V. S. Iyengar

The flattening of the X-ray spectrum of Sco X-1 at energies beyond 40 keV

The measurements of the hard X-ray spectrum of Sco X-1 in the energy interval 20–150 keV in three balloon flights from Hyderabad, India are reported. These results show conclusively that the spectrum of Sco X-1 is very flat in the energy interval 40–150 keV and the measured fluxes beyond 60 keV are several orders of magnitude higher than those expected on the basis of an extrapolation of bremsstrahlung spectrum from a thin hot plasma at a temperature corresponding tokT=5 keV, which is applicable for Sco X-1 for energies <40 keV. The results are compared with those of other investigators of hard X-rays from Sco X-1, and the implication of the results is briefly discussed.

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.

Sudden Changes in the Intensity of High Energy X-Rays from Sco X-1

WE report the observation of sudden changes in the intensity of Sco X-1 by a factor of about three in the energy interval 29.9–52.3 keV on December 22, 1968, between 0427 h and 0553 h UT. The observation was made with an X-ray telescope in a balloon flight from Hyderabad (latitude 17.6° N, longitude 78.5° E), India. The telescope consisted of an NaI(Tl) crystal with an area of 97.3 cm2 and thickness 4 mm surrounded by both active and passive collimators. The passive collimator was a cylindrical graded shield of lead, tin and copper and the active collimator was a plastic scintillator surrounding the shield. The field of view of the telescope at f.w.h.m. was 18.6°. The axis of the telescope was inclined at an angle of 32° to the zenith. The telescope was mounted on an oriented platform programmed to look in four specified directions successively, spending about 4 min in each direction during a cycle of period about 16 min. The four specified directions were N(ϕ = 0), SW(ϕ=110°, with the convention ϕ = 90° being due west), S(ϕ = 180°, due south) and NE(ϕ = 310°). In this flight the orienter performed satisfactorily and the telescope picked up Sco X-1 in the south direction, Cyg X-1 in the direction NE(ϕ = 310°) and a new source in the direction SW(ϕ = 110°). In the north direction (ϕ = 0) there was no source during the period of observation and therefore the information on background X-rays was obtained from this direction. The observations of the short term changes of intensity of Sco X-1 are discussed here—the results from the other data will be published separately. A pair of crossed flux gate magnetometers provided information every 8.2 s on the azimuth of the telescope. The pulse heights from the X-ray detector were sorted into nine contiguous channels extending from 10 to 120 keV. Counting rates of several channels were combined for analysis to improve the statistics. A 241Am source came into the field of view of the telescope once in 15 min for about 30 s to provide in-flight calibration of the detector and this indicated that there was stable operation and no drift in the channel positions during the flight. The balloon was launched at 0200 h UT and reached the ceiling altitude at 0435 h. Between 0435 and 0530 h, the ceiling altitude was 7.6 ± 0.3 g cm−2. Just before the balloon reached the ceiling Sco X-1 was in the field of view of the telescope for 3 min 41 s when the balloon was ascending from 9.2 to 8.3 g cm−2. After the balloon reached ceiling, Sco X-1 was in the field of view on five occasions between 0443 and 0553 h. During the last observation, 0551–0553 h, however, the floating altitude of the balloon was a little lower, 8.5 g cm−2. The meridian transit of Sco X-1 was at 0454 h and the flight was terminated at 0615 h.

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.

A balloon borne instrument for the study of cosmic x-rays

Design features of a baloon borne instrument for the study of cosmic X-rays, are described. Advantages of equatorial location for investigations in X-ray astronomy are discussed. Details of the X-ray telescope and associated instrumentation are described. A summary of the performance of the instrument and results obtained in three balloon flights are presented.

The absorption of soft X-rays from the Crab nebula

The X-ray spectrum of the Crab nebula has been determined in the energy range 0.5 10 keV using thin window proportional counters carried aboard a Centaur IIA rocket launched from TERLS, India. The spectrum can be well represented by a power law with an exponent−2.1 beyond 2 keV. The absorption of the soft X-ray component below 2 keV is clearly seen in the experiment. Attempts to understand quantitatively the spectral features in terms of interstellar absorption lead to a column density of hydrogen in the iirection of the Crab nebula of 3.5×1021 H atoms cm−2, if we adopt a revised version of the interstellar absorption coefficients of Brown and Gould to include the contributions of heavier elements, especially of iron. This value of density is a factor of 2 higher than the density obtained from 21 cm radio observations, but falls well within the range of values for atomic and total hydrogen deducible from UV measurements with satellites and the measured visual extinction coefficients for the Crab nebula. It is concluded that it is not necessary to consider anomalous abundance of elements like carbon or neon either in the source or in the interstellar medium as suggested by some authors. The absorption of X-rays in the interstellar dust in the light of current dust models is presented.

An orientation system for balloon-borne payloads

A new, simple system to orient a balloon-borne X-ray telescope about a vertical axis is described. The telescope is mounted in servo controlled gimbals resulting in a low moment of inertia for the criented part. The servo uses a flux-gate magnetometer as a direction sensor. Pointing accuracy of ±1 o has been obtained in flight.

Energy Spectra of Several Discrete X-ray Sources in the 20–120 keV Range

In this paper we report on our observations of hard X-rays from several X-ray sources in the energy range 20–120 keV. The results were obtained from the data collected during two balloon flights made from Hyderabad, India (latitude 17.6°N, longitude 78.5°E). The first flight was made on April 28, 1968, and the balloon reached a ceiling of about 5.3 g cm−2 residual atmosphere and floated from 0230 to 0800 hrs. IST (Indian Standard Time). The second balloon was launched on December 22, 1968 and floated at about 7.5 g cm−2 of residual air from 1000 to 1130 hrs. IST.