Engineering topics

Schaefer, B. E.

Publications and source records attributed to Schaefer, B. E..

At least 19 records

NASA NTRS1995

BATSE observations of gamma-ray burst spectra. 2: Peak energy evolution in bright, long bursts

We investigate spectral evolution in 37 bright, long gamma-ray bursts observed with the Burst and Transient Source Experiment (BATSE) spectroscopy detectors. High-resolution spectra are chracterized by the energy of the peak of nu F(sub nu), and the evolution of this quantity is examined relative to the emission intensity. In most cases it is found that this peak energy either rises with or slightly precedes major intensity increases and softens for the remainder of the pulse. Interpulse emission is generally harder early in the burst. For bursts with multiple intensity pulses, later spikes tend to be softer than earlier ones, indicating that the energy of the peak of nu F(sub nu) is bounded by an envelope which decays with time. Evidence is found that bursts in which the bulk of the flux comes well after the event which triggers the instrument tend to show less peak energy variability and are not as hard as several bursts in which the emission occurs promptly after the trigger. Several recently proposed burst models are examined in light of these results and no qualitative conflicts with the observations presented here are found.

Ford, L. A.↗

NASA NTRS1992

A possible galactic association of bursts showing cyclotron lines

Fifteen KONUS bursts and one HEAO burst showing cyclotron lines have well measured positions. If these are due to strongly magnetized neutron stars then we expect them to be associated with the galactic disk with a scale height of about 500 pc as expected for old neutron stars. The simplest way to test this hypothesis would be to test for a concentration along galactic latitude /b/ = 0. We got a marginal indication of such a concentration, and the probability that the 16 events are isotropically distributed is 0.02. We also show that this result is not in conflict with the present BATSE data set.

De Jager, O. C.↗

NASA NTRS1988

Periodicities in gamma-ray burst light curves

Only one case is known where a gamma-ray burst (GRB) light curve indisputably exhibits a periodic behavior. However, a number of GRB light curves have claims for periodicity of varying degrees of plausibility. A vital part of each claim is some mathematical calculation of the significance of the period when compared to some appropriate hypothesis where no modulation is present. For the period search techniques of Fourier transforms, periodograms, and model fitting, well known statistical procedures allow the significance to be evaluated. However, a fourth period search technique, so called 'pattern recognition', does not have any published means of estimating confidence levels. Here, a series of Monte Carlo calculations is presented which will allow evaluation of a period's significance. These results are then applied to periods proposed for 16 bursts.

Schaefer, B. E.↗

NASA NTRS1987

Optical flash background rates

Flash background rates are measured from August 18-September 5, 1985 using the Lowell 0.61-m telescope at CTIO and the 28-cm f/10 Schmidt-Cassegrain telescope at ESO. During 230.1 hours of observing 40 flash events were detected, and it was observed that the background events were dominated by meteors and satellites passing through the field-of-view. The measured flash rate is compared with that of Pedersen et al. (1984), and it is determined that the background flash rate that is applicable to the data of Pedersen et al. for February 8, 1984 is 0.023 events/hr. The morphology of that flash, which appears to be controlled by a gamma-ray burster, is examined.

Schaefer, B. E.↗

NASA NTRS1986

R Coronae Borealis stars and planetary nebulae

IRAS observations of R Coronae Borealis type stars (R CrB's) suggests that a subset of these is inside planetary nebulae (PNs). In most cases, the PN is confirmed by the finding of a visible nebula around the star. These nebular R CrB's are identified as being the results of a final helium shell flash on the central star of old PNs. The majority of the R CrB's formed after the coalescence of a binary consisting of CO and He white dwarfs. Also presented in this paper are the results of a survey of 52 R CrB's. The normal R CrB's have power-law spectra which imply that the grain absorption coefficient varies linearly with frequency. It is estimated that R CrB's eject about 300 clouds per year, each of which subtend an angle of about 30 sq deg.

Schaefer, B. E.↗

NASA NTRS1986

IRAS observations of binaries with compact objects

The infrared emission data, obtained on 260 binary systems by the all-sky IRAS survey in wavelengths between 12 and 100 microns, are reported. Of all the 260 sources, which contained compact objects including white dwarfs, neutron stars, or possibly black holes, only 32 contained detectable IR radiation. The X-ray emitting Be-type stars (gamma-Cas and X Per) were found to have their energy flux proportional to frequency in the range of the log nu values of 12.7-14.7. However, the GS304-1 flux distribution is unique, in that its flux rises by several orders of magnitude as the wavelength changes from 4000 A to 60 microns. A static dust cloud was detected, with a radius of about 1 AU, which has formed around the classical nova RR Pic since its 1925 eruption. The post-eruption far-IR light curve of a classical nova provides strong evidence for IR emissions from both dust grains formed during the eruption and dust grains existing from previous eruptions.

Schaefer, B. E.↗

NASA NTRS1986

H 0323 + 022 - A new BL Lacertae object with extremely rapid variability

Results of a multiband, multiepoch series of observations are reported, demonstrating beyond reasonable doubt that H 0323 + 022 is a remarkably variable BL Lac object. The optical nebulosity around the stellar object is confirmed, the stellar optical radiation is polarized, the radio characteristics and the overall electromagnetic spectrum resemble those of other BL Lac objects. Variability is seen in all bands, including radio and broad-band optical variations in days, optical polarization changes on time scales of tens of minutes, and X-ray changes on time scales of about 30 s. H 0323 + 022 is an active galactic nucleus with much of the observed emission emerging from a region less than about 10 to the 12th to 10 to the 14th cm in size.

Feigelson, E. D.↗

NASA NTRS1986

Searches for low-energy counterparts of gamma-ray bursters

The results of six studies concerning the identification of low-energy counterparts of gamma-ray bursters (GRBs) are reported. From these results, it is concluded that the near-infrared observations of GRBs from IRAS pose serious difficulties for several GRB models. No precedent or plausible explanation is found for the optical flash detected on February 8, 1984 by Pedersen et al. (1984) other than that the flash originated on a GRB. The star in the middle of the 1944 optical transient error box has an ultraviolet excess which is significant at the 95 percent confidence level, a result which strongly suggests that the object is indeed the quiescent GRB optical counterpart. If true, then GRBs are probably low-mass contact binaries caused by thermonuclear or accretion instabilities at distances of roughly 1 kpc.

Schaefer, B. E.↗

NASA NTRS1985

Search for infrared counterparts of gamma-ray bursters

The result of two searches for infrared counterparts of Gamma-ray Bursters (GRB's) is reported. The first search was made using data from the Infrared Astronomy Satellite and covered 23 positions. The second search was made with the Kitt Peak 1.5 m telescope and covered 3 positions. In neither of these two searches was any infrared candidate detected.

Schaefer, B. E.↗

NASA NTRS1985

Gamma-ray burster recurrence time scales

In the decade since gamma-ray bursters (GRBs) were discovered by Klebesadel et al. (1973), many models have been proposed to explain the GRB phenomenon. A difficulty is related to the small number of predictions which would make it possible to evaluate the models. One verifiable prediction is the recurrence time scale, tau-gamma. One method to measure tau-gamma is to look for possible cases of recurrence as indicated by overlapping error boxes. The analysis considered in the present investigation is composed of three procedures. One of these involves a search through known error regions for cases where the error regions overlap. In addition, the number of overlaps expected by chance coincidence alone has been determined, and a calculation has been performed regarding the number of overlaps which are expected due to recurrence for various assumed tau-gamma and luminosity functions.

Schaefer, B. E.↗

NASA NTRS1985

Gamma-ray bursters

Current theoretical models developed to explain the observational data (from spaceborne detectors) on gamma-ray bursters are summarized and illustrated with drawings, diagrams, graphs, and photographic images. Although the data are fragmentary and often flawed by instrument defects, models involving neutron stars with strong magnetic fields are generally favored, and it is assumed that most observed bursters lie within the Galaxy. The neutron-star origin of the bursts is suggested by their intensity and rapid variability (implying a very compact high-energy source) and the presence in some burster spectra of a line at 420 keV which is explained by the combination of electron-positron annihilation and gravitational reddening. Consideration is also given to optical flashes observed to occur about once per year in the direction of gamma bursters, and the need for further searches for lower-energy emissions from bursters is stressed.

Schaefer, B. E.↗

NASA NTRS1984

Two probable optical flashes from gamma-ray bursters

Two images on archival photographic plates which are most likely records of optical flashes from gamma-ray bursters (GRBs) were examined. One of these images appears on a 1901 plate in the field of the Nov. 5, 1979 GRB, while the other is in the field of the Jan. 13, 1979 GRB on a plate exposed in 1944. The 1901 optical transient image is circular in shape, while all normal star images are trailed by 8 in. No optical transients are found in a control region which is 34.3 times larger than the GRB error regions examined. Independent limits on the optical flash rate from the sky yield a probability of less than 0.0001 that any one of the optical transients is due to a background flash. A total exposure of 2.7 years was examined for GRB flashes at known GRB locations on the Harvard plates and a total of three GRB flashes were seen, that the average recurrence time scale for optical flashes is roughly one year. The optical fluence of these optical flashes was measured. For the three currently known GRB optical flashes, the ratio of gamma-ray fluence (from a modern burst) to the optical fluence (from a archival burst) were measured to be 800, 900, and 900.

Schaefer, B. E.↗

NASA NTRS1984

Gamma-ray burster recurrence timescales

Three optical transients have been found which are associated with gamma-ray bursters (GRBs). The deduced recurrence timescale for these optical transients (tau sub opt) will depend on the minimum brightness for which a flash would be detected. A detailed analysis using all available data of tau sub opt as a function of E(gamma)/E(opt) is given. For flashes similar to those found in the Harvard archives, the best estimate of tau sub opt is 0.74 years, with a 99% confidence interval from 0.23 years to 4.7 years. It is currently unclear whether the optical transients from GRBs also give rise to gamma-ray events. One way to test this association is to measure the recurrence timescale of gamma-ray events tau sub gamma. A total of 210 gamma-ray error boxes were examined and it was found that the number of observed overlaps is not significantly different from the number expected from chance coincidence. This observation can be used to place limits on tau sub gamma for an assumed luminosity function. It was found that tau sub gamma is approx. 10 yr if bursts are monoenergetic. However, if GRBs have a power law luminosity function with a wide dynamic range, then the limit is tau sub gamma 0.5 yr. Hence, the gamma-ray data do not require tau sub gamma and tau sub opt to be different.

Schaefer, B. E.↗

NASA NTRS1984

Two probable optical flashes from gamma-ray bursters

Two images on archival photographic plates which are most likely records of optical flashes from gamma-ray bursters (GRBs) were examined. One of these images appears on a 1901 plate in the field of the 5 Nov. 1979 GRB, while the other is in the field of the 13 Jan. 1979 GRB on a plate exposed in 1944. The 1901 optical transient image is circular in shape, while all normal star images are trailed by 8 in. No optical transients are found in a control region which is 34.3 times larger than the GRB error regions examined. Independent limits on the optical flash rate from the sky yield a probability of less than 0.0001 that any one of the optical transients is due to a background flash. A total exposure of 2.7 years was examined for GRB flashes at known GRB locations on the Harvard plates and a total of three GRB flashes were seen, that the average recurrence time scale for optical flashes is roughly one year. The optical fluence of these optical flashes was measured. For the three currently known GRB optical flashes, the ratio of gamma-ray fluence (from a modern burst) to the optical fluence (from a archival burst) were measured to be 800, 900, and 900.

Schaefer, B. E.↗

NASA NTRS1983

More on possible outburst objects Hertzsprung and Popovic

Hertzsprung (1927) and Popovic (1982) report on three unusual images found on archival photographs which could have been caused by optical transients. The original photograph on which the Hertzsprung object appears has been examined, and it is concluded that it is a plate defect. Photographs have been located which were exposed several hours after Popovic's photograpahs which show no unusual objects at or near the transient positions.

Schaefer, B. E.↗

NASA NTRS1983

Implications of the Three GRB Optical Flashes

The implications of three optical transients associated with gamma ray bursts are discussed. Data on the duration, fluence, E sub gamma/E sub opt, optical recurrence time scale, presence of optical precursors or afterflows, and the identification of quiescent gamma ray burst candidates are addressed.

Schaefer, B. E.↗

NASA NTRS1983

Size of a gamma-ray burster optical emitting region

Measured limits on the quiescent optical and IR flux of the Nov. 19, 1978 gamma ray burster event are presented and used to establish a lower limit to the size of the optically emitting region. A total of 305 min of exposure was made, using a CCD with a UV cutoff at 3800 A. It was determined that the size of the optical emitting region is more than an order of magnitude larger than a neutron star. The size of the optical emitting region did not, however, coincide with that of the gamma ray emitting region, indicating that the optical emissions originate from a different area of the region, i.e., from a companion star or accretion disk rather than a neutron star.

Schaefer, B. E.↗

NASA NTRS1983

A large-amplitude photometric periodicity on a T Tauri star

B, V, and R light curvea are presented for the years 1970-1972, during whith the T Tauri star SY Chamaeleontis was observed to undergo precisely periodic brightness oscillations with a 1.6 mag B amplitude and a 6.129-day period. While SY Cha has shown small amplitude irregular variability since 1901, and in 1971 exhibited a sinusoidal light curve in the 13.4-14.2 mag range, this range was found to increase during 1971-1972 to 12.76-14.34 mag. Between 1973 and 1978, the odd behavior pattern was resumed. It is suggested that a localized bright spot formed on, or near, the surface of SY Cha, and that stellar rotation has caused the spot to appear and disapear from the vantage point of the earth.

Schaefer, B. E.↗