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new calibration files available for ASCA
There are new ASCA SIS Background datasets and SIS Response Matrices
available in the Calibration Database on legacy.gsfc.nasa.gov. These
datasets have been provided by Koji Mukai of the ASCA GOF.
The Background datasets are stored in the directory
/caldb/data/asca/sis/bcf/bgd/94nov and supercede all Background datasets
found in /caldb/data/asca/sis/bcf/bgd/94apr.
The SIS Response Matrices are stored in the directory
/caldb/data/asca/sis/cpf/94nov9 and supercede all matrices found in
Info files for both sets of data are included in their respective
directories and are appended below.
HEASARC Calibration Database
README for SIS Response Matrices
This directory contains 54 SIS response matrices
These are all made with version 0.8 of the SIS response generator,
sisrmg. Since most ASCA users have access to this software (and the
script, sisrsp, which provides a friendlier interface), only a representative
set is provided here.
File names are constructed from the following components.
Instrument: s0 or s1
Chip: c0, c1, c2 or c3
Event grade: g0234 or g02
Split threshold: p40
Echo correction: e0 [Bright 2 mode] or e2 [Bright or Fast mode]
No of channels: 1024 or 512
Channel type: h [PHA] or i [PI]
XRT response?: rmf [no] or rsp [yes]
For a more complete description, please refer to the release note which
has been sent out to the communty.
README FOR SIS BACKGROUND FILES
* New for 94nov directory: all the PI columns have been filled using the
best available calibration (as of late Oct) to a uniform 3.65 eV/channel
* This directory contains SIS background event files:
* Usage: Select one of the files that is appropriate for your data
(i.e., one that has the same cut-off rigidity criterion applied
as you used for your source). Read the file into XSELECT,
_then_ 'set phaname PI' if you wish a PI spectrum of the background
(current XSELECT resets itself to using PHA when you read in a SIS
event file). Apply the same region filter as used for the source
(in detector coordinates), extract spectrum, then save. To obtain a
512 channel spectrum, answer 'yes' to the question "rebin or group?";
for a 1024 channel spectrum, answer 'no' and then, outside of XSELECT,
% rbnpha <in.pha> <out.pha> 1024 bright2linear
These files contain events from 3 deep field observations without
any obvious sources and with a total exposure of about 120 ks. The
events were cleaned using the "select mkf" command in XSELECT
(with ELV_MIN > 20 and SAA = 0). Hot and flickering pixels were
removed using the "sisclean" command in XSELECT (sisclean method
2, with cell size 5, log probability -5.24, background level 3). Finally,
any spurious events which remained were removed by manual
cleaning based on the light curve (i.e. excluding spikes and drop-
The background rate is a weak function of the lower limit of cut-off
COR all 6 8 10 12 14
SIS-0 0.174 0.173 0.172 0.170 0.163 0.152
SIS-1 0.202 0.198 0.197 0.194 0.189 0.184
For a 40 ksec observation or longer, the distribution of cut-off
rigidity is likely to be similar to the blank field data, while
shorter observation may have a very different distribution. About
half the good data have COR>10, the other half COR<10. COR<6
data suffer from significantly higher background rate.
We typically recommend COR minimum of 6. For this, background rate
in different energy bins are:
E 0.4-1 1-2 2-6 6-12 0.4-12 All
SIS-0 0.0357 0.0447 0.0396 0.0196 0.139 0.173
SIS-1 0.0379 0.0461 0.0422 0.0270 0.153 0.198
"All" includes a lot of 'junk' above 12 keV.
In order to prevent the extraction of astrophysical information, the
files have a spurious pointing direction of RA =0, dec=0. This means
that X=DETX and Y=DETY. TIME is set to DBLE(PHA) and the events
are sorted in ascending order of DETX
SIS background consists of four components: extragalactic, Galactic
and instrumental (of which hot/flickering pixels are treated
(1) Hot/flickering pixels: The "sisclean" procedure removes the vast
majority of flickering pixels. However, low duty cycle flickering pixel
events cannot be cleaned out. Moreover, how many of the low level
flickering pixels are left depends on observing conditions (Sun angle,
CCD temperature etc.), the exposure time and the cleaning criteria
used. The background event files should be a reasonable indication of
the flickering pixel background, but not completely accurate.
(2) Extragalactic X-ray background: This component dominates the
SIS background in the range 2-6 keV, where it is well-determined
and relatively independent of pointing directions. At lower energies,
the different Galactic Nh does make the determination unreliable.
Note also that there could be unresolved, or barely resolved
background sources in the background and/or target field of view.
(3) Galactic X-ray background: No attempt has been made to create
background event files that would enable subtraction of this
component, as it depends entirely on the pointing direction. This
component is very important around 1 keV.
(4) Internal background (excluding hot/flickering pixels): The
background event files should give some indication of the overall
level of this component. However, its contribution depends on the
observing conditions and cleaning criteria, as well as the clocking
mode of CCD. The total level is around 0.008 cps/CCD in 4-CCD mode
(which is the case for the background event files), 8% higher in 2-CCD
mode and 15% higher in 1-CCD mode. This component is dominant at
high energies, and has fluorescent lines of Al, Fe, Ni and Au.
This component dominates in the range 6-12 keV.
The total background rate is about 0.05 cps per CCD chip (or 0.014
cps per 3.5 arcmin radius circular beam), and the use of the
background files will allow estimations accurate to ~0.005 cps per
chip (0.0015 cps per 3.5 arcmin beam). At the lowest energies, this
procedure leaves uncertainties due to the variable absorption of
extragalactic background, the Galactic background, and the variable
efficiency in removing flickering pixels. At the highest energies, the
instrument (non-flickering) component leaves uncertainties
particularly where it has lines.
For questions, please write to Koji Mukai: