Engineering topics

Bradt, H. V.

Publications and source records attributed to Bradt, H. V..

At least 19 records

NASA NTRS1994

The X-ray selected cataclysmic variables H0459 + 246 and H0857 - 242

We report the discovery of two cataclysmic variables located with the assistance of X-ray positions from the HEAO 1 Modulation Collimator and the Large-Area Sky Survey. Each case is distinguished by evidence of two periodic modulations that appear to represent the orbital period and the spin period of the white dwarf, respectively. The first case, H0459 + 246, has been observed optically during faint states (V approximately 16), in which there are spectral features of both an accretion disk and a K star. The light curves in the V and I bands are consistent with ellipsoidal variations in the secondary stars with a binary period of 9.952 hr. X-ray observations with EXOSAT reveal a strong pulsation with a period approximately 62 minutes. This result confirms an 'intermediate polar' classification for H0459 + 246. The pulsation is observed at 63.2 minutes in the optical I band. The long orbital period opens the possibility that H0459 + 246 is a relatively young intermediate polar that might evolve into a polar. The second case, H0857--242, shows radial velocity modulations at 1.78 hr along with photometric variations at a period approximately 49 minutes. The latter are observed only during the decay phase of dwarf nova outbursts (13 less than V less than 17), which apparently recur frequently. Given the lack of X-ray monitoring observations and the absence of proof that the 49 minute periodicity is coherent over long time-scales, we regard H0857--242 as a candidate intermediate polar. Photographic records from th e Harvard Observatory Plate Library further reveal superoutbursts for H0857--242 (V approximately 11). A bright X-ray source that is both an intermediate polar and a continually cycling dwarf nova may provide an effective means of measuring the time delay for the arrival of accreting matter at the white dwarf surface, relative to the onset of optical brightening.

Remillard, R. A.↗

NASA NTRS1993

X-ray timing explorer mission

The XTE will address a number of fundamental questions about the nature of the cosmos. The large effective area (about 0.8 sq m total) and broad band of sensitivity (2-200 keV) of its three instruments make it especially valuable for timing of intensity variations and for the determination of broadband spectra from high-energy sources. For the first time, studies of variability ranging from about 1 microsecond to several years will be carried out. XTE's design and flexibility of operations will allow it to respond rapidly to changes in the X-ray sky (within hours) and will facilitate multifrequency observations. It is scheduled for launch on a Delta II vehicle prior to April 1996.

Bradt, H. V.↗

NASA NTRS1992

H0538 + 608 (= BY Camelopardalis) - An asynchronously rotating AM Herculis binary?

Evidence from a concurrent X-ray/optical observing program is presented which indicates that the orbital period of the anomalous AM Her-type cataclysmic variable H0538 + 608 (BY Cam) differs from the rotational period by 1.3 +/- 0.1 percent. This finding would explain much of the unusual behavior previously exhibited in optical photometry and polarimetry. The model proposed by Ishida et al. (1991) is developed further using observations of accretion and the primary characteristics of optical and X-ray light curves and radial velocity measures of H-alpha. An optical QPO with a period of about 30 min was seen on two nights and possible causes of it are discussed.

Silber, A.↗

NASA NTRS1991

A rapid energetic X-ray flare in the quasar PKS0558 - 504

A recent observation from the Ginga satellite of the quasar PKS0558 - 504 is reported during which the X-ray flux increased by 67 percent in the space of only three minutes. There was no significant change in the spectrum. Comprehensive analysis of the data strongly indicates that this was a genuine X-ray flare originating in the quasar. The implied rate of change in luminosity in the 2-10 keV range, assuming a Hubble constant of 70 km/s/Mpc, and a cosmological deceleration parameter q0 = 0.5, is 3.2 x 10 to the 42nd erg/sq s, the highest value measured for a quasar. When photon scattering is considered, this is about 16 times greater than could be produced, with a three-minute rise time, in an isotropically emitting plasma. It is argued that the apparent luminosity must be enhanced by relativistic beaming. This is the first indication of beaming in an 'ordinary' unpolarized quasar.

Remillard, R. A.↗

NASA NTRS1991

Ginga observation of an AM Herculis type source H0538 + 608

Results from a Ginga observation of the AM Her type object H0538 + 608 are presented. During the first half of the observation (pulsing state) the source showed periodic modulations caused by the white dwarf rotation, and the modulation profile was characterized by a flat top and a flat bottom. During the second half, the source was in a remarkable flaring state and the periodic modulation disappeared. The dramatic change of the X-ray light curve is interpreted in terms of a change of accretion pattern between an eclipsing spot and a noneclipsing spot. The spectrum is well described throughout by a thin thermal model including an iron K-line with equivalent width of some hundred eV, supporting the standard model for the hard X-ray emission region. The temperature in the pulsing state was found to change with the spin phase of the white dwarf. In the flaring state, the temperature showed a positive correlation with the source luminosity.

Ishida, M.↗

NASA NTRS1991

The X-ray Timing Explorer

The capabilities and projected mission objectives of the X-ray Timing Explorer (XTE) are set forth with attention given to the components of the space explorer. The XTE instrument complement is reviewed and specific data and graphic descriptions are presented regarding the large-area proportional counter array, the all-sky monitor, and the high-energy crystal scintillation experiment. The XTE provides low-background continuous 2-200 keV response with large apertures, rapid maneuverability, and detailed energy-resolved studies of phenomena close to neutron stars. The synergism of the mission capabilities is shown to permit a powerful mechanism for timing and broad-band spectral studies after deployment by the NASA Explorer Platform in approximately 1995.

Bradt, H. V.↗

NASA NTRS1990

1H 0709-360 - A cataclysmic variable with an orbital period inside the 'period gap'

The discovery is reported of the second cataclysmic variable with a binary period lying well inside the nominal 2-3 hr period gap. The object, 1H 0709-360, is an X-ray source which has been identified on the basis of positional data from the instruments of the HEAO 1 survey. The X-ray source was also detected by the Uhuru and Ariel 5 surveys. Multicolor photometry has revealed eclipsing behavior at an orbital period of 2.444 hr. Radial velocity measurements show modulations at the same period, with an offset between the spectroscopic and photometric conjunctions. Both the eclipse profile and a clear rotational disturbance in the radial velocities of emission lines during eclipse provide unambiguous evidence for the existence of an accretion disk. Thus the object cannot be an AM Herculis variable, a conclusion supported by the absence of circular polarization. Instead, the evidence suggests that the object could be a DQ Herculis magnetic variable that is close to synchronism. The evolutionary implications of this rare object are discussed.

Tuohy, I. R.↗

NASA NTRS1990

The X-ray Timing Explorer

The capabilities of the X-ray Timing Explorer (XTE) are described with particular attention paid to current scientific problems it will address, from galactic neutron star systems to active galactic nuclei. It features a low-background continuous 2-200 keV response with large apertures (a 0.63 sq m proportional counter array and a 0.16 sq m dual rocking NaI/CsI scintillation array). Rapid response (in hours) to temporal phenomena, e.g., transients, is obtained by virtue of a scanning all-sky monitor and rapid maneuverability. The XTE will carry out detailed energy-resolved studies of phenomena close to neutron stars (e.g., quasi-periodic oscillations) because of its submillisecond timing (to 10 microsec), its high telemetry rates (to 256 kbs), and the high throughput of its data system (to greater than about (200,000 c/sec).

Bradt, H. V.↗

NASA NTRS1990

The X-ray Timing Explorer

The capabilities of the X-ray Timing Explorer (XTE) are described with particular attention paid to current scientific problems it will address from galactic neutron star systems to active galactic nuclei. It features a low-background continuous 2-200 keV response with large apertures (a 0.63 sq m proportional counter array and a 0.16 sq m dual rocking NaI/CsI scintillation array). Rapid response (in hours) to temporal phenomena, e.g. transients, is obtained by virtue of a scanning all-sky monitor and rapid maneuverability. XTE will carry out detailed energy-resolved studies of phenomena close to neutron stars (e.g. QPO's) because of its submillisecond timing, its high telemetry rates, and the high throughput of its data system.

Bradt, H. V.↗

NASA NTRS1987

Discovery of three new RS Canum Venaticorum-like counterparts to HEAO I X-ray sources

The identification of three high-latitude HEAO I Scanning Modulation Collimator X-ray sources with the chromospherically active RS CVn-like stars HD 113816, HD 146413, and HD 39576 is reported. Optical observations, including coude spectroscopy and broad-band and narrow band photoelectric photometry are presented. The Ca II emission strength of all three stars shows that they are chromospherically active. HD 146413 and HD 39576 exhibit variable X-ray emission in the 1-13 keV energy range, while HD 113816 is a softer and steadier source. The level of X-ray flux detected from these three stars is some one to two orders of magnitude higher than predicted empirically from the Ca II emission fluxes. It is proposed that this emission results from flarelike activity.

Buckley, D. A. H.↗

NASA NTRS1986

Identification of two southern X-ray emitting cataclysmic variables

The optical identification of two faint HEAO 1 X-ray sources with previously uncataloged cataclysmic variables is reported. The two cataclysmic variables, 1H 0542 - 407 and H0534 - 581, have average V magnitudes of about 15.7 and about 14.9, and have similar high excitation spectra with moderate to strong He II 4686 A emission. X-ray measurements and multicolor optical photometry for both objects establish 1H 0542 - 407 as a new DQ Herculis magnetic variable having a white dwarf rotation period of about 1920 s and an orbital period near 6.2 hr. 1H 0542 - 407 is unusual among DQ Her systems in having a large soft X-ray pulse fraction of about 70 percent semiamplitude below 1 keV. No definitive X-ray or optical periodicities were detected from H0534 - 581, but it appears to be either a nova-like variable or a DQ Her system. Both cataclysmic variables have very hard X-ray spectra, with evidence for iron line emission at about 6.7 keV from 1H 0542 - 407.

Tuohy, I. R.↗

NASA NTRS1986

Simultaneous X-ray and optical observations of GX 339-4 in an X-ray high state

Optical emission, soft X-rays, and hard X-rays have been observed simultaneously from the black hole candidate GX 3339-4 in a typical high state. Each of these components is interpreted as arising from a characteristic region of the accretion disk. Considered as a black hole, GX 339-4 lacks the variable 2 keV blackbody component that would be emitted from the 'solid' surface of a neutron star in the low-mass neutron star binaries. The X-ray emission in the high state is therefore extremely soft and stable in time. The intermediate disk region is optically thick and geometrically thin. The observed soft X-ray component with a 'disk blackbody' spectrum comes from this region. The innermost disk region is dynamically unstable due to the effect of general relativity and possibly due to radiation pressure. This region is responsible for the generation of the hard X-ray tail through Comptonization.

Makishima, K.↗

NASA NTRS1986

H0538 + 608 - A bright AM Herculis-type X-ray source

The discovery of an AM Herculis-type magnetic variable, an accreting binary system that contains a magnetic white dwarf and a late-type companion, is reported. The proposed optical counterpart to the detected X-ray source exhibits all the optical emission characteristics of its class: circular polarization, strongly modulated brightness variations, intense flickering on time scales of seconds, historical 'low states', and intense emission lines, including very strong lines of He II. The modulations in the polarization and the photometric magnitude suggest an orbital period of 3.1 + or -0.2 hr.

Remillard, R. A.↗

NASA NTRS1986

The discovery of eight bright active galactic nuclei observed with the HEAO 1 scanning modulation collimator

The eight 14th- and 15th-magnitude active galactic nuclei (AGNs) considered include one QSO and seven type 1 Seyferts; all were discovered in the course of a systematic program for the identification of the fainter X-ray sources of the HEAO 1 survey. While the X-ray and optical luminosities, and many of the optical emission features of the AGNs resemble other bright Seyfert 1s nearby, three of the galaxies exhibit very strong Fe II emission, and two show very weak forbidden line spectra. Photometric magnitudes and selected emission line intensities are given for each of the AGNs.

Remillard, R. A.↗

NASA NTRS1985

Optical observations of new X-ray emitting cataclysmic variables

Two X-ray emitting cataclysmic variables were discovered as a result of a program to identify faint X-ray sources located by the HEAO-A3 modulation collimator experiment. The objects, H0534-581 and 1H0542-407, have similar high excitation spectra with intense emission lines showing broad and narrow components. Photometry shows that H0534-581 and 1H0542-407 have average magnitudes of V=14.9 and 15.7 respectively, leading to Lx/Lopt ratios of 1.5 (typical of dwarf novae in quiescence). While pronounced variability of delta B or = 1.5 mag is observed from both systems, there is no clear evidence for repeatable orbital periods between 1 and 4 hr.

Tuohy, I. R.↗

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

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 NTRS1982

Circinus X-1 - X-ray observations with SAS 3

Eight observations of Cir X-1 with SAS 3, each lasting 1-6 days, have yielded a variety of new phenomena, viz., a luminous state of steady emission, rapid large-intensity dips, an extremely rapid X-ray transition, and bright flares. Through searches for periodic X-ray pulsations were carried out on data trains of duration up to 6 days; upper limits for pulsations with periods greater than 250 microsec range down to 0.3%. Aperiodic variability with characteristic times of 0.4-1.0 sec was observed but is not well characterized by a simple shot noise model. No millisecond bursts were observed during 40,000 sec in three separate observations. Spectral parameters derived before and after several X-ray transitions indicate that the transitions are not due to absorption of X-rays by intervening gas. Models previously proposed for the Cir X-1 system do not easily provide explanations for all the complex phenomena reported herein.

Dower, R. G.↗