High-Resolution Camera (HRC)) to study unresolved questions about the spectral nature of remnant features, such as the limb-brightening of the X-ray halo and the bright knot in the northern part of the halo. The Chandra analysis favours the non-thermal interpretation of the limb. Its spectrum is well fit with a power-law model with a photon index Γ = 2.13 (1.94-2.33) and a luminosity of L x (0.5-8 keV) = (2.3±0.6)×10 33 erg s −1 (at an assumed distance of 5.0 kpc). An srcut model was also used to fit the spectrum between the radio and Xray energies. While the absence of a shell in the radio still prohibits constraining the spectrum at radio wavelengths, we assume a range of spectral indices to infer the 1 GHz flux density and the roll-off frequency of the synchrotron spectrum in X-rays, and find that the maximum energy to which electrons are accelerated at the shock ranges from ∼60-130 TeV (B/10µG) −1/2 , where B is the magnetic field in units of µG. For the northern knot, we constrain previous models and find that a two-component power-law (or srcut) + pshock model provides an adequate fit,

Pulsar wind nebulae (PWNe) studies with the Chandra X-ray Observatory have opened a new window to address the physics of pulsar winds, zoom on their interaction with their hosting supernova remnant (SNR) and interstellar medium, and identify their powering engines. We here present a new 70 ks, plus an archived 18 ks, Chandra ACIS observation of the SNR CTB 87 (G74.9+1.2), classified as a PWN with unusual radio properties and poorly studied in X-rays.We find that the peak of the X-ray emission is clearly offset from the peak of the radio emission by ∼100 ′′ and located at the southeastern edge of the radio nebula. We detect a point source -the putative pulsar -at the peak of the Xray emission and study its spectrum separately from the PWN. This new point source, CXOU J201609.2+371110, is surrounded by a compact nebula displaying a torus-like structure and possibly a jet. A more extended diffuse nebula is offset from the radio nebula, extending from the point source to the northwest for ∼250 ′′ . The spectra of the point source, compact nebula and extended diffuse nebula are all well described by a power law model with a photon index of 1.1 (0.7−1.6), 1.2 (0.9−1.4) and 1.7 (1.5−1.8), respectively, for a column density N H = 1.38 (1.21−1.57) × 10 22 cm −2 (90% confidence). The total X-ray luminosity of the source is ∼1.6 × 10 34 erg s −1 at an assumed distance of 6.1 kpc, with ∼ 2% and 6% contribution from the point source and compact nebula, respectively. The observed properties suggest that CTB 87 is an evolved (∼5−28 kyr) PWN, with the extended radio emission likely a 'relic' PWN, as in Vela-X and G327.1−1.1.To date, however, there is no evidence for thermal X-ray emission from this SNR, and the SNR shell is still missing, suggesting expansion into a low-density medium (n 0 < 0.2 D −1/2 6.1 cm −3 ), likely caused by a stellar wind bubble blown by the progenitor star.Subject headings: ISM: individual (CTB 87, CXOU J201609.2+371110) -ISM: supernova remnants -X-rays: ISM

We report on sensitive new 1.4‐GHz Very Large Array radio observations of the pulsar wind nebula G21.5−0.9, powered by PSR J1833−1034, and its environs. Our observations were targeted at searching for the radio counterpart of the shell‐like structure seen surrounding the pulsar wind nebula in X‐rays. Some such radio emission might be expected as the ejecta from the ≲1000 yr old supernova expand and interact with the surrounding medium. We find, however, no radio emission from the shell, and can place a conservative 3σ upper limit on its 1‐GHz surface brightness of 7 × 10−22 W m−2 Hz−1 sr−1, comparable to the lowest limits obtained for radio emission from shells around other pulsar wind nebulae. In addition, our wide‐field radio image also shows the presence of two extended objects of low surface brightness. We re‐examine previous 327‐MHz images, on which both the new objects are visible. We identify the first, G21.64−0.84, as a new shell‐type supernova remnant, with a diameter of ∼13 arcmin and an unusual double‐shell structure. The second, G21.45−0.59, ∼1 arcmin in diameter, is likely an H ii region.

1WGA J1346.5−6255 is a ROSAT X-ray source found within the radio lobes of the supernova remnant (SNR) G309.2−00.6. This source also appears to coincide with the bright and early-type star HD 119682, which is in the middle of the galactic open cluster NGC 5281. The radio morphology of the remnant, consisting of two brightened and distorted arcs of emission on opposite sides of the 1WGA J1346.5−6255 source and of a jet-like feature and break in the shell, led to the suggestion that 1WGA J1346.5−6255/G309.2−00.6 is a young analog of the microquasar SS 433 powering the W50 nebula. This motivated us to study this source at X-ray and optical wavelengths. We here present new Chandra observations of 1WGA J1346.5−6255, archival XMM-Newton observations of G309.2−00.6, and optical spectroscopic observations of HD 119682, in order to search for X-ray jets from 1WGA J1346.5−6255, study its association with the SNR, and test for whether HD 119682 represents its optical counterpart. We do not find evidence for jets from 1WGA J1346.5−6255 down to an unabsorbed flux of 2.6×10 −13 ergs cm −2 s (0.5-7.5 keV), we rule out its association with G309.2−00.6, and we confirm that HD 119682 is its optical counterpart. We derive a distance of 1.2±0.3 kpc, which is consistent with the distance estimate to NGC 5281 (1.3±0.3 kpc), and much smaller than the distance derived to the SNR G309.2−00.6. We discuss the nature of the source, unveil that HD 119682 is a Be star and suggest it is a new member of the recently proposed group of γ-Cas analogs. The Chandra and XMM-Newton X-ray lightcurves show variability on timescales of hundreds of seconds, and the presence of a possible period of ∼1500 s that could be the rotational period of an accreting neutron star or white dwarf in this γ-Cas analog.