B. V. Sreekantan

A total absorption spectrometer for measuring the energies of nuclear interacting particles of the cosmic radiation in the energy range of a few GeV to a few hundred GeV

Abstract An instrument referred to as a “total absorption spectrometer” (of area 1.4 m 2 and solid angle = 0.25 steradians) has been constructed for measuring the energies of nuclear interacting particles of the cosmic radiation in the energy range of a new GeV to a few hundred GeV. The general features of the design, construction and the procedure used for calibration are discussed as also the factors which govern the accuracy (≈20%) of energy measurement.

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 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.

Very High Energy Gamma Rays from the Vela Pulsar

Four independent data sets with slightly different gamma ray energy thresholds are separately analysed and the phasograms are derived using the pulsar elements derived from radio measurements. Each phasogram shows a weak double pulse structure at the same Dhase bins separated by 0.45 of a phase, where the first pulse coincides with the expected position of the optical first Dulse. However the statistical significance of the signal improves when lower energy showers only are chosen, based on the pulse height information or the source hour-angle. When the first three 3 phasograms after applying this cut are summed, the resultant phasogram shows 2 peaks at 4.0 σ and 1.5σ level. If one multiplies the significances calculated for individual phasograms and after allowing for various degrees for freedom, one obtains a value of 8.0x10-4, for the probability that all the excesses are statistical fluctuations. From the excesses observed at different gamma ray energy thresholds an integral energy spectrum of VHE gamma rays from Vela is derived. The estimated slope of this power law energy spectrum is -(1.8±0.2)

Time variations of hard X-rays from Sco X-1

Simultaneous hard X-ray and optical observations of Sco X-1 were carried out on 1971 May 1 at Hyderabad, India, when Sco X-1 was optically bright. The X-ray intensity observed by balloon-borne counter telescopes increased in coincidence with optical enhancements, while the plasma temperature derived by fitting the X-ray spectrum in the energy range 20–40 keV to the thermal bremsstrahlung spectrum did not appreciably change over the whole period of observation.

Triggered spark counter arrays of large area (square meters) for experiments on very high energy cosmic ray particles

SummaryThe constructional and operational features of triggered spark counters of large area are described. These counters are being used, in conjunction with nuclear emulsions, in current experiments in this laboratory on high energy nuclear interacting particles which arrive simultaneously over large areas, sometimes associated and at other times unassociated with air showers. For these experiments the spark counters and emulsions are being used in arrays covering an area of a few square meters.RiassuntoSi descrivono le caratteristiche costruttive di contatori a scintillazione di grande superflcie. Questi contatori vengono usati, in unione con emulsioni nucleari, per esperimenti correnti in questo laboratorio su particelle ad alta energia da interazioni che arrivano simultaneamente su larghe aree, alcune volte in associazione con sciami dell’aria ed altre volte senza associazione con questi ultimi. Per questi esperimenti i contatori a scintillazione e le emulsioni sono usati in disposizioni che coprono un’area di alcuni metri quadrati.

High energy X-ray timing and spectral observations of SCO X-1, CYG X-1, GX 5-1, CYG X-3 and GX 1+4

Abstract A high energy X-ray astronomy payload with phoswich scintillation detectors was flown from Hyderabad, India on December 18, 1984. Five galactic X-ray sources, Sco X-1, Cyg X-1, Gx 5-1, Cyg X-3 and GX I+4 were observed. The timing data, with a high time resolution of 40 μs, were analysed for periodicities and quasi-periodic oscillations (QPOs). GX 1+4 was in a low luminosity state and showed pulsations with a period of 95.9 s. Its period derivative was calculated. Sco X-1 was found not to exhibit QPOs in 18–40 keV X-rays with a 90% confidence limit of 0.20 on amplitude. Spectra are presented for Sco X-1, Cyg X-1 and GX 5-1.

A high sensitivity phoswich scintillator X-ray telescope for hard X-ray (20 – 120 keV) astronomy from balloon platform

Abstract A large area (400 cm 2 ) low background X-ray telescope consisting of four collimated NaI/CsI scintillator phoswich detectors (each 100 cm 2 ) was built and successfully flown several times during 1980–1984. The phoswich configuration enables one to eliminate X-ray background produced by high energy atmospheric and cosmic gamma-rays. Combined with large area, small field of view (5° × 5°) and large exposure time that was possible due to an on-board telescope pointing programmer, for the 20 – 120 Kev energy range, 3σ sensitivity achieved was 5 × 10 −6 photon/cm 2 sec Kev for exposure time of 10 4 sec at observational altitude of 4 g/cm 2 . The Instrument characteristics, relevant details on pointing system, detector system, associated electronics and telemetry and in-flight performance in 1983 and 1984 are presented.

Cosmic-ray intensity at great depths and neutrino experiments

SummaryA group from the Tata Institute of Fundamental Research, Bombay, has recently completed measurements on cosmic-ray intensities deep underground up to depths of 8400 m w.e. On the basis of these observations it is suggested that, for investigations on naturally occuring neutrinos, there are great advantages to be gained by performing the experiments at considerable depths — around 9000 ft. — below the earth‘s surface, essentially because of the very low background which is encountered at such depths. Some features relevant to neutrino physics which emerge from the measurements already made and the design of future experiments to detect cosmic ray produced and extra terrestrial neutrinos are discussed.RiassuntoUn gruppo del Tata Institute of Fundamental Research, Bombay, ha recentemente completato le misure delle intensità dei raggi cosmici sotterra sino a profondità di 8400 m a.e. Sulla base di queste osservazioni si suggerisce che si potrebbero ottenere grandi vantaggi eseguendo esperimenti a considerevoliprofondità — circa 9000 ft. — sotto la superficie, essenzialmente grazie al bassissimo fondo che si riscontra a queste profondità. Si discutono alcune caratteristiche, interessanti per la fisica del neutrino, che emergono dalle misure già fatte ed il progetto di nuovi esperimenti per rivelare i raggi cosmici prodotti ed i neutrini extra terrestri.

Studies on extensive air showers

SummaryAn experiment carried out at sea level on the lateral distribution of nuclear-interacting particles in air showers and the variation of their number with the size of the showers, is described; the showers recorded ranged in size from 104 to 2.5·106 particles. The results indicate that the number of nuclear-interacting particles is proportional toNe0.45±0.05 for showers of sizeNe less than about 6·105 particles, and the number is proportional to ∼Ne1.2 for larger sizes. What has been obtained in this experiment and in similar experiments conducted by other groups (Nicol’skyet al. andLehaneet al.), is theaverage number of nuclear-interacting particles in a large number of showers of a given size. The possibility that such an average may not be meaningful if large intrinsic fluctuations (other than normal statistical variations) exist, is pointed out.RiassuntoSi descrive un esperimento effettuato al livello del mare sulla distribuzione laterale delle particelle che interagiscono coi nuclei negli sciami dell’aria e sulla variazione del loro numero con la dimensione dello sciame; gli sciami registrati variavano in dimensione da 104 a 2.5·106 particelle. I risultati indicano che il numero di particelle che interagiscono coi nuclei è proporzionale aNe0.45±0.05, per sciami di dimensioneNe inferiore a circa 6·105 particelle, e proporzionale a ∼Ne1.2 per dimensioni maggiori. Ciò che si è ottenuto con questo esperimento ed altri simili eseguiti da altri gruppi (Nikol’skyet al. eLehanet al.), è il numeromedio di particelle che interagiscono coi nuclei in un gran numero di sciami di una data dimensione. Si mette in rilievo la possibilità che una tale media sia priva di significato se esistono ampie fluttuazioni intrinseche (diverse dalle normali variazioni statistiche).