HST/Time Series of gamma Cas GHRS observations

This page brings you to an ultraviolet grayscale timeseries of difference spectra for gamma Cas in the neighborhood of the SiIV resonance lines. This is a unique kind of data set for the GHRS.

The spectral data were recorded for over 21 hours while gamma Cas was in HST's "continuous viewing zone" on 1996 March 14-15 using the GHRS 2" Large Science Aperture (LSA). This mode of observing permitted observations to be made continuously except for a few minutes of wavelength recalibration at the end of each orbit and except for interruptions of the South Atlantic Anomaly, which became non-negligible toward the end of the observing sequence. The observations covered 1045 out of a total of 1280 minutes. The data were obtained at a cadence of one spectrum per second in the ACCUM mode (8100 counts per second per pixel) with the G160M grating and were later rebinned to 1 spectrum per minute. The ACCUM mode permitted only a modest resolution (30 km/s) but provided the ability to integrate fluxes unobstructed by an aperture and construct a pseudo-continuum light curve from "non-line pixels." The spectral range covers approximately 1382--1417 Ångstroms.

Several instrumental effects had to be removed to obtain the final spectra. These effects included included fixed pattern noise artifacts from the detector, differential spacecraft velocities, and shifts of the spectrum in both detector coordinates caused by changes in the Geomagnetic field (so-called "GIMP") as the spacecraft traced its orbit. The removal of the velocity effect gives a slight residual wavy appearance to sharp features in the grayscale plot. Finally, photometric variations in a 140 pixels to the red of the Si IV line complex were removed and are presented as a separate data file. Finally, a maximum-spectrum template was obtained from the spectra by submtracting the maximum flux at each pixel from each spectrum of the time series. The data shown are from difference spectra from that template in units of continuum = 1. Full details of the data reductions and of the continuum light curve are given in Smith, Robinson, and Hatzes (1998).

The spectra were taken as part of a simultaneous campaign with the RXTE satellite and contemporaneously with IUE and optical observations two months earlier. A full analysis of the details of the X-ray characteristics are given in Smith, Robinson, and Corbet (1998). The analysis of the quasi-continuum light curve obtained from these and wavelength-integrated IUE data are discussed in terms of occulting magnetic clouds by Smith, Robinson, and Hatzes (1998). The analysis of the variations of the photospheric Si IV lines, the blue-to-red moving (striated) features, and many other faint features away from the Si IV complex is published by Smith and Robinson (1999). An analysis of the Si IV DAC features in the context of X-ray illumination is given by Cranmer, Smith, and Robinson (2000, in preparation).

The "Available Data" consist of spectral and photometric data files. The photometric file lists time from start in minutes and the flux normalized to unity for (normalized for the first two orbits) is a UV pseudo-continuum light curve The spectral files consist of two-column ascii files listing the wavelengths (Ångstroms) and flux in continuum units. The one line identifying header must be stripped before numbers are read in the wavelength and flux files. Postscript and files of the grayscales are scaled in intensity to bring out the global development of the dominant, Discrete Absorption Features of Si IV and also the faint absorptions and striations ("migrating subfeatures") that are ubiquitous in this region of the spectrum. These figures show wavelength from left to right and time in minutes from bottom to top.

Available Data

Copyright Statement: The data presented here were published in the Astrophysical Journal. and appears with the permission of the American Astronomical Society and the authors cited above. Reuse or redistribution of these data is subject to the copyright policies of the American Astronomical Society.