arXiv subjects

George Sonneborn

Publications and source records attributed to George Sonneborn.

39 records · Page 3Linked to original sources

arXiv1997

STIS Near Ultraviolet Time-Tagged Spectra of the Crab Pulsar

We present the spectrum and pulse profile of the Crab Pulsar in the near ultraviolet (1600-3200 Angstroms) observed with the Space Telescope Imaging Spectrograph (STIS) during the Hubble Space Telescope (HST) second Servicing Mission Orbital Verification (SMOV) period. The two-dimensional Near-Ultraviolet Multi-Anode Microchannel Array (NUV MAMA) was used in time-tag mode with a 2 arcsec by 2 arcsec aperture and the low dispersion grating, G230L, to obtain a cube with axes of slit position, wavelength, and time. The observation-derived pulse profile is consistent with radio measurements, and the pulse profile agrees well with previous NUV broadband measurements by the High Speed Photometer. The pulsar spectrum includes the 2200 A dust absorption feature, plus several interstellar absorption lines. Dereddening the spectrum using the Savage-Mathis model with E(B-V)=0.55+- 0.05 leads to a good fit to a power law with slope of -0.3+-0.2. Spectra of the main pulse, the interpulse, and the individual rising and falling edges are similar to the total spectrum within the limits of photon statistics. The pulse profile is stable across the NUV spectral range. Histogram analysis reveals no evidence for the superpulses seen at radio wavelengths. The interstellar absorption line equivalent widths of Mg I, Mg II and FeII are lower than expected based upon the implied HI column density from E(B-V)=0.5. While several explanations are possible, additional studies will be necessary to narrow the options.

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arXiv1997

The structure of the circumstellar gas of SN 1987A

Recent observations of the rings around SN 1987A are discussed and modeled, with particular emphasis on HST observations of the inner ring by the SINS team (P.I.: R. P. Kirshner). It is found that the lowest density detected in the ring is roughly (1-2)E3 cm-3. The geometry of the inner ring is constrained by its different size in [N II] and [O III]. The implications of this on the distance to the supernova are discussed and we find an upper limit of 54.2 +/- 2.2 kpc, which is in agreement with recent RR Lyrae and Cepheid measurements. In addition, preliminary results are presented for improved calculations of the supernova breakout.

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arXiv1996

Non-LTE model atmosphere analysis of the early ultraviolet spectra of nova OS Andromedae 1986

We have analyzed the early optically thick ultraviolet spectra of Nova OS And 1986 using a grid of spherically symmetric, non-LTE, line-blanketed, expanding model atmospheres and synthetic spectra with the following set of parameters: $5,000\le$ T$_{model}$ $\le 60,000$K, solar abundances, $ρ\propto r^{-3}$, $\v_{max} = 2000\kms$, $L=6 \times 10^{4}\Lsun$, and a statistical or microturbulent velocity of 50 $\kms$. We used the synthetic spectra to estimate the model parameters corresponding to the observed {\it IUE} spectra. The fits to the observations were then iteratively improved by changing the parameters of the model atmospheres, in particular T$_{model}$ and the abundances, to arrive at the best fits to the optically thick pseudo-continuum and the features found in the {\it IUE} spectra. The {\it IUE} spectra show two different optically thick subphases. The earliest spectra, taken a few days after maximum optical light, show a pseudo-continuum created by overlapping absorption lines. The later observations, taken approximately 3 weeks after maximum light, show the simultaneous presence of allowed, semi-forbidden, and forbidden lines in the observed spectra. Analysis of these phases indicate that OS And 86 had solar metallicities except for Mg which showed evidence of being underabundant by as much as a factor of 10. We determine a distance of 5.1 kpc to OS And 86 and derive a peak bolometric luminosity of $\sim$ 5 $\times$ 10$^4$ L$_{\odot}$. The computed nova parameters provide insights into the physics of the early outburst and explain the spectra seen by {\it IUE}. Lastly, we find evidence in the later observations for large non-LTE effects of Fe{\sc ii} which, when included, lead to much better agreement with the observations.

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