Exosat observation of the candidate X-ray counterpart of Geminga

Abstract

During a search for X-ray counterparts of the COS B≥100 MeV γ-ray sources (see refs 1, 2), observations using the Einstein Observatory revealed a new X-ray source, 1E0630 + 178, inside the ∼1/2 square degree error box of the γ-ray source 2GC195+4, ‘Geminga’³. On the basis of its interesting properties, this source has been proposed as the X-ray counterpart of Geminga. A faint (m_v∼21), star-like object lying at the inner boundary of the HRI 98% error box has been suggested^4–7 as the optical counterpart of the X-ray (and thus the γ-ray) source. No radio counterpart has been detected, despite several searches^7,12,13. The region of Geminga was observed with the European Space Agency (ESA) Exosat satellite on 9–10 September 1983, from 22.30 to 07.55 UT. Due to malfunctions in orbit, both Position Sensitive Detectors of the low energy (LE) telescopes were not operable, and only the Channel Multiplier Arrays (CMA1 and 2) gave useful data. The medium energy (ME) and Gas Scintillation Proportional Counter (GSPC) instruments also recorded data although their sensitivity is insufficient to detect this soft ≃0.1 UFU source, also not detected by Einstein's Monitor Proportional Counter (MPC).