  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 Gamma = 2.8 and luminosity
L_X \approx 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 2
L_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/Edot ~ 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.
