Dear ASCA PIs,

To coincide with the opening of the ASCA archive, the ASCA GOF has made
available a new set of GIS background datasets. These files are
applicable to observations made in the early part of the PV phase when
the GIS Spread Discriminator was not turned on. PLEASE NOTE THAT THE
GIS BACKGROUND DURING THIS PERIOD IS VERY DIFFERENT FROM THE NOMINAL,
"SPREAD DISCRIMINATOR ON" BACKGROUND.

The new asca GIS background datasets are available via anonymous FTP at
legacy.gsfc.nasa.gov in the directory:

  /caldb/data/asca/gis/bcf/bgd/sp_off

They are complementary to the current set of background datasets.

Additional information can be found in the accompanying README file
which is appended below.

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

Ron Zellar, HEASARC Calibration Database
Charles Day, ASCA GOF

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              README FOR GIS SPREAD_OFF BACKGROUND FILES

The files in this directory, listed below, contain GIS background data
(blank sky observations) taken during the early PV phase when the GIS
Spread Discriminator was 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 xenon atoms, and the electrons are
accelerated and emit UV light.  The UV-photons are detected by the
position sensitive photo-multiplier-tube (PMT). The GIS has an on-board
event discriminator based on the spatial distribution of the UV light.
The distribution of UV light caused by X-rays is spatially concentrated,
whereas that of particle events tends to have more extended
distributions. Hence, by setting appropriate discriminator for the UV-
light spatial extension (Spread Discriminator), background events can be
effectively rejected.

In the early phase of the ASCA mission the GIS was operated,
experimentally, with the Spread Discriminator 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 BACKGROUND 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 background for GIS data analysis
during the period  when the Spread Discriminator was off.

Each file in this directory is a superposition of three blank sky fields
observed during the period when the Spread Discriminator was off. The
files having *all* in their name (one each for the two detectors) include
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).

The RTI (Rise Time Invariance) column is filled, thus particle background
is efficiently removed with the 'gisclean' command in XSELECT, which
applies a PHA dependent RTI window to the event lists. IF YOU USE RTI 
(VIA GISCLEAN) TO REJECT BACKGROUND IN YOUR SOURCE, YOU SHOULD ALSO USE 
IT ON YOUR BACKGROUND.


             CASES FOR WHICH THE SPREAD DISCRIMINATOR WAS OFF 

The Spread Discriminator was off for the following sequences, times and
targets:

  o ALL the observations up to 1993 May 28 (80015000, Hydra A).

but also:

  o 40007000, 1993 June 11-12, Cyg X-3, 
  o 60038000, 1993 June 12-13, NGC 253
  o 20000000, 1993 June 13-14, 30 Dor (but a part of this observation
      was carried out with S_DSCR=ON).

All subsequent observations were carried out with S_DSCR=ON. The first
observation of this period is 80016000, 1993 June 14, Coma Cluster.

In any case, it is a good idea to verify the Spread Discriminator
setting by reading th the FITS keyword 'S_DSCR' in the event file
primary header.

Ken Ebisawa, 16 November 1994

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

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