Early observations, 1965

The X-ray background was not anticipated. It was discovered in 1962 during the rocket flight which first detected Sco X-1, the first successful attempt to detect X-rays from sources other than the Sun or Earth. An uncollimated detector viewing about 10 000 square degrees of the sky was used. Giacconi et al. (1962) concluded that the background was of ‘diffuse character’ and due to X-rays of about the same energy as those from Sco X-1. The observed diffuse signal in this detector could have been generated by a few moderately strong point sources spread over the sky. The next observations, however, with detectors collimated to observe only 100 square degrees, showed the background to be indeed diffuse and of uniform brightness to at least 10 per cent.

There was no doubt that this background existed. The signals observed were strong and unmistakable. When detectors in rocket payloads were uncovered, pointed at any part of the night sky, the count rate always increased. All early observations, without exception, showed a few bright sources embedded in a uniform X-ray glow. The night sky at X-ray wavelengths was uniformly bright! Sources appeared superposed on this background, rather like stars viewed with the naked eye on a night with a full moon; when the faint stars disappear into the background of moonlight scattered from the atmosphere. Because no structure was observed and the emission was apparently uniform, this phenomenon has been called the ‘diffuse X-ray background’.