  We have identified a new externally irradiated Herbig-Haro (HH) jet, HH 1158,
within ~2 pc of the massive OB type stars in the sigma Orionis cluster. At an
Lbol ~ 0.1 Lsun, HH 1158 is the lowest luminosity irradiated HH jet identified
to date in any cluster. Results from the analysis of high-resolution optical
spectra indicate asymmetries in the brightness, morphology, electron density,
velocity, and the mass outflow rates for the blue and red-shifted lobes. We
constrain the position angle of the HH 1158 jet at 102+/-5 degree. The mass
outflow rate and the mean accretion rate for HH 1158 using multiple diagnostics
are estimated to be (5.2 +/- 2.6) x 10^(-10) Msun/yr and (3.0 +/- 1.0) x
10^(-10) Msun/yr, respectively. The properties for HH 1158 are notably similar
to the externally irradiated HH 444 -- HH 447 jets previously identified in
sigma Orionis. In particular, the morphology is such that the weaker jet beam
is tilted towards the massive stars, indicating a higher extent of
photo-evaporation. The high value for the Halpha/[SII] ratio is also consistent
with the ratios measured in other irradiated jets, including HH 444 -- HH 447.
The presence of an extended collimated jet that is bipolar and the evidence of
shocked emission knots make HH 1158 the first unique case of irradiated HH jets
at the very low-luminosity end, and provides an opportunity to learn the
physical properties of very faint HH jet sources.
