Vela Satellites Research Articles

Observations of cosmic gamma-ray bursts

The occasional occurrence of brief but intense bursts of cosmic gamma-rays was disclosed through a systematic search of data acquired from the Vela satellites. Confirmation of the nature of the events and additional detail of their characteristics has subsequently been provided by 15 other groups of experimenters with instruments on 13 spacecraft. Thirty-nine such events have been identified from data spanning a period of four and a half years. The record of intensity as a function of time varies considerably for different events, with total durations ranging from 0.1 to ∼ 60s. Time-integrated flux density ranges from ∼10−6 to 10−3 erg cm−2. Spectral measurements have been accomplished by several groups of experimenters, showing a broad maximum in the energy distribution at about 150 keV. The distribution of source directions implies either near galactic or extragalactic locations. The existing data are not sufficient to distinguish between the various models proposed to explain the phenomenon; no model is completely consistent with all observed characteristics.

Analysis of narrow spikes in two cosmic gamma-ray bursts

An analysis has been made of the narrow intensity spikes occurring in two cosmic gammaray bursts which were observed with a fast time resolution germanium spectrometer on board the low altitude polar orbiting satellite 1972-076B (Imhofet al., 1974a, 1975; Nakanoet al., 1974a). The bursts on 18 December, 1972 and 21 July, 1973, were also recorded on the Vela satellite system. The durations of three spikes were observed to be ∼0.06 s with the limits being 0.03 to 0.10 s. Four other narrow spikes were measured to have time width limits between 0.001 and 0.10 s. An additional eight spikes had widths less than 0.9 s. During the spikes, the gamma-ray intensities increased by factors of ∼2 to ∼10, with a median value of ∼3. These and other characteristics of the fast time structure are presented.

Expected Rate of Transient Events from Stellar Deaths in Other Galaxies

view Abstract Citations (5) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Expected Rate of Transient Events from Stellar Deaths in Other Galaxies Talbot, Raymond J., Jr. Abstract The expected rates for 7-ray, microwave radio, neutrino-antineutrino, or gravitational wave bursts are discussed under the assumption that they are produced with rate proportional to the rate of observed optical supernovae VSN (from Tammann). For bursts with energy E = 1048E48 ergs, a detection of 1 event per year per 4 steradians requires a sensitivity of S = 8.4 x 10- 5 48(ViVsN)213 ergs cm-2. The 7-ray bursts from the Vela satellites are discussed. The frequency and distribution in the ky sare shown to be consistent with their being extragalactic, and that interpretation provides the constraint 1. ;{$ (f )8/2 Subject headings: galaxies: stellar content - gamma rays: bursts - stars: supernovae Publication: The Astrophysical Journal Pub Date: April 1976 DOI: 10.1086/154307 Bibcode: 1976ApJ...205..535T full text sources ADS | data products NED (7)

Gradients of solar protons in the high-latitude magnetotail and the magnetospheric electric field

Energetic particle detectors on the Vela satellites have the capability of making three-dimensional measurements of particle fluxes. We report observations, using these instruments, of directional anisotropies of solar proton flux in the high-latitude magnetotail (i.e., the lobes of the magnetotail). The anisotropies last for several minutes and are associated with sudden changes in the intensity of the proton flux. The measurements suggest that the gradients in the proton flux are perpendicular to the magnetic field and move past the satellites toward the midplane of the tail, i.e., toward the plasma sheet. Our observations constitute the first measurements of the E × B drift of energetic protons in the tail lobes and provide a means for measuring the associated electric field. Measured values of the electric field fall in the range ∼0.5–1.3 mV/m, which would drive convection at ∼30–80 km/s in a typical magnetotail magnetic field of ∼15 γ. The observations are consistent with an open model of the magnetosphere which has magnetic field lines from the earth's polar regions extending into the solar wind and, along with solar proton fluxes that are associated with these field lines, being convected into the magnetosphere as they are pulled downstream by the solar wind.

Proton flow measurements in the magnetotail plasma sheet made with Imp 6

Patterns of proton flow in the magnetotail plasma sheet are described. They are derived from observed anisotropies of the flux of protons measured with the Los Alamos Scientific Laboratory (LASL) plasma probe on Imp 6. These observations, made in the geocentric distance range rapprox. =25--32 R/subE/, are entirely consistent with those made at rapprox. =18 R/subE/ with Vela satellites and reported earlier. In particular, the substorm thinning and thickening of the plasma sheet are seen and are accompanied by antisunward, then sunward, flow of plasma as they are at the Vela orbit. During one substorm the Goddard Space Flight Center magnetometer on the satellite clearly showed the Z component of the magnetic field in the plasma sheet to be southward throughout the expansion phase, while antisunward flow prevailed, and then to become suddenly northward again as the flow turned sunward at the substorm's recovery. This observation of the magnetic field behavior provides important confirmation of the view, regarding neutral line formation and movement during substorms, that has been established earlier on the basis of plasma flow measurements made by the Vela satellites. A dawn-to-dusk cross-tail proton flow comprising a proton current of approx.10$sup 7$ protons/cm$sup 2$ -s(approx.1.6x10$sup -12$ A/cm$sup 2$)more » was observed as the magnetometer indicated that the satellite was crossing the magnetic 'neutral sheet.' This observation further supports an earlier suggestion that dawn-to-dusk cross-tail proton flows, measured occasionally with the Vela satellites, constituted the magnetotail neutral sheet current. (AIP)« less

Complete plasma dropouts at vela satellites during thinning of the plasma sheet

Five satellite years of Vela data are examined for plasma sheet thinnings. Complete proton disappearances (plasma dropouts) are the main subject here. During such times, the Vela satellite is temporarily in the high-latitude tail lobe. The distribution of such plasma dropouts within the magnetotail suggests that the semithickness of the plasma sheet near midnight seldom reaches less than 1 RE during substorms and that the dawn and dusk portions of the plasma sheet remain thicker than the midnight portion. But it is also shown that the plasma sheet occasionally becomes very thin near the dusk magnetopause. This seems to occur when the magnetopause near the dawn and dusk meridians lies further away from the earth than is normal. Extension of the magnetopause to these greater distances does not seem to be related to magnetically quiet periods nor to the Z component of the interplanetary magnetic field (IMF). No such severe thinnings of the plasma sheet are found near the dawn magnetopause. Plasma dropouts can occur regardless of the sign of the Z component of the IMF, but their frequency of occurrence seems to be greater when the Z component is negative. The decreased frequency of occurrence with northward IMF may reflect weaker thinning of the plasma sheet at ∼ 18 RE during the weaker substorms that occur then. Weaker thinning of the plasma sheet may also imply a greater than normal distance from the earth to the magnetic neutral line thought to form in the magnetotail at substorm onset. Three plasma dropouts which occurred in the midnight sector at unusually large distances from the estimated position of the neutral sheet were observed during geomagnetic storms. It is likely that the midnight sector of the plasma sheet can become very thick (∼18 RE) at certain times during the main phase of storms. Detailed measurements in the plasma sheet were obtained near the beginning of a geomagnetic storm whose sc triggered a substorm. A compression of the plasma sheet at XSM ≈ −15 RE occurred about 10 min after the sc onset at the earth and about 5 min after the start of plasma sheet thinning associated with the sc-related substorm. If compression-thinning of the plasma sheet initiated this substorm, the triggering action must have occurred earthward of XSM ≈ −15 RE.

Size distribution of cosmic gamma-ray bursts

WE report on the combined results of three balloon-borne searches for small cosmic γ-ray bursts, carried out by the Imperial College Space Physics Groups. We suggest that our results are incompatible with the extragalactic source postulated for the large events previously seen by satellites. No decisive directional information has been obtained from the timing measurements, carried out by the Vela satellites on the large (10–100 photon cm−2s−1 (> 100 keV)) events, although a weak correlation with the local spiral arm exists1.

Results of X-ray and optical monitoring of Scorpius X-1 in 1970

Scorpius X-1 was monitored at optical and X-ray wavelengths from 1970 April 26 to 1970 May 20. The optical observations were made at six observatories around the world, and the X-ray observations were made by the Vela satellites. There was a tendency for the object to show greater variability in X-ray emission when the object was optically bright. The intensity histograms for both the optical and X-ray observations are discussed, as well as periodic variations in the optical intensity.

Response of the plasma sheet at ∼18REto sudden southward turnings of the interplanetary magnetic field

Simultaneous measurements of the interplanetary magnetic field (IMF) by Explorer 35 and of the magnetotail plasma by Vela satellites are examined for the period August–October 1970. Nineteen cases are found in which the IMF turns southward more than 40° (|Δθ| > 40°) in less than 10 min and a Vela satellite, operating in a high time resolution mode, is in the plasma sheet. Twelve of these cases show a large decrease of plasma pressure, or the so-called plasma sheet thinning, during the subsequent period of southward IMF. Polar magnetic substorms occur at the earth in conjunction with these twelve plasma sheet thinnings. On the other hand, seven cases show only slight plasma sheet thinnings, and there was no indication of substorms at the standard auroral zone stations. Thus it is likely that the southward IMF alone does not reduce significantly plasma pressure in the plasma sheet and that a large decrease of plasma pressure is associated with a substorm.

Observations of Moon‐plasma interactions by orbital and surface experiments

Extensive magnetic field observations together with crucial plasma measurements by the Explorer 35 lunar orbiter and Apollo surface and orbital experiments have established the basic nature of the moon's interaction with the solar wind and interplanetary magnetic field. The effective absorption of the incident solar wind by the moon creates a plasma void or cavity behind the moon. The cavity‐associated magnetic signature is characterized by an enhancement in magnetic field magnitude B within the cavity as compared with the mean level of B in the surrounding interplanetary plasma and dips or decreases in B near the cavity boundaries with the solar wind. The axis of the lunar wake is aberrated from the moon‐sun line by the relative velocity of the solar wind with respect to the moon, and the cross section of the wake is elliptical, reflecting the anisotropic propagation of magnetoacoustic waves in the solar wind. Enhancements in B are often observed just external, i.e., on the solar wind side, of the rarefaction‐associated dips in the field magnitude. These are of perturbation magnitude at the altitude of Explorer 35, whereas at the low height of the subsatellites they are often large field increases or limb compressions that occur either just upstream of or directly above lunar limbs. Particular surface regions of the moon seem more effective than others as a source of the limb compressions; the pertinent regional property is likely to be remanent magnetization. Propagation of the limb disturbances downstream from the source and the efficacy with which a regional property can create a disturbance in the solar wind flow past the moon's limb depend on properties of the plasma and interplanetary magnetic field. The main enhancement of B in the shadow region behind the moon is a more conspicuous feature of the magnetosheath interaction, whereas the field strength dips and the limb compressions become more difficult to identify in the highly variable state of the magnetic field. In the lobes of the geomagnetic tail, remanent lunar magnetic fields are readily detected at the subsatellite altitude. There is a distinct structure associated with the average field magnitude measured by the Apollo 15 subsatellite in the plasma sheet of the geomagnetic tail; B is enhanced over the day side quadrant of the subsatellite orbit. This variation in the average magnitude of the magnetic field may be associated with the drift of plasma sheet particles toward the earth, creating a wake region centered approximately on the day side orbital quadrant of the subsatellite. Alternatively, flux tube depletion earthward of the moon may explain the observed day side enhancement of average magnetic field magnitude. Apollo particle and field measurements on the lunar surface have provided evidence of a regional interaction of the highly conducting solar wind with lunar remanent magnetic fields. Simultaneous Ogo 5, Vela 5, and Apollo 12 spectrometer data of ion density and velocity taken when the Ogo and Vela satellites were in the solar wind and the Apollo spectrometer was on the lunar day side also exposed to solar wind plasma show that the proton velocity is smaller and the proton density is larger at the Apollo site than in the free stream solar wind, a result of proton deceleration at the site by an electric field established via the plasma‐remanent magnetic field interaction. Simultaneous plasma and magnetic field data, from the spectrometer and the lunar surface magnetometer at the Apollo 12 location, show the compression of the local remanent field by large solar wind and magnetosheath plasma dynamic pressures.

An attempt to detect the effects of cosmic gamma-ray bursts in the lower ionosphere

DETECTION of gamma-ray bursts of cosmic origin by detectors onboard Vela, OGO, IMP-6 and OSO-7 satellites1–3 was one of the major surprises of observational astronomy recently. The bursts were observed over the energy range 7 keV to 1.5 MeV and were found to have time durations ranging from less than a second to about 80 s with integrated flux density between a few times 10−6 and 3 × 10−4 erg cm−2 for different events. From the 20 events that have been detected so far (from data accumulated over 5 yr) it seems that these events are distributed almost isotropically on the celestial sphere and occur at frequencies of about four to five each year, at the level of sensitivity of Vela satellites. In view of the absence of definitive positive associations with well known transient phenomena1,4 such as supernovae, galactic radio noise spikes, rapid atmospheric fluorescence increases, Cygnus X-3 radio flares or even with gravitational radiation events, continuous patrol for detecting these events through as many independent techniques as possible is very important to understand the nature of their origin.

Interpretation of double structure in the celestial γ-ray bursts

THE essentially isotropic distribution of the Vela satellite γ-ray bursts1 implies that the sources are either very local and galactic, or very remote and extragalactic. Of the theories for a remote origin there are two schools of thought, either that these γ-rays originate in shock waves during the formation of type II supernovae2 or that they represent transient pulses of blackbody emission associated with the collapse of a white dwarf to a neutron star3.

Prolonged tailward flow of plasma in the thinned plasma sheet observed atr≈ 18REduring substorms

Measurements of plasma and energetic electrons made during substorms by Vela satellites within ≲1 RE of the magnetotail neutral sheet are reported. Rapid antisunward flow of plasma is observed to start at the onset of the expansion phase of a substorm (i.e., at breakup) and to continue until the expansion phase ends and substorm recovery begins, as is indicated by auroral zone magnetograms. When substorm recovery begins, the plasma flow reverses direction, and rapid sunward flow begins. Fluxes of energetic electrons are low, and the temperature of the plasma is reduced during the interval of antisunward flow. The lowest fluxes (i.e., complete dropouts) of energetic electrons are observed just before the reversal of plasma flow to the sunward direction. After the reversal the energetic electron flux intensity rises rapidly to high values as the plasma sheet recovers. These observations are regarded as added evidence that a neutral line forms between the earth and the Vela orbit (r ≈ 18 RE) at substorm breakup and then moves suddenly to distances beyond the Vela orbit when substorm recovery starts. Throughout the expansive phase there exists a thin residual plasma sheet beyond the neutral line in which plasma continually flows rapidly downstream into the more distant tail. The particle dropout just preceding the flow reversal and plasma sheet recovery is thought to be the signature of the passage of the neutral line near the Vela satellite as it moves rapidly outward in the tail. Spin modulation of the proton flux observed continuously for about 1 hour during very quiet conditions preceding one substorm may constitute a direct measurement of the proton component of the cross-tail electric current that maintains the taillike configuration of the magnetotail field.

A Preliminary Catalog of Transient Cosmic Gamma-Ray Sources Observed by the VELA Satellites

A Preliminary Catalog of Transient Cosmic Gamma-Ray Sources Observed by the VELA Satellites

Association of plasma sheet thinning with neutral line formation in the magnetotail

Magnetotail electron observations by Vela satellites (at a radial distance of about 18 RE) and tail magnetic field observations by Explorer 34 (when its distance is beyond 25 RE) are combined. They support the belief, advanced earlier, that during substorms a neutral line is formed in the near-tail region, earthward of the Vela orbit, and its formation leads to the rapid (further) thinning of the plasma sheet beyond its distance. The intervals with the neutral line and the associated thin plasma sheet have a duration of 0.5 to 1 hour, and they appear intermittently after intervals of energy storage in the magneto tail. In the ground magnetograms from the auroral zone the formation and the disappearance of the neutral line are reflected most sensitively around the premidnight region and correspond, respectively, to the intensification and the peak (and beginning of recovery) of the negative bay activity.

Cosmic Gamma-Ray Burst Detected with an Instrument on Board the OGO-5 Satellite

Gamma-ray bursts of cosmic origin have recently been detected by instruments on the Vela satellites. We now confirm the detection of the June 30, 1971 event with an instrument on board the OGO-5 satellite. The intensity of this burst is calculated to be approximately 100-200 photons per sq cm/sec for photons of energy greater than 150 keV with an upper limit of 50 photons per sq cm/sec for the intensity above 5 MeV. An upper limit of one-third of the intensity of the June 30, 1971 event is set for 10 other events studied.

Gamma Rays: From Neutron Stars, Supernovas, or "Beebees"?

The gamma-ray bursts first detected by the Vela satellites are discussed and six theories as to their origin are summarized. (WDM)

Many models proposed to explain cosmic gamma‐ray bursts

The various wavelength regimes of observational astronomy seem to generate their maximum excitement when rapid time‐varying phenomena are first observed. For example, radioastronomy gave us pulsars; x‐ray astronomy has its flares and pulsating sources. Now it is the turn of gamma‐ray astronomy. Ray Klebesadel, Ian Strong and Roy Olson (Los Alamos) recently reported finding 16 short bursts of cosmic gamma radiation in three‐years‐worth of data from the Vela satellite program.

Magnetotail and boundary layer plasmas at a geocentric distance of ∼18RE: Vela 5 and 6 observations

At the distance of the Vela satellite orbits (r ≈ 18 RE) there are three distinct plasma environments associated with the earth's magnetotail: the plasma sheet, the high-latitude tail (or tail lobes), and the magnetotail boundary layer. This paper reports measurements of the plasma properties in these three regions made with electrostatic analyzers on Vela satellites 5A, 5B, 6A, and 6B. Evidence presented suggests that the plasma temperature profile across the thickness of the plasma sheet may be nearly uniform and very cool (proton average energy ≈1 kev) after long geomagnetically quiet periods; the profile may be nearly uniform and very hot (proton average energy ≈20–30 kev) immediately after a substorm. Except during these extreme cases the central part of the plasma sheet is hottest, and there is a gradual but substantial cooling toward the upper and lower edges. The magnetotail boundary layer, containing plasma less dense than that in the magnetosheath and flowing antisunward at less than magnetosheath flow speeds, is often very thin near the tail's magnetic midplane, but it is thousands of kilometers thick away from the midplane. We suggest that it may extend completely around the top and the bottom surfaces of the tail. The very low density plasma in the tail lobes is rather uniform over that portion of the height of the lobes explored with the Vela satellites. Comparisons of these observations with a magnetospheric model suggest that the boundary layer is the magnetic projection of the dayside cusps and may be a site of injection of magnetosheath plasma into the plasma sheet.

Soft Gamma-ray Bursts from Accreting Compact Objects

THE recent report that Vela satellites have detected 16 soft γ-ray bursts1 has generated great interest in the possible sources of such bursts2,3. Here we consider the possibility that the γ-ray bursts may originate from fleeting episodes of accretion onto compact galactic objects. We first describe the characteristics of the observations which make this general class of models an attractive one and then discuss a particular example which appears promising, namely the triggering of bursts by solar-type flares in a binary companion.