Observation of cosmic soft X-rays

Cosmic soft X-rays in the energy range between 0.14 and 7 keV were observed with thin polypropylene window proportional counters on board a sounding rocket. The field of view crossed the galactic plane in the Cygnus-Cassiopeia region at a large angle and reached the galactic latitudes of –55° and +30°. Referring also to the result with Be window counters, we obtained the energy spectrum of Cyg XR-2, the flux from the Cas A region and the distribution of the intensity of diffuse X-rays over the scanned region. The turn-over of the Cyg XR-2 spectrum at about 1 keV indicates that the distance of the Cyg XR-2 source lies between 600 and 800 pc, if the turn-over is due entirely to interstellar absorption. The flux from the Cas A region is obtained as 0.23±0.05 photons cm^–2 sec^–1 in the energy range between 1.1 and 4.1 keV. The intensity of diffuse soft X-rays depends on the galactic latitude more weakly than expected from the interstellar absorption of extragalactic X-rays and shows asymmetry with respect to the galactic equator, thus suggesting a contribution of galactic X-rays. The spectrum of extragalactic X-rays is approximately represented by a power lawE ^–1.8.     

The spectrum of diffuse cosmic X-rays in the 20–125 keV range

Diffuse cosmic X-rays in the energy range 20–125 keV were measured in four balloon flights from Hyderabad, India during 1968–70 using almost identical X-ray telescopes mounted on oriented platforms. The results from these flights show that the spectrum of the diffuse cosmic X-rays can be represented by the form dN/dE=29E ^–2.1±0.3 photons/(cm² sr s keV) in 20–125 keV interval after corrections for photoelectric absorption and Compton scattering effects in the atmosphere. The best fit spectrum of all published results in the energy interval 20–200 keV can be represented by the form dN/dE=36E ^–2.1±0.1 photons/(cm² sr s keV) after similar corrections are effected, and there is no need for a change of spectral index in this energy interval. The intensity at 20 keV obtained from the above spectrum agrees well with that given by the spectral form dN/dE=10E ^–1.7±0.1 photons/(cm² sr s keV) in the energy interval 1–20 keV in several rocket experiments. Therefore it is concluded that if there is a break in the spectrum, it occurs between 10 and 20 keV with a change of spectral index by about 0.5, or the index is continuously changing from 1.7±0.1 to 2.1±0.1 in 10–20 keV interval. The implications of the results are briefly discussed.     

Anomalous Cosmic-Ray Helium,Nitrogen and Oxygen in 1996 – Measurements of the ERNE Instrument on-board SOHO

The energy spectra of the anomalous components of helium, nitrogen and oxygen have been measured by the ERNE experiment on board the SOHO spacecraft. During February 28–April 30, 1996, the maximum intensity of anomalous helium was found to be 3.8 × 10^-5 cm^-2 sr^-1 s^-1 (MeV nucl^-1)^-1 in the energy range 10–15 MeV nucl^-1. During the period January 26–April 30, 1996, the maximum oxygen intensity was 1.2 × 10^-5 cm^-2 sr^-1 s^-1 (MeV nucl^-1)^-1 at 4–7 MeV nucl^-1, and the maximum nitrogen intensity 1.7 × 10^-6 cm^-2 sr^-1 s^-1 (MeV nucl^-1)^-1 at 4–9 MeV nucl^-1. These peak intensities are at the same level as two solar cycles ago in 1977, but significantly higher than in 1986. This gives observational evidence for a 22-year solar modulation cycle. A noteworthy point is that the spectra of anomalous nitrogen and oxygen appear to be somewhat broader than in 1977.     

Large-Scale Electric Fields in Solar Flare Regions

A method of separating electric field in the flare region in the potential and vortex (induced) parts is discussed. According to the proposed model, the motion of flare ribbons from the central line of the flare region is caused by the vortex component of the coronal electric field, while the motion of bright spots within the flare region towards the central line is driven by the potential component of that field. The intensity of both the components of the flare region electric field is estimated to equal approximately 1–3 V cm^–1, which provides the input of the electromagnetic energy into the active region at a rate of about 10¹⁰ erg cm^–2 s^–1.     

Line features from Cygnus X-1 and the Crab nebula in the energy range 30–270 keV

A balloon-borne gemanium spectrometer was flown in an attempt to detect line-emission from Cyg X-1 and the Crab nebula in the energy range 30–270 keV. The experiment was carried out on 29–30 September, 1982. A line feature at 145 keV was observed from Cyg X-1. The intensity is (1.34±0.31)×10^–2 photons cm^–2 s^–1 and the width is 14.3 keV FWHM. From the Crab nebula, a weak line feature with 1.8 excess was found around 78 keV.     

Energy spectrum and time variations of hard X-rays from Cyg X-1

Experimental results on the intensity, energy spectrum and time variations in hard X-ray emission from Cyg X-1 based on a balloon observation made on 1971, April 6 from Hyderabad (India) are described. The average energy spectrum of Cyg X-1 in the 22–154 keV interval on 1971 April 6 is best represented by a power law dN/dE=(5.41±1.53)E ^{–(1.92±0.10)} photons cm^–2s^–1 keV^–1 which is in very good agreement with the spectrum of Cyg X-1 derived from an earlier observation made by us on 1969 April 16 in the 25–151 keV band and given by dN/dE=(3.54±2.44)E ^{–(1.89±0.22)} photons cm^–2s^–1 keV^–1. A thermal bremsstrahlung spectrum fails to give a good fit over the entire energy range for both the observations. Comparison with the observations of other investigators shows that almost all balloon experiments consistently give a spectrum of E ^–2, while below 20 keV the spectrum varies fromE ^–1.7 toE ^–5. There is some indication of a break in the Cyg X-1 spectrum around 20 keV. Spectral analysis of data in different time intervals for the 1971 April 6 flight demonstrates that while the source intensity varies over time scales of a few minutes, there is no appreciable variation in the spectral slope. Analysis of various hard X-ray observations for long term variations shows that over a period of about a week the intensity of Cyg X-1 varies upto a factor of four. The binary model proposed by Dolan is examined and the difficulties in explaining the observed features of Cyg X-1 by this model are pointed out.     

The ionization state of oxygen ions in anomalous cosmic rays: Results from the ANURADHA experiment in Spacelab-3

We report the first results on the determination of the ionization states of oxygen ions in the anomalous cosmic rays (ACR) from the measurements of their flux in the cosmic-ray experiment in Spacelab-3 (SL-3) mission of NASA flown at 350 km altitude during 29 April–6 May, 1985. The detectors used were specially prepared CR-39 plastics of very high sensitivity for recording tracks of ions withZ>2. The measured orbit averaged flux of ACR oxygen is (2.9±1.3)×10^–4 particles m^–2sr^–1s^–1 (MeV N^–1) at an energy of 23 MeV N^–1. We made an independent estimate of the expected ACR oxygen flux at SL-3 orbit from interplanetary data and compared this with the measured flux to infer the ionization states of ACR oxygen ions. The flux and energy spectra of ACR oxygen at 1 AU outside the magnetosphere is obtained from the data of Voyager-2, during the same epoch as the SL-3 flight, and using the measured radial intensity gradient of 15%/AU for ACR oxygen between 1–17 AU. We calculate the geomagnetic transmission factors for ACR oxygen ions of charge states O⁺¹, O⁺², etc., from the known cut-off rigidities in the world grid and using the SL-3 trajectories for 116 orbits in the 6-day mission to obtain the expected flux at SL-3 for different charge states. When these flux values are compared with our measured flux, the averge ionization state of ACR oxygen ions in the energy interval of 20–26 MeV N^–1 is obtained as O⁺¹.     

A comparison of spectral reduction techniques in X-Ray astronomy

We report the results of a numerical investigation of three methods of cosmic X-ray spectral reduction, with an emphasis on the apodization technique of Dolan (1972). The study is carried out in the energy range 10–100 keV on pulse height distributions which were synthesized from simple X-ray source spectra with predetermined parameters. The spectral response function used was that of a balloon-borne, large area, xenon-filled proportional counter; however, we believe that our results are generally relevant to other non-dispersive systems such as scintillators and low energy (1–10 keV) proportional counters. We agree that the ill-conditioned energy resolution matrix cannot be inverted using standard procedures without propagating unacceptable errors. The apodization solution was investigated as a function of counter resolution, fluorescence escape probability, extent of energy window, number of pulse height channels, dataS/N ratio, and the so-called apodization instrument factor. We conclude that in most circumstances apodization should be the preferred reduction technique, but that the present widely and improperly used minimum –x ² method may be useful as the final step in obtaining refined spectral parameters estimates.     

Diffuse X-ray background measurements in the energy range 2–18 keV

Rocket measurements, of the diffuse X-ray background in the energy range 2–18 keV, conducted from Thumba Equatorial Rocket Launching Station (TERLS), India, are presented. The estimates of the cosmic background are derived by the method which employs the Earth and its atmosphere as a shutter to intercept the celestial X-rays. The results are shown to be consistent with a power law photon spectrum.13.6 _–3.3 ^+4.3 E ^{–1.73±0.15} photons/cm²-sec-keV-ster the spectrum being much flatter than that observed at higher energies.     

Seasonal variations of diffuse solar radiation at Ilorin (8°32′N,4°46′E), Nigeria

Hourly diffuse solar irradiance (H_d) and the corresponding global irradiance (H) falling on a horizontal surface at Ilorin (8°32N,4°46E), Nigeria has been measured for a period of fifteen months commencing from February, 1993. As expected, each monthly mean diurnal variation of H_d exhibits a mid-day maximum, while that of the ratio H_d/H exhibits a minimum. The month to month variations of these parameters are consequent on the dominant atmospheric conditions observed in the months concerned. The monthly average daily total of diffuse radiation (H_dm) was found to be highest in February, 1994 with a value of (3347 ± 45) Wm^–2day^–1, mainly due to the effects of Harmattan haze and thin clouds, while it was found to be lowest in November, 1993 and January, 1994 with a value of about 2500 Wm^–2day^–1. Similarly, the ratio of the monthly average daily total of diffuse radiation to the monthly average daily total of global radiation (H_dm/H_m) had the lowest value of 0.49 in the relatively clear sky month of November, 1993 and the highest value of 0.74 in the mainly cloudy month of August, 1993. On an annual average, the diffuse component constituted about 60% of the global solar radiation arriving on the ground at Ilorin, Nigeria primarily due to clouds and Harmattan haze which are seasonal, in addition to the constant background of molecular scattering.     

Characteristics of soft solar X-ray bursts

The burst component of the solar X-ray flux in the soft wavelength range 2 < < 12 Å observed from Explorer 33 and Explorer 35 from July 1966 to September 1968 was analyzed. In this period 4028 burst peaks were identified.The differential distributions of the temporal and intensity parameters of the bursts revealed no separation into more than one class of bursts. The most frequently observed value for rise time was 4 min and for decay time was 12 min. The distribution of the ratio of rise to decay time can be represented by an exponential with exponent -2.31 from a ratio of 0.3 to 2.7; the maximum in this distribution occurred at a ratio of 0.3. The values of the total observed flux, divided by the background flux at burst maximum, can be represented by a power law with exponent -2.62 for ratios between 1.5 and 32. The distribution of peak burst fluxes can be represented by a power law with exponent - 1.75 over the range 1–100 milli-erg (cm² sec)^–1. The flux time integral values are given by a power law with exponent -1.44 over the range 1–50 erg cm^–2.The distribution of peak burst flux as a function of H importance revealed a general tendency for larger peak X-ray fluxes to occur with both larger H flare areas and with brighter H flares. There is no significant dependence of X-ray burst occurrence on heliographic longitude; the emission thus lacks directivity.The theory of free-free emission by a thermal electron distribution was applied to a composite quantitative discussion of hard X-ray fluxes (data from Arnoldy et al., 1968; Kane and Winckler, 1969; and Hudson et al., 1969) and soft X-ray fluxes during solar X-ray bursts. Using bursts yielding measured X-ray intensities in three different energy intervals, covering a total range of 1–50 keV, temperatures and emission measures were derived. The emission measure was found to vary from event to event. The peak time of hard X-ray events was found to occur an average of 3 min before the peak time of the corresponding soft X-ray bursts. Thus a changing emission measure during the event is also required. A free-free emission process with temperatures of 12–39 × 10⁶K and with an emission measure in the range 3.6 × 10⁴⁷ to 2.1 × 10⁵⁰ cm^–3 which varies both from event to event and within an individual event is required by the data examined.Now at Department of Astrophysical Sciences, Princeton University, Princeton, New Jersey.     

A continuous spectrum of a white-light flare

White-light continuum was observed at the Norikura Solar Observatory in a 2B flare of 10 September 1974 in the spectral region between 3600 Å and 4000 Å. The duration of continuum emission was 8–12 min. The continuum shows a Balmer free-bound component, but the main contribution to the continuum between 3646 Å and 4000 Å is H^– emission. The white-light continuum, therefore, is thought to be of photospheric origin. The energy loss in the continuum is 10²⁷ erg s^–1.     

X-ray observation of the Jovian impacts of comet Shoemaker - Levy 9

An ASCA observation of the Jovian impact of the comet Shoemaker-Levy 9 is reported. Four impacts of H, L, Q1 and R were observed and four impacts of B, C, G, and Q2 were observed within 60 minutes after their impacts. No significant flaring of X-ray emission was observed. Upper limit X-ray fluxes of 90 % confidence level, averaged 5 minutes just after the impacts, were 2.4 × 10^–13 erg sec^–1 cm^–2, 3.5 × 10^–13 erg sec^–1 cm^–2, 1.6 × 10^–13 erg sec^–1 cm^–2 and 2.9 × 10^–13 erg sec^–1 cm^–2 for the impacts of H, L, Q1 and R, respectively, in the 0.5(0.7 for H and Q1)–10 keV energy range. However, a hint of X-ray enhancement around Jupiter from July 17 to July 19 was detected with about 2 6 × 10^–14 erg sec^–1 cm^–2 in the 0.5–10 keV energy range.     

Cosmic neutrinos of ultra-high energies and detection possibility

The fluxes and spectra of galactic and extragalactic neutrinos at energy 10¹¹–10¹⁹ eV are calculated. In particular, the neutrino flux from the normal galaxies is calculated taking into account the spectral index distribution. The only assumption that seriously affects the calculated neutrino flux atE _v 10¹⁷ eV is the power-like generation spectrum of protons in the entire considered energy region.The normal galaxies with the accepted parameters generate the metagalactic equivalent electron component (electrons+their radiation) with energy density _e8.5×10^–7 eV cm^–3, while the density of the observed diffuse X-ray radiation alone is 100 times higher. This requires the existence of other neutrino sources and we found the minimized neutrino flux under two limitations: (1) the power-law generation spectrum of protons and (2) production of the observed energy density of the diffuse X-an -radiation. These requirements are met in the evolutionary model of origin of the metagalactic cosmic rays with modern energy density _M83.6×10^–7 eV cm^–3.The possibility of experiments with cosmic neutrinos of energyE _v 3×10¹⁷ eV is discussed. The upper bound on neutrino-nucleon cross-section <2.2×10^–29 cm² is obtained in evolutionary model from the observed zenith angular distribution of extensive air showers.In Appendix 2 the diffuse X-and -ray flux arising together with neutrino flux is calculated. It agrees with observed flux in the entire energy range from 1 keV up to 100 MeV.     

Evolution of the high-temperature plasma in the 15 June 1973 flare

The analysis of the high temperature plasma in Fe xxiii–xxiv in the 15 June 1973 flare is presented. The observations were obtained with the NRLXUV spectroheliograph on Skylab. The results are: (1) There was preheating of the active region in which the flare occurred. In particular, a large loop in the vicinity of the flaring region showed enhanced brightness for many hours before the flare. The loop disappeared when the flare occurred, and returned in the postflare phase, as if the energy flux which had been heating the large loop was blocked during the flare and restored after the flare was gone. The large magnetic fields did not change significantly. (2) The flare occurred in low-lying loop or loops. The spatial distribution of flare emission shows that there was a temperature gradient along the loop. (3) The high temperature plasma emitting Fe xxiii and xxiv had an initial upward motion with a velocity of about 80 km s^–1. (4) There was large turbulent mass motion in the high temperature plasma with a random velocity of 100 to 160 km s^–1. (5) The peak temperature of the hot plasma, determined from the Fe xxiii and xxiv intensity ratio, was 14 × 10⁶ K. It decreased slightly and then, for a period of 4 min, remained at 12.6 × 10⁶ K before dropping sharply to below 10 × 10⁶ K. The density of the central core of the hot plasma, determined from absolute intensity of Fe xxiv 255 Å line, was of the order of 10¹¹ cm^–3.The persistence of the high level of turbulence and of the high temperature plateau in the decaying phase of the flare indicates the presence of secondary energy release. From the energy balance equation the required energy source is calculated to be about 3 to 7 ergs cm^–3 s^–1.Ball Brothers Research Corporation.     

Additional electron flux detection after magnetic perturbations

An additional electron flux at an energy above 100 MeV was observed in the experiments carried out with high-altitude balloons flown at geomagnetic latitudes 46° and 49°, in the upper layers of the atmosphere, on the days following magnetic perturbations.Its intensity, equal to 6 × 10^–2 cm^–2s^–1 sr^–1, decreased over 20–30 hours. The effect observed confirms the presence of high-energy electrons in the regions of the trapped radiation.     

CRRES Measurements of Energetic Helium During the 1990–1991 Solar Maximum

During the 1990–1991 solar maximum, the CRRES satellite measured helium from 38 to 110 MeV n^–1, with isotopic resolution, during both solar quiet periods and a number of large solar flares, the largest of which were seen during March and June 1991. Helium differential energy spectra and isotopic ratios are analyzed and indicate that (1) the series of large solar energetic particle (SEP) events of 2–22 June display characteristics consistent with CME-driven interplanetary shock acceleration; (2) the SEP events of 23–28 March exhibit signatures of both CME-driven shock acceleration and impulsive SEP acceleration; (3) below about 60 MeV n^–1, the helium flux measured by CRRES is dominated by solar helium even during periods of least solar activity; (4) the solar helium below 60 MeV n^–1 is enriched in ³He, with a mean ³He/⁴He ratio of about 0.18 throughout most of the CRRES mission `quiet' periods; and (5) an association of this solar component with small CMEs occurring during the periods selected as solar `quiet' times.     

Abundance enhancements of silicon to iron in solar energetic particles and their implications

Differential energy spectra of low abundant elements between silicon and iron of energetic solar particles (SEP) in the August 4, 1972 event were measured in the energy region of 10 to 40 MeV amu^–1 using rocket-borne Lexan detectors. The relative abundances of elements were determined and abundance enhancements, i.e., SEP/photospheric ratios, and their energy dependence were derived in 10–40 MeV amu^–1 interval. It is found that there are four types of abundance enhancements as a function of energy as follows: (a) silicon, iron, and calcium show fairly strong energy dependence which decreases with increasing energy and at 20–40 MeV amu^–1 reaches photospheric values; (b) in case of sulphur enhancement factors are independent of energy and the values are close to unity; (c) argon shows energy independent enhancements of about 3 to 4 in 10–40 MeV amu^–1; (d) titanium and chromium show weakly energy-dependent, but very high abundance enhancement factor of about 10 to 40. These features are to be understood in terms of the atomic properties of these elements and on the physical conditions in the accelerating region. These are important not only for solar phenomena but also to gain insight into the abundance enhancements of cosmic-ray heavy nuclei.on leave from Tata Institute of Fundamental Research, Bombay, India.     

Altitude variation of ultraviolet oxygen night airglow

Night airglow of oxygen 130.4 and 135.6 nm emissions was measured by a spectrophotometer aborad an S520 sounding rocket, launched at 19:50 JST (10:50 UT) on 14 February, 1982 from Kagoshima, Japan. The altitude variation of the emissions was obtained from 110 to 266 km at zenith angles of 35.5°±4°. The emission intensity around 260 km was about 160R and is roughly compatible with model calculations taking account of O⁺+e ^– radiative recombination as well as O⁺–O^– mutual neutralization. Some excess of about 50R, compared to the model calculation, was observed around 200 km. Possible explanations of the excess are: (i) remnant oxygen ions during the transition period from day to night and (ii) diffuse radiation from the background sky. Model calculations taking account of remnant oxygen ions were also performed by adding an excess electron density to the original density profile. However, it was found that an unreasonably large electron density is required around 200 km (5×10⁵ cm^–3) to produce the observed intensity. It is also probable that some contribution from the background sky is present in the observed intensity.     

A search for neutrons of solar origin using balloon borne detectors 1967–69

A series of telescopes having approximately a 30° half opening angle and responding to neutrons in the energy range 50 MeV to 350 MeV has been flown to the top of the atmosphere on balloons released from an equatorial launching site at Kampala, Uganda, between 1967 and 1969. The aim of the experiment was to attempt to detect solar neutrons during periods of enhanced solar activity. No neutrons of solar origin were detected, but an upper limit of the order of 30 neutrons m^–2 s^–1 at the Earth has been placed on the continuous solar neutron flux in the above energy range, and a limit of four photons m^–2 s^–1 has also been placed on the corresponding -ray flux above 80 MeV. Limits have likewise been placed on the total emission from various flares. For a 1B flare the values were 23 × 10⁴ neutrons m^–2 and 6 × 10⁴ photons m^–2.