X-ray emitting class I protostars in the Serpens dark cloud
Astronomy and Astrophysics, 428, 569-577 (2004)
We analyze a set of three individual XMM X-ray observation
of the Serpens dark cloud.
In addition to the 45 sources already reported in the analysis
of the first of these XMM observations by Preibisch (2003),
the complete combined data set leads to the detection of X-ray emission
from four of the 19 known class~I protostars in the region.
The set of three observations allows us to study the variability
of the sources on timescales from minutes to several months.
The lightcurves of two of the four X-ray detected class I protostars
show evidence for significant variability; the data suggest at least four
flare-like events on these objects. This relatively high level of
variability in the X-ray emission from the class I protostars is
in qualitative agreement with the result by Imanishi et al.~(2001), who
the class I protostars in the rho Ophiuchi dark cloud show a higher level
than that of more evolved class II and class III young stellar objects.
This may support non-coronal X-ray emission mechanisms
for class I protostars
and is in agreement with the predictions of models that assume magnetic
the protostar and its surrounding disk as a source of high-energy emission.
We also find a
strong variation (by a factor of ~10) in the X-ray luminosity
of the class II object EC~74 between the three observations, which may
be explained by a long duration flare or by rotational modulation.
Finally, we find no evidence for X-ray emission from the five
class 0 protostars in the region.
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