The stars examined here were primarily culled from two
lists of X--ray sources in Orion. The objects in the lists were
detected by pointed observations using the ROSAT X--ray
satellite. The observations, made by Walter (1993, 1994, 1995),
cover two regions in Orion. One region is six degrees in north--south aspect
and ten degrees east--west, centered on the belt of Orion.
The second region is two degrees in
extent east--west and ten degrees north--south with 
Orionis
being near the northern edge.
X--ray satellites have been proven to be extremely efficient in finding
PMS stars
which would have gone unnoticed by other search techniques such as
objective prism imaging (Walter 1986). Near the dark clouds of the
Taurus--Auriga star forming
region, it has been estimated that the nTTs (detected primarily via their
X--ray flux) outnumber cTTs by a ratio of nearly 10 to 1 (Walter et al.
1988). More recent work in Taurus-Auriga gives similar
(a ratio of 9:1) results and suggests an age dependence
of the ratio (Neuhäuser et al. 1995).
The ROSAT point source lists provided over 800 X--ray sources
with fluxes at least three
above the background located in Orion north
of M42. To limit this to a more tractable number, I chose
to concentrate on the regions densest in X--ray sources. In this way,
the observing program could be very efficient by observing many X--ray
sources in every exposure. By selecting the best regions to
monitor, I could measure rotational periods for the highest possible
number of PMS stars. In the end, two main regions were
chosen for monitoring: stars near 
Orionis and stars northwest
of the belt of Orion. The group of sources associated with 
Orionis are within the OB1b association. It was chosen since it represented
an extreme of compactness with over 50 sources in the central 25
arcminutes. The total
area of this region is about 900 minutes of arc. This region was
observed by ROSAT during 2 pointed observations. The first of these was
25 Kseconds of exposure time using the Position Sensitive Proportional
Counter (PSPC). Supporting ribs obscure part of the PSPC's nominal
1
radius field of view. Problems caused by the obstruction are small,
since the central field of view is clear and the ribs move relative to
the sky due to spacecraft wobble. A larger problem with the PSPC is
scattered light from the bright central X--ray source (
Orionis), which raises the local
background and makes detection of faint sources difficult.
A followup 15 Ksecond exposure was obtained by M. Freyberg
(1994) using the High
Resolution Imager (HRI), which had an unobstructed view of
25 arcminutes centered on 
Orionis and higher resolution.
These data were processed
using SASS processing and analyzed using IDL/RX software
(Walter 1993). Source extractions were performed using the SEX procedure.
Sixty--three sources were identified within 25
of 
Orionis.
While both PSPC and HRI data were available, the HRI was the primary source of
targets, because it was much more complete near 
Orionis.
The HRI detected 18 sources that PSPC missed.
Sixteen of these sources are within 6
30
of 
Orionis, so they
were probably missed due to scattered light.
The PSPC identified 22 sources within 15
of 
Orionis in addition to
the sources listed in Table 1, giving a total of
85 X--ray sources in the 
Orionis region.
It is not surprising that the PSPC would
detect additional sources since it is about 5 times more sensitive than
the HRI. Of the 22 sources, 8 had no obvious optical counterpart
and one was the bright star SAO 132412. This left 13 sources additional sources
to be monitored. All the X--ray sources near 
Orionis with optical
counterparts are listed in Table 1.
A second region to the northwest of the belt in the OB1a was also chosen because it was expected that this group would represent somewhat more evolved stars.
This area was
initially observed by the PSPC using a 7 Ksecond exposure. The data were
analyzed in a similar manner to those near 
Orionis.
One PSPC field covers
over 3000 arcminutes of field. This is too much to monitor with the
same time resolution as the 
Orionis field.
The area of analysis was limited
to the 1350 arcminutes which was densest in X--ray sources. These
sources are listed in Table 2
