An optical counterpart to the anomalous X-ray pulsar 4U0142+61

• Published in: Nature (London) Vol. 408; no. 6813; pp. 689 - 692
• Main Authors: van Kerkwijk, M. H, Hulleman, F, Kulkarni, S. R
• Format: Journal Article
• Language: English
• Published: London Nature Publishing 07.12.2000; Nature Publishing Group
• Subjects: Accretion; Astronomy; Earth, ocean, space; Emissions; Energy; Energy sources; Exact sciences and technology; Late stages of stellar evolution (including black holes); Magnetic fields; Pulsars; Stars; Wavelengths; X-rays; Optical observation; X ray pulsar; Optical counterpart; Two color diagram; Energy distribution; Color-magnitude diagrams
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/35047024

The energy source of the anomalous X-ray pulsars (AXPs) is not understood, hence their designation as anomalous. Unlike binary X-ray pulsars, no companions are seen, so the energy cannot be supplied by accretion of matter from a companion star. The loss of rotational energy, which powers radio pulsars, is insufficient to power AXPs. Two models are generally considered: accretion from a large disk left over from the birth process, or decay of a very strong magnetic field (1015 G) associated with a 'magnetar'. The lack of counterparts at other wavelengths has hampered progress in our understanding of these objects. Here we report deep optical observations of the field around 4U0142+61, which is the brightest AXP in X-rays. The source has no associated supernova remnant, which, together with its spin-down timescale of ∼105 yr (ref. 5), suggests that it may be relatively old. We find an object with peculiar optical colours at the position of the X-ray source, and argue that it is the optical counterpart. The optical emission is too faint to admit the presence of a large accretion disk, but may be consistent with magnetospheric emission from a magnetar.