new asca gis background files available

[zellar@timaeus.gsfc.nasa.gov (Ron Zellar)]

New asca GIS background datasets are available via anonymous ftp from
legacy.gsfc.nasa.gov.  The datasets are located in the directory
/caldb/data/asca/gis/bcf/bgd/sp_off and are complementary to the current
set of background datasets.

Additional information about these datasets can be found in the README file
located in the sp_off directory and is appended below.

These datasets were provided by Ken Ebisawa of the ASCA GOF.

Ron Zellar
HEASARC Calibration Database

-----------------------------------------------------------------------------

README FOR GIS SPREAD_OFF BACKGROUND FILES 11/15/94 written by Ken Ebisawa

The files in this directory, listed below, contain GIS background data (blank
sky observations) taken during the early PV phase when GIS Spread
Discriminator is off.

blanksky_g2_sp_off_10cor12.evt      blanksky_g3_sp_off_10cor12.evt
blanksky_g2_sp_off_12cor14.evt      blanksky_g3_sp_off_12cor14.evt
blanksky_g2_sp_off_14cor.evt        blanksky_g3_sp_off_14cor.evt
blanksky_g2_sp_off_4cor6.evt        blanksky_g3_sp_off_4cor6.evt
blanksky_g2_sp_off_6cor8.evt        blanksky_g3_sp_off_6cor8.evt
blanksky_g2_sp_off_8cor10.evt       blanksky_g3_sp_off_8cor10.evt
blanksky_g2_sp_off_all.evt          blanksky_g3_sp_off_all.evt
blanksky_g2_sp_off_cor4.evt         blanksky_g3_sp_off_cor4.evt

In the GIS Gas cell, X-rays ionize Xe atoms, and the electrons are
accelalated and emitts UV lights. The UV-photons are detected by the
position sensitive photo-multiplier-tube (PMT). GIS has a on-board event
discriminator based on the spatial distribution of the UV-lights.
Distribution of UV lights by X-rays is spatially concentrated, but
on the other hand, particle BGDs tend to have more extended UV-light
distribution. Hence, by setting appropriate discriminator for the UV-light
spatial extension (Spread Discriminator), BGD events are effectively rejected.

In the early phase of the ASCA mission, experimentally, GIS is
operated with the Spread Discriminator being OFF.  This condition is
shown with the FITS keyword S_DSCR='OFF' in the event file primary header.
(See below for the exact period of the S_DSCR='OFF'.)
GIS BGD during this period is very different from
that when the Spread Discriminator is ON.  The files in this directory are
made for Guest Observers to subtract/estimate BGD for GIS data analysis
during the period Spread Discriminator = OFF.

Each file in this directory is a superposition of 3 blank sky fields
observed during the period when spread discriminator = OFF.
The files having *all* in their name
(each for the two detectors) includes all the events, regardless of the
magnetic cut-off rigidity (COR) values.  Since the GIS internal
background increases as COR decreases, other files are sorted by COR
values (in units of GeV/c).

RTI (Rise Time Invariance) column is filled, thus
particle background is efficiently removed with the 'gisclean'
command in XSELECT, which applies PHA dependent RTI window to the
event lists.

## Sequence/Period/Target/ when the Spread Discriminator = OFF ##

* All the observations up to May 28, 1993, 80015000, Hydra A.

* 40007000 Cyg X-3, June 11-12, 1993
* 60038000 NGC253,  June 12-13, 1993
* 20000000 30 Dor,  June 13- 14,1993, but a part of this observation
  was carried out with S_DSCR=ON.

  All the following observations were carried out with S_DSCR=ON
  (the first observation of this period is 80016000, Coma Cluster,
  June 14, 1993).

  In any case, please find the FITS keyword 'S_DSCR' in the
  event file primary header to see if the Spread Discriminator
  is ON or OFF.

################################################

If you have questions, please write to Ken Ebisawa at:

ebisawa@gsfc.nasa.gov (English)
ebisawa@astro.isas.ac.jp        (Nihongo)