Recurrent X-ray outbursts from Aquila X-1
Abstract
A HIGH degree of variability has been associated with Aquila X-1 (Aql X-1) (3U1908 + 00) since its occasional observation in early rocket surveys1-3, and its appearance as a variable source of moderate brightness in the Uhuru catalogue4 (S ~ 0.07-0.2 Scrab). Later observations of the source have been made from OSO-7 (ref. 5) during 1971-73 which suggest the existence of `low' (S ~ 0.02 Scrab) and `high' (S ~ 0.5 Scrab) intensity states, and from OAO Copernicus, where the source was reported at levels at least an order of magnitude below the Uhuru range during intermittent exposures over a 2-yr period through 1975 May (ref. 6). This apparent quiescent state was interrupted in 1975 June by a sudden flare7 (factor of >20 increase) to the level of the Crab nebula. Observations of the declining source by the Sky Survey Instrument (SSI) on Ariel V revealed an ~10% sinusoidal modulation8 (2-18 keV) at the period p = 1.3+/-0.04 d. By August the flux had decayed back to the pre-outburst intensity (<0.003 Scrab) as measured by the SSI (ref. 9), which instrument observed the same low flux from Aql X-1 again in 1976 January. In 1976 June, the All-Sky Monitor (ASM) on Ariel V detected an additional outburst of this source10, which was confirmed by the SSI (M. G. Watson, personal communication). We report here ASM observations of Aql X-1 obtained between the launch of Ariel V in 1974 October and 1976 November, and compare them with the previously cited additional data available. We suggest that the totality of these measurements imply that Aql X-1 may provide a plausible connection between dwarf novae and the very bright, long-duration transient X-ray source11.
- Publication:
-
Nature
- Pub Date:
- February 1977
- DOI:
- 10.1038/265606a0
- Bibcode:
- 1977Natur.265..606K
- Keywords:
-
- Binary Stars;
- Dwarf Stars;
- Novae;
- X Ray Sources;
- Ariel Satellites;
- Spaceborne Astronomy;
- Variable Stars;
- X Ray Astronomy;
- X Ray Stars;
- Astrophysics