THE NATURE OF X-RAYS FROM YOUNG STELLAR OBJECTS IN THE ORION NEBULA CLUSTER - A Chandra HETGS Legacy Project

This first release provides the community with a first cut of confusion cleaned X-ray spectra of the Orion Nebula Cluster observed with the HETG onboard the Chandra X-ray Observatory. The data were taken starting in 1999 until 2021.

The confusion cleaning is based on several aspects of sources for confusion, which includes cluster point sources intersecting with grating dispersions, grating arms intersecting each other in CCD space, as well as grating dispersion overlaps prohibiting proper order sorting. The latter is a major effect and resulted in sometimes severe data losses. In the first release, our automated procedure took care of the vast majority of point sources and grating arm intersections. With respect to the dispersion arm overlaps, in this release we took a statistical approach optimizing the agreement of all four grating dispersion arms in the merged data to agree within a 1 sigma statistical uncertainty over 90% of the bandpass between 2 and 15 Angstrom. For that we used the zero order flux fractions of the interfering sources as the driving parameter.

There are still many caveats and rooms for improvement, which we will address in upcoming releases, which include the treatment of the increasing background at high dispersion, improve extraction efficiency, exclude observations with non-detections before confusion cleaning, include possible new detections, investigate the 5 A excess we observe in the HEG, though at low statistics, spotcheck individual observations for any residual issues.

Release 1 provides the community with an excellent starting point for addressing our identified science projects. Out of the 46 sources that were extracted, 37 resulted in valid spectral data. 7 sources have less than 1000 counts in 1st order, some of those may not yet be very useful.

Each directory contains the merged cleaned spectrum and responses for
one source.  The file "pha2" is a Type II PHA file (multiple spectra)
containing the four first order spectra, HEG -1, HEG +1, MEG -1, and
MEG +1.  Headers have been edited indicate the object (OBJECT), and
start and stop times for the set of observations.  Since the exposure
depends on order, due to the cleaning process, EXPOSURE is a column in
the data table.  Some other keywords now say "MERGED" since they can
vary with observation.

There is one effective area file per order (".arf" files).  These have
also been merged by zeroing out the same regions as excluded in the
count spectra, and summed weighting by exposure.  They also have
similar header edits as for the spectra.

While the exposures in headers may say 2 Ms, the actual exposure at
any wavelength may be much less.  This is not explicitly known, but is
implicit in the ignored wavelength regions in the merged counts and
responses.

There is one grating response matrix (".rmf" files) per order.  Since
all spectral extractions of all sources used the same cross-dispersion
region, there is no change in these files between sources.  One set
suffices for all extractions.  These are in the directory "RMFs", and
also for convenience have symbolic links in each source directory.

HETG background files have also been provided, one PHA file per first
order, in directory HETG_Background.   These have been derived from
long observations of blank fields.  Details are provided in the
accompanying memo, hetg_background.pdf.

Headers have not been designed for auto-loading of responses (that is
CORRFILE, RESPFILE, and BACKFILE are set to 'none').