AAS Meeting #193 - Austin, Texas, January 1999
Session 74. Supernova Remnants
Display, Friday, January 8, 1999, 9:20am-6:30pm, Exhibit Hall 1

## [74.05] Superluminous Extragalactic Supernova Remnants

C.H.R. Chen, Y.-H. Chu (U.Ill)

Extragalactic supernova remnants (SNRs) are conventionally surveyed by optical emission-line images, using the [S II]/H\alpha line ratio to diagnose SNRs. The majority of the optically identified extragalactic SNRs are too faint to be confirmed at X-ray or radio wavelengths. Conversely, extragalactic SNRs that are initially identified by X-ray or radio observations are all superluminous, e.g., the X-ray SNR in NGC 6946 (Blair & Fesen 1994, ApJ, 424, L103) and the radio SNR in NGC 5471 (Skillman 1985, ApJ, 290, 449).

NGC 5471 is a giant H II region in M101. Optical echelle observations of the SNR in NGC 5471 have detected high-velocity gas with a FWZI of at least 350 km/s. Decomposing the narrow H II component and the broad SNR component in the H\alpha velocity profile, Chu & Kennicutt (1986) derived a total mass of 6500±3000 M\odot and a kinetic energy of a few \times1050 ergs. Using archival ROSAT X-ray observations, Williams & Chu (1995) measured an X-ray luminosity of ~1 \times 1038 ergs/s for NGC 5471. Apparently, the SNR in NGC 5471 is superluminous at all wavelengths.

To determine the physical conditions and nature of the superluminous SNR in NGC 5471, we have obtained HST WFPC2 images of NGC 5471 in the H\alpha and [S II] lines and two continuum bands. These high-resolution images reveal a [S II]-enhanced shell with a diameter of ~60 pc. A recent 180-ks ROSAT High Resolution Imager image of M101 shows that the X-ray emission from NGC 5471 peaks at this SNR shell. We are thus confident in the identification of the superluminous SNR in NGC 5471.

Are superluminous SNRs produced by particularly powerful supernova explosions? Are they associated with gamma-ray bursters? Are their luminosities caused by dense interstellar environment? We will report the detailed physical properties of the SNR in NGC 5471, compare it to the other superluminous SNRs, and address these questions.