We were intrigued by the story “X-ray Outburst Reveals a Supernova Before It Explodes” ( Physics Today 0031-9228 61 8 2008 21 https://doi.org/10.1063/1.2970204 https://doi.org/10.1063/1.2970204 ), which describes the likely discovery of a core-collapse supernova by Alicia Soderberg and colleagues. 1 The story’s figure 2 resembles a similar x-ray light curve, reported by collaborators at Los Alamos National Laboratory, 2 from an x-ray outburst that occurred on 7 July 1969 and preceded by two days the x-ray nova Centaurus XR-4. 3
The spin periods of the Vela satellites that recorded the 1969 event were roughly 1 minute, and any location within the instruments’ field of view would be sampled for 2 or 3 seconds out of that period, followed by subsequent samplings every 60 seconds or so. When first observed, the precursor to the Cen XR-4 nova was already at its highest level, but the subsequent decline is almost identical to that of SN 2008D.
The outburst was discernable above background for seven minutes; 2 the Physics Today item indicates a similar duration for the outburst of SN 2008D. The x-ray nova part of the transient Cen XR-4 was observed two days later on 9 July 1969, the next time the satellites’ detector scanned that part of the sky.
An article about the original discovery of Cen XR-4 was published right around the time the nova phase was rapidly declining. By 24 September 1969, the source was no longer visible above background. In a second article covering the known life of the Cen XR-4 x-ray nova, 3 we stated that there was no definite optical identification of Cen XR-4; a nova outburst had not been reported at the location of the source.
It is not clear whether Cen XR-4 was a core-collapse supernova as the similarities between it and SN 2008D suggest. But it is certainly clear that the occurrence of x-ray precursors to energetic cosmic processes was documented in the 1972 event and again in 2008.
