Hard X-rays from Coma Constellation

Abstract

THE region of the sky defined by RA=190° to 330° and δ= –20° to +50° has been scanned for high energy X-ray sources in a series of five balloon flights carried out from Hyderabad since April 1968 using oriented X-ray telescopes^1–3. The sky map of hard X-ray intensity in the energy range 20 to 150 keV obtained from these flights is plotted in celestial coordinates in Fig. 1. Each circle in the figure represents one observation on a part of the sky in which the telescope axis passed through a celestial bin 10° × 10°/cos δ enveloping the circle. The different observations correspond to those made either at different times in the same flight or in different flights on different dates. The size of the circle represents the number of standard deviations σ by which the counting rate of the telescope is either greater than (filled circles) or less than (open circles) the background counting rate for that particular flight. The legend by the side of the figure gives the relation between the size of the circle and the σ value. Only those points for which the observation time was in excess of 40 s have been included in the plot; the positions of known X-ray sources are indicated by star symbols. The dotted circle represents the typical field of view of the telescope which corresponded to an FWHM of 18.5° in some flights-the circle slightly distorts into an ellipse for higher declinations. In other flights the field of view was 12°. The clustering of large diameter filled circles (high σ values) in the regions of known sources like Sco X-1, Cyg X-1 and GX 9+9 is clearly evident. There are two other regions, one enclosed by RA=200° to 225° and δ = +10° to −10° and another by RA= 195° to 215° and δ=25° to 50°, which also show significant clustering of high positive σ values. Details of the first region, which falls in the Virgo constellation, have been presented elsewhere3 together with results for the various known sources. We wish to present here the results for the second region of enhancement, which falls in the Coma constellation. This part of the sky was scanned on April 16, 1969, and May 5, 1970, and the evidence for enhanced emission has come from these two flights. The plots of the counting rates in celestial coordinates from these two flights for the relevant periods of observation are given in Figs. 2 and 3. (Complete azimuthal plots and celestial plots for the period of observation are available in the thesis of R. K. Manchanda, submitted to Bombay University.)