Search

This paper reports on a new satellite sensor, the Geostationary Earth Radiation Budget (GERB) experiment. GERB is designed to make the first measurements of the Earth's radiation budget from geostationary orbit. Measurements at high absolute accuracy of the reflected sunlight from the Earth, and the thermal radiation emitted by the Earth are made every 15 min, with a spatial resolution at the subsatellite point of 44.6 km (north–south) by 39.3 km (east–west). With knowledge of the incoming solar constant, this gives the primary forcing and response components of the top-of-atmosphere radiation. The first GERB instrument is an instrument of opportunity on Meteosat-8, a new spin-stabilized spacecraft platform also carrying the Spinning Enhanced Visible and Infrared (SEVIRI) sensor, which is currently positioned over the equator at 3.5°W. This overview of the project includes a description of the instrument design and its preflight and in-flight calibration. An evaluation of the instrument performan...

During its close approach to Earth, comet C/Hyakutake 1996 B2 was observed at extreme ultraviolet and x-ray wavelengths with the Rœntgen X-ray Satellite and Rossi X-ray Timing Explorer. The emission morphology was symmetric with respect to a vector from the comet's nucleus toward the sun, but not symmetric around the direction of motion of the comet with respect to interplanetary dust. A slowly varying emission and a large impulsive event that varied on time scales of 1 to 2 hours were observed. An interaction between the comet and the solar wind/solar magnetic field seems to be the most likely mechanism for the observed emission.

During 1990-91 the Wide Field Camera (WFC), on the ROSAT satellite, performed the first allsky survey at EUV wavelengths. The survey was conducted in two wavebands, S1 and S2, with passbands covering the ranges 60–140 A and 110–200 A. It was fully imaging, with effective spatial resolution of about 3 arcmin FWHM, and point source location accuracy of typically better than 1 arcmin. An initial analysis produced a published Bright Source Catalogue (BSC) of 383 sources. Of those optically identified in the BSC, 181 were found to be late-type stars, 119 hot DA white dwarfs, 17 CVs and 7 AGNs. Recently the database has been completely reprocessed to take advantage of several significant developments over what was possible during the course of the BSC work. Compared to the BSC, this has increased (i) the number of EUV sources by ∼ 25% and (ii) the number of detection pairs, ie. sources seen in both wavebands, from ∼ 60% to ∼ 80%. We describe the contents of the new “2RE” catalogue in terms of sky exposure, optical identifications, source counts and time-series analysis. Further, we outline our plans for a public archive of these data.

The first all-sky survey for cosmic sources of extreme-ultraviolet radiation has been carried out with the UK Wide Field Camera on ROSAT. A first reduction of the survey data has yielded a catalogue of 383 relatively bright EUV sources, forming the WFC Bright Source Catalogue. This represents a 30-fold increase in the number of astrophysical objects detected in the ∼ 60-200 eV energy band and covers a flux range, in each of the two survey bands, of more than 2000. A search of the (typically ∼ 1-arcmin) error circles of the WFC sources, using a variety of catalogues and the SIMBAD data base, has identified probable optical counterparts of ∼73 per cent, including many active stars, white dwarf stars and a variety of other galactic and extragalactic objects

Following a successful launch on 1990 June 1, the ROSAT spacecraft and its payload, consisting of an X-ray and an EUV telescope, underwent two months of in-orbit calibration and detailed performance checks. A preliminary observation of a small section of the sky, carried out over July 11-16 (the mini-survey) showed all ROSAT systems to be functioning well and has allowed predictions to be made on the ultimate productivity of both ROSAT all-sky surveys. An analysis of the mini-survey data from the UK Wide Field Camera, has revealed 35 EUV sources, including several white dwarf stars, a variety of active cool stars and several other objects. In all, 23 sources have probable optical counterparts

Results obtained during the first six months in orbit of Ariel V with the Leicester Sky Survey are reviewed. Among 80 sources found by a scan of the Milky Way, 16 are new, and 11 UHURU sources in the scanned region are not detected. Some of these sources may be transient. The light curve of Cen X-3 in a binary cycle shows a dip between phase 0.5, and 0.75, and a secondary maximum at the centre of the dip. The dip and the maximum get progressively weaker in the succeeding cycles. These features are interpreted in terms of the stellar wind accretion model. Cyg X-1 observation for 14 days gives a broad minimum around superior conjuction. Four bright transient sources of nova-like light curves have been observed. The light curves and the spectra are given for TrA X-1 (A1524-62) and Tau X-T (A0535+26).

Results are reported for a deep survey of 65 radio quasars with the Ariel V Sky Survey Instrument, which was undertaken to search for X-ray emission with the aid of a detector system covering the energy range from 2 to 17 keV. Three-sigma upper limits are placed on the X-ray fluxes and luminosities of most of the candidate quasars. It is estimated that the upper limits for quasars with redshifts of less than 1.5 are larger than the likely Compton X-ray fluxes by factors of 10 to several hundred, assuming that there are no strong evolutionary effects on the spectra or luminosities of the quasars.

We report a measurement of the background spectrum based on 10000 counts observed in the energy range 2–10 keV. The rocketborne detector system was optimised for cosmic ray noise rejection. A best fit power law spectrum $$\frac{{dN}}{{dE}} = 16E^{ - 1.8} photons{\text{ }}cm^{ - 2} s^{ - 1} sr^{ - 1} keV^{ - 1} .$$ resulted from the analysis. At 10 keV this result is consistent with recently assessed higher energy data. We show therefore that the change in spectral index between 5 and 50 ke V is approximately −0.2.

THE position of GX3+1 has previously been reported by Schnopper et al.1 with sufficient accuracy to allow the times of lunar occultations to be predicted to within about ±3 min2. This uncertainty in the predicted time can be covered by the relatively brief observation period of the flight of a sounding rocket. Two of the present cycle of lunar occultations were predicted for the rocket range at Woomera, Australia, and were successfully observed: the first on September 27, 1971, by the Leicester X-ray Astronomy Group, and the second on October 24, 1971, by the Mullard Space Science Laboratory, UCL.

Evidence has been published recently for variability in both the intensity and spectrum of X-rays from the massive elliptical galaxy M87. This article reports a new spectral measurement obtained in a rocket flight on November 20, 1970 and re-examines the evidence for variability over the period 1965–70.

THE Leicester University Sky Survey instrument with its field of view perpendicular to the spin axis was able to continue the study of A0535+26 after the spin axis experiments (see accompanying papers) were manoeuvred away on April 29. The source was close to the centre of our field of view on May 2, May 19 and June 1. On the first occasion the instrument was in a single channel mode (no spectral information) but eight-channel spectra between 2 and 18 keV were obtained on the second and third observations.

The Ariel 6 X-ray instruments observed 0115+63 during 1980 December 16–30, when the mean flux was ~ 150 millicrab. Analysis of the ~ 3.6s pulsations given a refined orbital period of 24.3155 ± 0.0002 days and a periastron angle of 47°.15 ±0 °.13, setting a limit on the rate of advance of periastron since 1978 of 0.11 yr−1. During the observation the pulse period was decreasing faster than during the 1978 outburst, but in the 3 year interval there had been a spin down.

X-ray spectra of the BL Lac type object Mkn 421 and several Seyfert type 1 galaxies; IIIZw2, MCG8-11-11 and NGC 4151, have been obtained using the Leicester University instrument on board the Ariel-6 satellite. The Mkn 421 spectrum is best represented by two powerlaw components, the soft component having α ~ 3.4 whilst the hard flux has α ~ 1.0. In MCG8-11-11 there is clear evidence for spectral variability between our observation in late 1979 and that of HEAO-1/A2 in 1977. The Ariel-6 spectrum of MCG8-11-11 can be fitted by a powerlaw of index α ~ 2.1 together with an iron line at ~ 6.2 keV with an equivalent width of ~ 1.6 keV. The first X-ray spectrum of IIIZw2 is also presented, fitting with a powerlaw we find an index of α ~ 1.7. With the exception of NGC 4151 there is no evidence for a significant column of cool material along the line of sight.

Observations of the quasar 3C273 made during 1975-77 have shown that the X-ray flux varied on a time scale of several months. No similar variation was present in radio and optical data taken at similar times. If this variation results from luminosity variations of a central compact object, then an upper limit of ≤4 x 1017cm can be set on its dimensions.

Whereas nearly all previous observations of Cyg X-1 with good time resolution have been made when the source was in its low emission state, Ariel-6 has been fortunate to observe it during 1980 June-July mainly in its high state, i.e. with enhanced low energy emission. The fluctuations were studied on all time-scales down to 2 ms and appeared similar to those previously reported for the low state. The crosscorrelation of rapid fluctuations between two bands above and below 3.5 keV however showed a small but significant asymmetry with the high energy emission delayed.

ON August 3 a faint new X-ray source was discovered (at 15 Ariel counts s−1) near the boundary of the Monoceros and Orion constellations and 6° from the galactic plane, during routine monitoring of the Milky Way with the Ariel V Sky Survey Experiment (SSE). During the following week, the source—designated A0620—00—increased rapidly in intensity to become the brightest in the sky (at 2–18 keV). The X-ray light curve from continued observation with the SSE to August 16 is shown in Fig. 1. The ordinate is in Ariel counts s−1 and for comparison, the corresponding count rates for the Crab Nebula and Sco X-1 are 400 s−1 and 3,200 s−1, respectively. The precursor peak on August 6 is real and is qualitatively similar to the feature seen1 in the rise to maximum intensity of the earlier Ariel transient A1524—62. Subsequent observations of A0620—00 with the Ariel V experiments pointing along the spin axis of the spacecraft have shown the intensity to have remained very high at least until August 27.

A REMARKABLE change in the X-ray spectrum of the transient source A0620—00 (ref. 1) was observed by the Ariel V Sky Survey Experiment (SSE) during the rise to maximum brightness, The SSE detector being used has an effective area (behind its field collimator) of 290 cm2 and covers the energy band 2–18 keV in 8 channels. The channel boundaries are marked in Fig. 1, which shows sample pulse height spectra taken as the source intensified. These raw data have not been corrected for the detector efficiency and at this stage include no input spectral assumptions. Two points emerge clearly: when first detected the X-ray spectrum of A0620—00 was hard and remained so as the overall intensity rose to the precursor peak and, secondly, the subsequent rise to maximum intensity was entirely due to a strong enhancement of X rays below 10 keV, giving a marked softening in the overall spectrum. In fact, the observed flux above 10 keV actually fell during this period.

The Ariel-5 satellite has monitored the X-ray light curve of A1524-62 almost continuously from 40 days prior to maximum light until its disappearance below the effective experimental sensitivity. The source exhibited maximum light on Dec. 4, 1974, at a level of 0.9 the apparent magnitude of the Crab Nebula in the energy band 3-6 keV. Although similar to previously reported transient sources with a decay time constant of about 2 months, the source exhibited an extended, variable preflare on-state of about 1 month at a level of greater than 0.1 maximum light. The four bright (greater than 0.2 of the Crab Nebula) transient sources observed during the first half-year of Ariel-5 operation are indicative of a galactic disk distribution, a luminosity at maximum in excess of 10 to the 37-th power ergs/sec, a frequency of occurrence which may be as high as 100/yr, and a median decay time which is less than 1 month.

The gamma-ray burst of 28 January 1976, one of 18 events thus far detected in interplanetary space with Helios-2, was also observed with the Vela-5A, -6A and the Ariel-5 satellites. A small source field is obtained from the intersection of the region derived from the observed time delays between Helios-2 and Vela-5A and -6A with the source region independently found with the Ariel-5 X-ray detector. This area contains neither any steady X-ray source as scanned by HEAO-A nor any previously catalogued X-ray, radio or infrared sources, X-ray transients, quasars, seyferts, globular clusters, flare stars, pulsars, white dwarfs or high energy gamma-ray sources. The region is however, within the source field of a gamma-ray transient observed in 1974, which exhibited nuclear gamma-ray line structure.

WE have previously given the results of two lunar occultations1 of the X-ray source GX3 +1. Only two optical objects were nearby, both outside the error box. The only one of these which looked to be a serious optical candidate has been shown2 to possess no unusual properties which could indicate that it had a strong X-ray emission.