J. L. Higdon — Page 2

arXiv1998

The gamma-ray burst of 980425 and its association with the extraordinary radio emission from a most unusual supernova

Supernova SN 1998bw exploded in the same direction and at the same time as the gamma-ray burst GRB 980425. Here we report radio observations of this type Ic supernova, beginning 4 days after the gamma-ray burst. At its peak the radio source is the most luminous ever seen from a supernova, nu L_nu = 4 x 10^38 erg/s at 5 GHz. More remarkably, the traditional synchrotron interpretation of the radio emission requires that the radio source be expanding at an apparent velocity of at least twice the speed of light, indicating that this supernova was accompanied by a shock wave moving at relativistic speed. The energy U_e associated with the radio-emitting relativistic electrons must lie between 10^49 erg <U_e< 10^52 erg, and thus represents a significant fraction of the total kinetic energy ~10^51 erg associated with supernova explosions. The presence of a relativistic shock in SN 1998bw suggests a physical connection with the gamma-ray burst GRB 980425. We argue that this represents a second class of gamma-ray burst, with much lower emitted energy ~10^48 erg in gamma-rays than the two powerful ~10^53 erg high-redshift gamma-ray bursts previously identified.

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arXiv1997

The Detection of Massive Molecular Complexes in the Ring Galaxy System Arp 143

We have imaged the kpc scale distribution of 12CO(J=1-0) emission in the ring galaxy Arp 143 (NGC 2444/5) using the OVRO millimeter array. We find two giant molecular complexes in the ring component (NGC 2445) and a bright central source. The ring complexes comprise 20-60% of the detected H_2 mass, depending on the relative I(CO)-N(H_2) for the ring & nucleus. Their individual H_2 masses and surface densities (S(H_2)) exceed typical spiral GMAs regardless of the conversion factor. Both are associated with a 6 kpc ridge of high S(HI) and massive star formation (MSF) activity. H-alpha imaging shows a patchy ring of HII regions situated along the HI ring's outer edge. The latter's kinematics show clear signs of expansion. A simple rotating-expanding ring model (V_exp = 118+-30 km/s) fits the data well, implying a ring age of >~60+-15 Myr. NGC 2445's ring is able to promptly form very large molecular complexes in a metal poor ISM and trigger MSF. Nearly 80% of the detected 12CO(1-0) flux originates in a resolved central source that is slightly offset from NGC 2445's starburst nucleus. We find an ordered velocity field in this component. Assuming an inclined disk, we argue that it is dynamically stable. The central S(H_2) significantly exceeds values typically found in normal spirals, but is much smaller than S(H_2) derived in similar sized regions of IR luminous galaxies. The nuclear H_2 may be the result of a previous encounter with NGC 2444. 12CO(1-0) emission in ring galaxies may be dominated by the nucleus, which could bias the interpretation of single-dish measurements.

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