Two X-ray periodicities from the vicinity of 4U1145–61

Abstract

EXTENDED Ariel V observations in September and December 1977 centred on 4U1145–61 reveal two X-ray periodicities at 4.95 and 4.86 min. The amplitude of both varied, the former by a factor 2, the latter by a factor 7. The shorter period decreased by 0.0015 min over an interval of 50 d while the longer increased by 0.0034 min in the same period. Measurements of the pulse phase revealed no evidence for orbital motion and upper limits to a_x sin i are given. It is not possible to determine whether these modulations both originate from 4U1145–61 or if there is an unreported source within 1° of 4U1145–61. The X-ray spectrum was unusual with a break at ∼9 keV. We have obtained spectra of the optical counterpart, HEN715, at 25 Å mm⁻¹ resolution in the range 3,700–5,300 Å and 6,200–7,200 Å. The Hα emission line was observed to change significantly in shape and central wavelength on a time scale of ∼30 min. We set an upper limit to pulsations in the Hα line of 5% relative to the continuum.