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Updated ASCA "Calibration Uncertainties"
Dear ASCA PIs and Archival Researchers:
An updated version of the "Calibration Uncertainties" is now available.
You can find it on the ASCA GOF WWW home page, the URL of which is:
The document contains an account of the current problems and limitations
of ASCA data and calibrations. Everyone who analyzes ASCA data should
For those without access to WWW, a text-only version is appended to this
Charles Day, ASCA GOF, NASA GSFC
Please send comments and questions to email@example.com
CALIBRATION UNCERTAINTIES AS OF 5 MARCH 1995
Analysis of extended sources.
Guest observers interested in doing spatially-resolved spectroscopy are
referred to the report by Takahashi et al. available at the ftp site
ftp.astro.isas.ac.jp in the file asca/report/xrt_US.ps.
Although GIS data can have time stamps precise to 0.061 msec, the actual
accuracy is considerably less. The time assignment at KSC is probably
accurate only to 0.2-0.3 msec. Other limiting factors are inaccuracies
in: (i) the on-board clock drift correction, (ii) the orbit-to-ground
data transmission delay, and (iii) the delay within the ground station.
As a result, data between two consecutive KSC contacts may keep relative
accuracy of time down to about 0.1 msec, while two data sets, reproduced
at different KSC contacts, may have relative shift of time up to 0.5
msec. However, absolute time seems to have a systematic offset of at
least a few tens msec. These limits have been derived from observations
of the Crab and PSR 0540-69 pulsars.
GIS gain: intensity dependence.
It is found that the GIS gain is dependent on the source flux when
the flux is very large (> 0.1 Crab, or about 100 counts per second). For
bright sources, the gain becomes larger up to 1.5 per cent according to
the source flux.
GIS gain: spatial dependence.
The GIS team now believes that the determination of how the gain varies
spatially across the GIS field of view is uncertain by about 1 per cent.
According to the GIS team, this uncertainty may actually represent a
fundamental limit based on the properties of the instrument. That is,
further calibration may not improve the accuracy of the spatial gain
calibration. Users should be aware of this uncertainty when analyzing
their data. Methods for dealing with it are being investigated. In
addition to the spatial uncertainty, the GIS team has found that the
current calibrations are not sufficient to track the gain variations
which arise as the satellite experiences variations in solar
illumination. The GIS team estimates that the amplitude of these gain
fluctuations is about 0.5 per cent. The spatial gain calibration of GIS3
is less accurate than that of GIS2.
SIS gain calibration.
The gain calibration of the two default chips (SIS0-chip1 and SIS1-
chip3) agree to within 0.5 per cent. The other chips are not as well
calibrated, and may differ by up to 2 per cent. Due to radiation damage,
the gain is changing with time. This drift can be compensated for by
using PI rather than PHA channels. The PI column in event files can be
populated by using the FTOOL SISPI for 1-CCD mode data. However, in 4-
CCD mode this does not work well.
SIS low-energy response.
>From various reports it appears that SIS spectral fits generally
overestimate the column density by up to 2E20, indicating a probable
overestimate of the effective area at low energies (<1 keV). In
addition, gain and offset in energy scales and the variation of the
Galactic soft X-ray background over the sky may contribute to
uncertainties. (For extended sources, where blank field data must be
used for background subtraction, the latter is a serious and unavoidable
problem.) Therefore, column measurements with the SIS should not be
believed at about the 2E20 level.
SIS energy scale offset.
Due to light leaks, the energy scale of both SIS has a variable offset
(Dark Frame Error, or DFE) of up to 30 eV. It is noticeable when: (i)
the Sun angle is less than 90 degrees during satellite day; and (ii) the
elevation angle above the Earth's bright (Sun-lit) limb is less than
30 degrees. For Faint mode data, this can mostly be corrected by using
the FTOOLS FAINTDFE and FAINT, except when the DFE is changing rapidly
(such data should be excluded from analysis). However, FAINTDFE is not
perfect, particularly for recent 4-CCD mode data: a 1-2 ADU (~5 eV)
offset may remain. This can cause a spurious features at various edges
where instrument throughput changes suddenly, such as 0.54 keV oxygen
Users can also investigate the effects of the energy scale offset on
their data by using the "gain" command in XSPEC. Since this command
works by shifting the energy boundaries of the response matrix, rather
than by changing the matrix elements themselves, it should be regarded
as an approximation. However, it does provide a simple first step toward
addressing the problem. The "gain" command takes as its arguments:
o The datafile number.
o The factor by which the slope of the gain is multiplied.
o The offset, in keV, by which the gain is shifted.
Thus the command:
XSPEC> gain 1 1 0.020
will multiply the slope of gain of the response of the first datafile by
a factor of 1 - i.e., leave it unchanged - but shift the offset by +20
eV. In practice, users should first fit their data with the original
gain, and then iterate toward the correct gain by adjusting the gain and
refitting the model until parameter values, especially of sharp
features, agree with their expected values and with their GIS values.
GIS and SIS response: gold edge residuals.
Both GIS and SIS have an artificial hump feature at around 2 keV, near
the gold edge energy (the reflecting surfaces of the XRT are coated with
gold). This feature has been incorporated into the ARF generator for the
SIS and GIS but may not have been completely modelled away. Please be
cautious when interpreting possible features around 2 keV.
GIS response: residuals in Crab spectrum.
The Crab nebula was observed for 50 ksec in September 1994, and 375
million photons(!) collected. The energy spectrum shows a clear
deviation from a power-law, which originates not in the data, but the
model (response). A wavy structure appears at 5 to 9 keV with the
amplitude +/- 4 per cent. This is the current limit of the GIS response.
GIS RMF: RTI cleaning always assumed.
The current GIS RMF is made for data which have passed through the
PHA-dependent RTI (Rise Time Invariance) window (data filtered with the
'gisclean' command in XSELECT). When RISE_TIME is not available (e.g.,
fast timing observations), gisclean is not usable, and events outside
the PHA-dependent RTI window cannot be rejected. Applying the current
GIS RMF to the data without the RTI filter is not precisely correct, and
will lead to a flux apparently larger by 1 to 5 per cent in the 1-10 keV
band (larger in higher energies). A modification to ASCAARF will be made
available soon to deal with this limitation.
GIS background uncertainty.
As a first approximation, GIS internal (non-X-ray) background is
proportional to the magnetic cut-off rigidity (COR). Blank sky data and
night-earth data sorted by COR have been released for GOs to
estimate/subtract GIS background. However, using only COR does not
perfectly describe time variation of GIS background. That is, there are
other minor parameters which control the GIS background. For a given
COR, the GIS non-X-ray background can be different by 20-30 per cent.
The GIS team is investigating the nature of the GIS background, and
trying to model the background more accurately.
XRT PSF: azimuthal dependence
The XRT point response function has an azimuthal energy dependence. Not
much quantitative information is available at the moment: there appears
a factor of two difference at 10 keV between the lobes of the "Maltese
GIS efficiency at low energy.
Both GIS have greater efficiency in the range 0.6-0.9 keV than predicted
by the response matrix. The difference is ~20 per cent at maximum and
will be corrected in the next version of the response.
The GIS tends to give larger (~20 per cent) absolute flux than the SIS.
The SIS flux seems to be reliable, so the problem is probably in the GIS
response (the plasma shield is the likely origin). GIS team is now
revising the GIS response.
SIS absolute efficiency.
The ground calibration of the SIS established the absolute uncertainty
in its efficiency to be 13 per cent.
Underestimated XRT effective area above 6 keV.
In the early phase of the mission the XRT model had less effective area
above 6 keV than is observed. The difference is ~40 per cent by 10 keV.
This discrepancy led to an apparent hard tail but has now been
incorporated into ASCAARF.
GIS gain map now covers full FOV.
The latest GIS gain maps, which are incorporated into ASCALIN, are now
accurate for the full GIS field of view (50 arcmin diameter). Data from
the edge of the FOV can be reliably analyzed, although particle
background remains high at the edge.
Gold edge fudged in XRT response.
The gold edge which appears around 2 keV has been incorporated into
ASCAARF for both the GIS and SIS. However, users should still be
cautious when interpreting spectral features around 2 keV.
SIS gain drift can be corrected for 1-CCD mode.
The FTOOL SISPI corrects the secular gain drift of the SIS by converting
PHA to PI. The correction works for 1-CCD data but is of limited
accuracy for 2-CCD and 4-CCD data.