G347.3-0.5 is one of three shell-type supernova remnants in the Galaxy whose X-ray spectrum is dominated by nonthermal emission. This puts G347.3-0.5 in the small, but growing class of SNRs for which the X-ray emission reveals directly the presence of extremely energetic electrons accelerated by the SNR shock. We have obtained new high-resolution X-ray and radio data on G347.3-0.5 using the Chandra X-ray Observatory and the Australia Telescope Compact Array (ATCA) respectively. The bright northwestern peak of the SNR seen in ROSAT and ASCA images is resolved with Chandra into bright filaments and fainter diffuse emission. These features show good correspondence with the radio morphological structure, providing strong evidence that the same population of electrons is responsible for the synchrotron emission in both bands in this part of the remnant. Spectral index information from both observations is presented. We found significant difference in photon index value between bright and faint regions of the SNR shell. Spectral properties of these regions support the notion that efficient particle acceleration is occurring in the bright SNR filaments. We report the detection of linear radio polarization towards the SNR, which is most ordered at the northwestern shell where particle acceleration is presumably occurring. Using our new Chandra and ATCA data we model the broad-band emission from G347.3-0.5 with the synchrotron and inverse Compton mechanisms and discuss the conditions under which this is a plausible scenario.

We present an X-ray study of three mixed-morphology supernova remnants (SNRs), HB 21, CTB 1 and HB 3, using archival ASCA and ROSAT data. These data are complemented by archival Chandra X-ray Observatory data for CTB 1 and XMM-Newton X-ray Observatory data for HB 3. The spectra from HB 21 and HB 3 are well-described with a single-temperature thermal plasma in ionization equilibrium, while a two-temperature thermal plasma is found in CTB 1. We found enhanced abundances in all three SNRs. The elemental abundance of Mg is clearly enhanced in CTB 1, while HB 21 has enhanced abundances of Si and S. The situation is not so clear in HB 3 -the plasma in this SNR either has significantly enhanced abundances of O, Ne and Mg, or it has marginally enhanced abundances of Mg and under-abundant Fe. We discuss the plausibility of mixed-morphology SNR models for the three SNRs and the presence of enhanced abundances. We revise a list of MM SNRs and their properties, compare the three SNRs studied here with other members of this class, and discuss the presence of enhanced elemental abundances in MM SNRs. We also report the ASCA detection of a compact source in the southern part of HB 3. The source spectrum is consistent with a power law with a photon index of ∼ 2.7, and an unabsorbed X-ray flux of ∼×10 −12 ergs cm −2 s −1 in the 0.5-10.0 keV band. The column density towards this source differs from that towards the SNR, and it is therefore unlikely they are related. Subject headings: radiation mechanisms: thermal -supernova remnants -ISM: individual(HB 21 (G89.0+4.7), CTB 1 (G116.9+0.2), HB 3 (G132.7+1.3)) -X-rays: ISM 1 current addres

We present the discovery of nonthermal radio and X-ray emission positionally coincident with the TeV gammaray source HESS J1813Ϫ178. We demonstrate that the nonthermal radio emission is due to a young shell-type supernova remnant (SNR), G12.8Ϫ0.0, and constrain its distance to be greater than 4 kpc. The X-ray emission is primarily nonthermal and is consistent with either an SNR shell or unidentified pulsar or pulsar wind nebula origin; pulsed emission is not detected in archival ASCA data. A simple synchrotron plus inverse Compton model for the broadband emission assuming that all of the emission arises from the SNR shell implies maximum energies of [B/(10 mG)] Ϫ0.5 TeV. Further observations are needed in order to confirm that the broadband emission has a common origin and to better constrain the X-ray spectrum.

A B S T R A C TPositions with subarcsecond accuracy have been measured for seven 22-GHz H 2 O masers associated with H II regions in the Large Magellanic Cloud (LMC); two of the masers are new detections. Initial position measurements were obtained with the 70-m antenna of the Canberra NASA Deep Space Network during a period of more than two years in which the antenna was used to monitor the maser emission. The positions were further improved using 22-GHz observations involving three antennas of the Australia Telescope Compact Array.The positions have been compared with those of 1.6-GHz continuum emission and other LMC masers (of OH and CH 3 OH). The H 2 O maser positions range from within 1 arcsec (270 mpc) of the centre of a compact H II component to beyond the boundary of significant continuum emission. Three of the four masers located near continuum peaks are close to OH masers. In two cases the positional agreement is better than 0.2 arcsec (53 mpc); in the third case the agreement is worse (0.9 arcsec) but the positions of the individual H 2 O features appear to be spread over more than 1 arcsec. The velocities of the OH masers are within the spread of the H 2 O velocities. The three H 2 O masers offset from continuum centres are located 3 -7 arcsec from optical or infrared phenomena probably associated with very early stages of star formation; no other molecular masers are known in these directions.