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Astron. Astrophys. 320, 783-786 (1997)

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Is RX J1856.5-3754 an old neutron star?

S. Campana 1, 2, S. Mereghetti 3 and L. Sidoli 3

1 Osservatorio Astronomico di Brera, Via Bianchi 46, I-22055 Merate (Lc), Italy (campana@merate.mi.astro.it)
2 Affiliated to I.C.R.A.
3 Istituto di Fisica Cosmica del C.N.R., Via Bassini 15, I-20133 Milano, Italy ((sandro, sidoli)@ifctr.mi.cnr.it)

Received 4 July 1996 / Accepted 30 September 1996


An unusual X-ray source, RX J1856.5-3754, has recently been discovered with ROSAT: its spectrum resembles that of supersoft sources, but the very high X-ray to optical flux ratio excludes the presence of a companion star, pointing to an isolated compact object. It has been proposed that RX J1856.5-3754 is an old, isolated neutron star accreting from the interstellar medium. Here we present a reanalysis of the ROSAT data. The HRI observation reveals an attitude reconstruction problem, resulting in a larger error box than previously reported. Deeper optical observations allow us to reveal a few optical counterparts inside the new error box, but the X-ray to optical flux ratio is still very high ([FORMULA]) and none of the candidates has peculiar colors.

Key words: stars: individual: RX J1856.5-3754 – accretion – X-rays: stars – stars: neutron

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998