Corrections

Dither flag

It is a known aspect of coded mask imaging that systematic errors arising
from the presence of bright sources in the field of view are spatially
correlated. This means that if two or more spacecraft pointings have the same
orientation on the sky (to within a few arcminutes) then fluctuations due to
systematics (either positive or negative) will tend to accumulate in a
particular location in the BAT field of view and hence at a particular
equatorial or galactic sky coordinate.

To mitigate this effect, starting on September 17, 2005, the Swift mission
operations team instituted a procedure known as "roll dithering." This means
that in successive pointings at the same target (same field center), the
spacecraft roll will be changed to a value within plus or minus one degree
of the original value. This is small enough so that it does not affect the
narrow-field instruments (NFIs) and ensures that systematic errors do not accumulate
in BAT images.

The roll dither procedure is done for most pointings. However, there
are certain situations in which roll dithering is not done. First, since the
dithering is commanded, there is no dithering for automatic targets (ATs):
GRBs or on-board fast transient response. Similarly there is not dithering
for targets of opportunity (ToOs). There are also other times when it is
decided not to do the dithering, either because the precise orientation of
source in the UVOT or XRT field is required, or for NFI calibration purposes.
Finally, for part of 2005 and early 2006, the dithering commands were
generated by hand, and sometimes this step was forgotten in calculating the
daily observing schedule.

The consequences for BAT transients of the "no dither" times is that
it is possible that small systematic errors
in individual pointings will add up when the daily averages are made. For
example a 2 standard deviation positive fluctuation in multiple single
pointings at the same sky location would accumulate and lead to a 7 or
higher sigma positive point in the daily averages. This means that the
significance of some positive daily average points is much higher than it
should be (or conversely the systematic error bars are underestimated). This
is only a problem for individual daily averages, since the spacecraft roll
is always changed each day. In other words, a positive source increase seen
on two or more days cannot be attributed to this systematic.

Starting on 6 December, 2007, we have added a "dither_flag" field to
the FITS and ASCII orbital light curves. A non-zero value of the
flag indicates that roll dithering was not in place for this pointing.
Although the adverse effects of no dithering are only seen in daily averages,
the dither flag itself has meaning only for an individual pointing since
most sources are seen in the BAT field of view for multiple different targets
and hence very different spacecraft orientations.

Bottom line: When you observe a high positive (or negative)
fluctuation for a source on a single day, please treat it with caution.
Look at the dither flag in the orbital light curves for that day and if most
of the pointings including the source are "no dither," then it is likely that
the positive fluctuation is not physical.

Error bars:

The error bars are a combination of statistical errors and systematic errors. Most systematic errors are corrected
in the analysis (see above) and the remaining systematics are accounted for in two systematic error terms as
described below.

Systematic error derived from blank sky positions

Since there are no sources in these locations the distribution of significances of counts from these locations should
follow a Gaussian distribution with zero mean and width of unity. As seen here ,
there are no systematic biases toward either high or low
significance, however, the width of the significance histogram is larger than one, which indicates that the statistical
errors underestimate the true distribution of errors. The statistical errors must therefore be increased by a
systematic factor which makes the width of the distribution unity. This correction was found to be 10% for the orbital
data and 23% for daily averages. It is as expected that systematic errors increase as the monitor duration increases.

This correction is a multiplicative factor which increases all statistical error values in the transient monitor
(included in the STAT_ERR column in the FITS and ASCII light curves.

Systematic error derived from the Crab light curves

This correction is found in the FITS and ASCII light curves in the SYS_ERR column and is derived by
multiplying the rate found in the transient monitor by the correction factors listed in the previous paragraph.

The full error reported in the ERROR columns and represented on the light curve plots is thus the
addition in quadrature of STAT_ERR and SYS_ERR.

It is important to note that there are strong spatial correlations in the BAT observations of a given source which can
place the Crab in the same location in the field of view for many days at a time. Swift is not a scanning survey
instrument. Its observing program is driven by the random location of gamma-ray bursts on the sky, and gamma-ray burst
afterglows are typically observed for many days. Thus in any given ~week long interval, the Crab is likely to be at
the same location in the BAT field of view and the same systematics will apply. This is a likely cause for the
coherent structure one can see in the light curve.

Hans A. Krimm

This page was last modified on Wed Aug 7 16:32:28 UTC 2013

The BAT Transient Monitor contact is:
Hans Krimm, hans.krimm@nasa.gov,
301-286-6955

• Curator: J.D. Myers
• NASA Official & Swift PI: Brad Cenko
• PAO Contact: Francis Reddy
• Questions/Comments/Feedback

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