Observations of A 0620 - 00, 1975 Nova Monocerotis, by Ariel V

A 0620 – 00, the transient X-ray source identified with 1975 Nova Monocerotis, was observed by the rotation modulation collimator experiment on Ariel V between 1975 August and October. The position of the X-ray source was measured as: $$(1950\cdot0)\,\text{RA}\,:\,{6}^\text{h}\,{20}^\text{m}\,{16}^\text{s}\cdot6\,\text{Dec}.:\,-{0}^{^{\circ} }\,{2}^{\prime}\,{24}^{\prime{}\prime}$$ with an error box including the optical and radio counterparts. The intensity in $$3-7\cdot5$$ keV X-rays had dropped to less than a third of its maximum value after 63 days, accompanied by an overall slow softening of the spectrum. After 48 days the source brightened by 15 per cent, an enhancement lasting for 12 days with no noticeable spectral change until the decay resumed. Flaring, associated with spectral hardening, was observed shortly after maximum light. A search for periodic fluctuations between 64 and 1024 s gave an upper limit of $$0\cdot14$$ per cent.