The spectral and temporal variability of the X-ray transient 4U1630-47

We present the results of four observations made by the European Space Agency's EXOSAT Observatory of the X-ray transient 4U1630-47 during its 1984 outburst. We observed marked spectral changes as the source decayed from a maximum observed intensity of 8×10^–9 erg/cm²/sec (1.5–10 keV). The spectrum could be modelled by a soft thermal-like component with a high energy power-law tail. The relative contribution of the soft to hard component decreased as the total luminosity decreased. We compare these changes with those observed from the black hole candidate Cyg X-1 when it transitions from a high to a low state. In addition we report the discovery of short timescale intensity variations (down to 50 msec) with a characteristic timescale of 20 sec. We present a precise position for this unidentified source.on leave from Università di Roma Dipartimento di Fisica G. Marconi.     

X-ray and optical observations of Nova Muscae 1983 during its nebular stage

The detection of X-rays from Nova Muscae 1983 (discovered on January 18, 1983) constitutes the first detection of X-rays from a classical nova during outburst. X-ray observations were carried out on 1984 April 20 and July 14 when Nova Mus had entered the nebular stage. During both observations no significant flux was observed with the medium energy detectors (2–50 keV). The source was detected with the low energy detector (.04–2 keV) using 3000 Å Lexan and Parlene- N-Aluminium filters; counting rates of (3.4 ± 1.2) × 10³ and (3.7 ± 1.2 × 10^-3 counts/sec were measured with the respective filters. The source was detected again on July 14 with about the same intensity. Either a shocked shell of circumstellar gas emitting 10⁷ thermal bremsstrahlung at 10³⁵ erg/sec intensity or a white dwarf remnant emitting 3.5 × 10⁵ blackbody radiation at 10³⁷ erg/sec luminosity are compatible with the measurements.Spectra taken in the visual spectral range show strong forbidden coronal emission lines of [FeVII] 6085, [FeX] 6374, and as never observed before in such a strength, [FeXIV] 5303 requiring excitation temperatures of 2 × 10⁶ °K.     

First detection of an X-ray burst and a one hour intensity dip in 4U1323-62

We report the results of a 1.4 10⁴s observation of the region of 4U 1323-62 with the EXOSAT ME. The source has a flux of 7–8 10^-11 erg/cm²s (2–10 keV) and a power-law spectrum with 1.1 < < 1.8. During our observation, the source showed a symmetric 60% dip in its X-ray flux of R~1 hr. The spectrum hardens during the dip. Inside the dip we observed an X-ray burst with a 2–10 keV peak flux of 7 10^-10 erg/cm²s. The burst spectrum is black-body, and shows evidence of cooling during the burst decay. The discovery of a burst from 4U 1323-62 settles the classification of the source; the observation of a dip suggests that we may be able to measure its orbital period in the near future.     

Composition of anomalous cosmic rays and implications for the heliosphere

We use energy spectra of anomalous cosmic rays (ACRs) measured with the Cosmic Ray instrument on the Voyager 1 and 2 spacecraft during the period 1994/157-313 to determine several parameters of interest to heliospheric studies. We estimate that the strength of the solar wind termination shock is 2.42 (–0.08, +0.04). We determine the composition of ACRs by estimating their differential energy spectra at the shock and find the following abundance ratios: H/He = 5.6 (–0.5, ⁺0.6), C/He = 0.00048 ± 0.00011, N/He = 0.011 ± 0.001, O/He = 0.075 ± 0.006, and Ne/He = 0.0050 ± 0.0004. We correlate our observations with those of pickup ions to deduce that the long-term ionization rate of neutral nitrogen at 1 AU is 8.3 × 10^–7 s^–1 and that the charge-exchange cross section for neutral N and solar wind protons is 1.0 × 10^–15 cm² at 1.1 keV. We estimate that the neutral C/He ratio in the outer heliosphere is 1.8(–0.7, +0.9) × 10^–5. We also find that heavy ions are preferentially injected into the acceleration process at the termination shock.     

The primordial Helium-4 abundance determination: systematic effects

By extrapolating to O/H = N/H = 0 the empirical correlations Y–O/H and Y–N/H defined by a relatively large sample of 45 Blue Compact Dwarfs (BCDs), we have obtained a primordial ⁴Helium mass fraction Y _p=0.2443±0.0015 with dY/dZ=2.4±1.0. This result is in excellent agreement with the average Y _p=0.2452±0.0015 determined in the two most metal-deficient BCDs known, I Zw 18 (Z /50) and SBS 0335–052 (Z /41), where the correction for He production is smallest. The quoted error (1) of 1% is statistical and does not include systematic effects. We examine various systematic effects including collisional excitation of hydrogen lines, ionization structure and temperature fluctuation effects, and underlying stellar Hei absorption, and conclude that combining all systematic effects, our Y _p may be underestimated by 2–4%. Taken at face value, our Y _p implies a baryon-to-photon number ratio =(4.7^+1.0 _–0.8)×10^–10 and a baryon mass fraction _b h ² ₁₀₀=0.017±0.005 (2), consistent with the values obtained from deuterium and Cosmic Microwave Background measurements. Correcting Y _p upward by 2–4% would make the agreement even better.     

LISM structure — Fragmented superbubble shell?

Small scale structure in local interstellar matter (LISM) is considered. Overall morphology of the local cloud complex is inferred from Ca II absorption lines and observations of H I in white dwarf stars. Clouds with column densities ranging from 2–100 × 10¹⁷ cm^–2 are found within 20 pc of the Sun. Cold (50 K) dense (10⁵ cm^–3) small (5–10 au) clouds could be embedded and currently undetected in the upwind gas. The Sun appears to be embedded in a filament of gas with thickness 0.7 pc, and cross-wise column density 2 × 10¹⁷ cm^–2. The local magnetic field direction is parallel to the filament, suggesting that the physical process causing the filamentation is MHD related. Enhanced abundances of refractory elements and LISM kinematics indicate outflowing gas from the Scorpius-Centaurus Association. The local flow vector and Sco data are consistent with a 4,000,000 year old superbubble shell at –22 km s^–1, which is a shock front passing through preshock gas at –12 km s^–1, and yielding cooled postshock gas at –26 km s^–1in the upwind direction. A preshock magnetic field strength of 1.6 G, and postshock field strength of 5.2 G embedded in the superbubble shell, are consistent with the data.Abbreviations LISM Local ISM - SIC Surrounding Interstellar Cloud - LIC Local Interstellar Cloud     

Simultaneous EXOSAT-IUE observations of NGC 4151

NGC 4151 was observed four times in Nov. 83. The results indicate that: a) there exists a correlation between the X-ray and UV fluxes on the long term; b) the soft X-ray excess between 0.1 and 1 keV is probably steeper than expected from the leaky absorber model by Holt et al (1980); c)the spectral fit to the ME data, after correction for a soft component, yields =1,73±0.27, N_H=(15.2±2.2)×10²² cm^–2, E.W.(Fe line)=0.208±0.084 keV, and does not require a strong overabundance of Fe in the absorber. The relationship between N_H and the strength of the broad emission lines is commented.     

Ultra high-energy gamma-ray observations of Geminga

Results of the observations of Geminga (2CG 195 + 4) in the energy range E 10¹² eV, carried out in 1979, 1981, and 1983 with the Tien Shan high-altitude facility for recording the erenkov flashes of extensive air showers are reported. The mean flux density averaged over the whole protracted data is (5.7 ± 2.5) × 10^–11 quanta cm^–1 s^–1. The flux is variable with a period 59 s. The character of the period variation with time is hard to be reconciled with earlier findings by other authors. The importance of further simultaneous observations at various energies is indicated.     

The discovery of a 25 min regular modulation in the X-ray flux from 2S0142+61

A 13 hr observation of 2S0142+61 on 1984 August 27 by EXOSAT shows the X-ray flux of 2S0142+61 to be modulated with a period of 1456+/-6 s. The 1–10 keV spectrum is two component with a 0.7 keV thermal and 0.0 energy index power law, with 30% of the total luminosity in the thermal component. The spectrum is absorbed by 1 × 10²² H cm^-2. Only the hard component is pulsed with a 3 to 10 keV peak to mean amplitude of 35%. Below 2 keV the modulation is less than a few percent. The total 1–10 keV luminosity is 3.5 × 1032 erg s^-1 for a distance of 100 pc. Possible optical counterparts are discussed.     

The Voyager infrared spectroscopy and radiometry investigation

The infrared investigation on Voyager uses two interferometers covering the spectral ranges 60–600 cm^–1 (17–170 m) and 1000–7000 cm^–1 (1.4–10 m), and a radiometer covering the range 8000–25 000 cm^–1 (0.4–1.2 m). Two spectral resolutions (approximately 6.5 and 2.0 cm^–1) are available for each of the interferometers. In the middle of the thermal channel (far infrared interferometer) the noise level is equivalent to the signal from a target at 50 K; in the middle of the reflected sunlight channel (near infrared interferometer) the noise level is equivalent to the signal from an object of albedo 0.2 at the distance of Uranus.For planets and satellites with substantial atmospheres, the data will be used to investigate cloud and gas composition (including isotopic ratios), haze scale height, atmospheric vertical thermal structure, local and planetary circulation and dynamics, and planetary energy balance. For satellites with tenuous atmospheres, data will be gathered on surface and atmospheric composition, surface temperature and thermal properties, local and global phase functions, and surface structure. For Saturn's rings, the composition and radial structure, particle size and thermal characteristics will be investigated. Comparative studies of the planets and their satellite systems will be carried out.Paris Observatory.Cornell University.Jet Propulsion Laboratory.University of Maryland.     

The einstein observatory medium sensitivity survey

Results are presented from an X-ray survey of 50 square degrees of the high galactic latitude sky at sensitivities in the range 7·10^–14 – 5·10^–12 erg/cm² sec (0·3–3·5 keV) carried out with the Imaging Proportional Counter (IPC) aboard the Einstein Observatory. The extragalactic sample consists of 48 sources which have been used to determine the number flux relation. The content of the sample is analyzed in terms of types of sources and is found to be significantly different from the content of similar samples selected at higher fluxes.     

Some problems of gamma-astronomy

The modern state of gamma-ray astronomy is reviewed, the paper being mainly devoted to the theoretical models that describe generation of gamma-ray emission under astrophysical conditions. Basic information on the processes of generation and absorption of gamma-rays, as well as the results of observations for various gamma-ray photon energies are reported.In the region of soft gamma-ray emission (i.e., for energies less than tens of MeV), where emission in gamma-ray lines dominates, we also discuss the nature of gamma-ray bursts, the origin of gamma-ray emission from the galactic centre, etc.Discrete sources and, in particular, the mysterious source Cyg X-3 are discussed in the region of very high (E > 10¹² eV) and ultra-high (E > 10¹⁵ eV) energy gamma-ray emission.A larger portion of the review is devoted to the analysis of cosmic-ray origin on the basis of the available gamma-ray data in the region from several tens of MeV to several GeV. The peculiarity of this energy range is, in particular, in the fact that the diffuse galactic emission was observed mainly there. We also discuss the problem of determination of the cosmic-ray density gradient from the gamma-ray data.The origin of high-latitude gamma-ray emission, the problem of galactic gamma-ray halo, etc., are discussed.The theoretical models explaining the nature of unidentified gamma-ray sources, as well as the results of measurements and theoretical estimations of a gamma-ray flux from SN1987A are analysed.List of Notations m electron mass, m = 9.108 × 10^–28 g, - M proton mass, M = 1.672 × 10^–24 g, - e electron charge, e = 4.803 × 10^–10 CGS - c velocity of light, c = 2.9979 × 10¹⁰ cm s^–1, - k Boltzmann constant, k = 1.380 × 10^–16 erg grad^–1, - e electron - p proton - gamma-ray photon - p antiproton - ^{0 0}-meson - -lepton - e ⁺ positron - r, , x radio-frequency, gamma-ray, and X-ray emission bands - E total energy of a particle - E _k kinetic energy - p particle momentum - spectral index for particles - spectral index for emission - n particle density (concentration) - H magnetic field strength - T temperature - _ph energy of low-energy photons - emission frequency - r _H Larmor radius of relativistic particles - k wave number - , z cylindric coordinates, in this case the coordinate (radius) along the galactic disk, z perpendicular to the galactic disk - M solar mass, M = 1.99 × 10³³ g.     

Auroral arc formation: Kinetic and MHD effects

Recent observations by the Dynamics Explorer satellites indicate that auroral field lines are not equipotentials. The field-aligned conductivity is the same for up and downgoing currents and of the order of several 10^–8 to several 10^{–9 –1}m^–2. This is not in agreement with models based on the resistance provided by the magnetic mirror force alone. It is suggested that the resistivity is due to damping of kinetic shear-Alfven wave packets. This damping may be nonlinear in nature or Landau damping. We briefly review the properties of kinetic Alfven waves.     

High energy X-ray spectrum of her X-1

Summary On May 8, 1980, we conducted a 90 minute observation on hard X-ray emission (15-200 keV) from Her X-1, using a large area ( 1500 cm²), low background balloon borne X-ray telescope. The energy resolution of the telescope was 17% FWHM at 60 keV. Her X-1 was at binary phase 0.0725 and 2.7 ± 0.5 days after turn on in the 35 day cycle.Average pulsation light curves were obtained by sorting data into 25 equal bins, according to pulse arrival time, modulo the 1.24 sec pulsation period. The width of the main pulse is energy dependent and in the 45–75 keV region about 30% smaller than in the range from 15 to 30 keV.The data have been analyzed by taking the Her X-1 pulse minus background spectrum, where the pulse count rate is defined in a pulse phase interval around the pulse maximum of the 1.24 sec period. The background spectrum was intermittently obtained by a chopping collimator system.A spectral feature is present in emission at an energy of 49.5 (+ 1.5, -3) keV and a FWHM of 18 (+ 6, -3) keV and in absorption at an energy of 29.5 (+ 1.7, -1.5) keV and a FWHM of 17.0 (+ 2.6, -2.8) keV. The intensity of this line feature in emission is (1.8 ± 0.4) photons/cm sec. The line excess in emission over the continuum (with kT = 6.75 (+ 0.2, -0.4) keV) is 7.     

Determination of the characteristics of the gamma-ray telescope Gamma-1

The Gamma-1 telescope has been developed through a collaboration of scientists in the USSR and France in order to conduct -ray astronomical observations within the energy range from 50 to 5000 MeV. The major characteristics of the telescope were established by Monte-Carlo simulations and calibrations made with the aid of electron and tagged -ray beams produced by an accelerator, and these have been found to be as follows: the effective area for photons coming along the instrument's axis varies from about 50 cm² at E = 50 MeV to approximately 230 cm² at E 300 MeV; the angular resolution (half opening of the cone embracing 68% events) is equal to 2.7° at E = 100 MeV, and 1.8° at E = 300 MeV; the energy resolution (FWHM) varies from 70% to 35% as the energy of the detected photons increases from 100 to 550 MeV; the telescope's field-of-view at the half-sensitivity level is 300–450 square degrees depending upon the spectrum of the detected radiation, and the event selection logic. Proceeding from the thus obtained characteristics it is demonstrated that a point source producing a photon flux J (E 100 MeV) = 3 × 10^-7 cm^-2 s^-1, can be detected with a 5 significance by observing it during 10⁶ s at the level of the Cygnus background, and a source having intensity J (E 100 MeV) = 10^-6 cm^-2 s^-1 can be detected to within a mean square positional accuracy of about 15.     

Recent results on the parameters of the interstellar helium from the ULYSSES/GAS experiment

Velocity and direction of the flow of the interstellar helium and its temperature and density have been determined from the measurements of the ULYSSES/GAS experiment for two different epochs: during the in-ecliptic path of ULYSSES, representing solar maximum conditions, and during the south to the north pole transition (11/94-6/95), close to the solar minimum conditions. Within the improved error bars the values are consistent with results published earlier.The determination of the density n of the interstellar helium at the heliospheric boundary from observations in the inner solar system requires knowledge about the loss processes experienced by the particles on their way to the observer. The simultaneous observation of the helium particles arriving on direct and indirect orbits at the observer provides a tool to directly determine the effects of the loss processes assumed to be predominantly photoionization and — for particles travelling close to the Sun — electron impact ionization by high-energy solar wind electrons.Such observations were obtained with the ULYSSES/GAS instrument in February 1995, before the spaceprobe passed its perihelion. From these measurements values for the loss rates and the interstellar density could be derived. Assuming photoionization to be the only loss process reasonable fits to the observations were obtained for an ionization rate = 1.1 · 10^–7 s^–1 and a density n 1.7 · 10^–2 cm^–3. Including, in addition, electron impact ionization, a photoionization = 0.6 · 10^–7 s^–1 was sufficient to fit both observations, resulting in a density n 1.4 · 10^–2 cm^–3.On leave from Space Research Centre, Warsaw, Poland.     

Observability of magnetically strongly shifted iron line emission from X-ray pulsars

As an application of our extensive calculations of energies and intensities of atomic lines in very intense magnetic fields of the order of 10¹¹-10¹³ G we discuss the possibility of observing magnetically strongly shifted iron lines in the spectra of pulsating X-ray sources. Careful estimates of the relevant parameters lead us to the conclusion that it would be profitable to look for magnetically shifted iron line enission in magnetic neutron stars of low luminosity using spectrometers working in the energy range 10 – 100 keV with sensitivities of 10^–4 cm^–2 s^–1 and energy resolutions E/E 10 – 100.This work was supported in part by the Deutsche Forschungsgemeinschaft (DFG).     

The gamma-ray telescope Gamma-1

The telescope Gamma-1 is designed to investigate cosmic gamma rays in the energy range from 50 MeV to 5000 MeV. The geometrical sensitive area of the telescope amounts to 1500 cm², the angular resolution in each direction is equal to 1.2° at the energy 300 MeV and is about 20 when including a coded mask in the telescope, the energy resolution changes from 70% at 100 MeV to 35% at 550 MeV. The characteristics of the telescope and its systems have been determined by the Monte-Carlo method as well as by accelerator calibrations. Discrete sources at the intensity level of 10^–7 quanta cm^–2 s^–1 may be recorded in a year of observations with the gamma-ray telescope Gamma-1 with a source location accuracy of 10 arc min.     

EXOSAT observations of the BL Lac object Mkn 421

The BL Lac object Mkn 421 was observed by EXOSAT four times over a period of six days in February 1984. Significant X-ray variability was apparent on a timescale of less than a day, but with no accompanying spectral change. The source exhibited a very soft power law X-ray spectrum with an extremely low intrinsic column density (N_H10²⁰ cm^–2). There was no evidence for an additional hard component attributable to synchrotron self-Compton emission. The observations when combined with other published data imply that significant changes occur in the form of the broad-band UV/X-ray continuum of this source.     

Starburst galaxies

Bursts of massive star formation can temporarily dominate the luminosity of galaxies spanning a wide range of morphological types. This review is concerned primarily with such events in the central 1 kpc region of spiral galaxies which result from bar driven inflows of gas triggered by interactions or mergers. Most of the stellar radiant luminosity of such bursts is absorbed by dust and re-emitted in the far-infrared and is accompanied by radio and X-ray emission from supernova remnants which can also act collectively to drive galaxy scale outflows. Both evolutionary stellar models and estimates of the gas depletion times are consistent with typical burst durations of 10^7–8 yr. Spatially-resolved studies of nearby starburst galaxies reveal that the activity is distributed over many individual star forming complexes within rings and other structures organized by interactions between bars and the disc over a range of scales. More distant and extreme examples associated with mergers of massive spirals have luminosities > 10¹³ L and molecular gas masses > 10¹⁰ M implying star formation rates > 1000 M yr^–1 which can only be sustained for 10⁷ yr. In the most luminous merging systems, however, the relative importance of starburst and AGN activity and their possible evolutionary connection is still a hotly debated issue. Also controversial are suggestions that starbursts in addition to a black hole are required to account for the properties of AGNs or that starbursts alone may be sufficient under certain conditions. In a wider context, starbursts must clearly have played an important role in galaxy formation and evolution at earlier times. Recent detections of high redshift galaxies show that star formation was underway at z 4 but do not support a continuing increase of the strong evolution in the co-moving star formation density seen out to z l. Primeval starburst pre-cursors of spheroidal systems also remain elusive. The most distant candidates are radio galaxies and quasars at z = 4–5 and a possible population of objects responsible for an isotropic sub-mm wave background tentatively claimed to have been detected by the COBE satellite.