X - Ray Emission from the Planet Pulsar B 1257 + 12

We report the detection of the millisecond pulsar B1257+12 with the Chandra X-ray Observatory. In a 20 ks exposure we detected 25 photons from the pulsar, with energies between 0.4 and 2.0 keV, corresponding to the flux F X = (4.4 ± 0.9) × 10 −15 ergs s −1 cm −2 in this energy range. The X-ray spectrum can be described by a power-law model with photon index Γ ≈ 2.8 and luminosity L X ≈ 2.5 × 10 29 ergs s −1 in the 0.3–8 keV band, for a plausible distance of 500 pc and hydrogen column density N H = 3 × 10 20 cm −2. Alternatively, the spectrum can be fitted by a blackbody model with kT ≈ 0.22 keV and projected emitting area ∼ 2000 m 2. If the thermal X-rays are emitted from two symmetric polar caps, the bolometric luminosity of the two caps is 2L bol ∼ 3 × 10 29 ergs s −1. We compared our results with the data on other 30 millisecond pulsars observed in X-rays and found that the apparent X-ray efficiency of PSR B1257+12, L X / ˙ E ∼ 3 × 10 −5 for d = 500 pc, is lower than those of most of millisecond pulsars. This might be explained by an unfavorable orientation of the X-ray pulsar beam if the radiation is magnetospheric, or by strong asymmetry of polar caps if the radiation is thermal (e.g., one of the polar caps is much brighter than the other and remains invisible for most part of the pulsar period). Alternatively, it could be attributed to absorption of X-rays in circumpulsar matter, such as a flaring debris disk left over after formation of the planetary system around the pulsar. Most of millisecond radio pulsars (MSPs) reside in binary systems, usually with a white dwarf companion. It is generally believed that they have been spun up to millisecond periods by accretion from their companions. About 20% of the known MSPs are solitary pulsars (Lorimer 2005 1), which either lost their binary companions or were born with short periods and low magnetic fields. Both binary and solitary millisecond pulsars can emit X-rays from their magnetospheres. Similar to " ordinary " (non-recycled) pulsars, this magnetospheric X-ray emission is usually characterized by a power-law (PL) spectrum with photon indices Γ ≈ 1–2 and sharp X-ray pulsations (e.g., Becker & Pavlov 2001; Zavlin 2006ab). …