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Scaled Map Transient Analysis Synopsis

Return to Main BAT Transient Monitor Page

For more information contact: Hans Krimm, NASA GSFC / USRA

We ask that when results from these analyses are used, they be referenced as "Swift/BAT transient monitor results
provided by the Swift/BAT team." Please also reference the following article: Krimm, H. A. et al., 2013, ApJSS 209, 14.
This team includes all those working on the BAT at the Goddard Space Flight Center, the Los Alamos National
Laboratory and other institutions in the United States and Japan.

Please Read Me

This web site provides an overview of Swift/Burst Alert Telescope (BAT) survey results for the convenience of users
in monitoring sources, preparing proposals and other work. The complete set of official Swift data products is
available via the Swift archive at the HEASARC at NASA's
GSFC, as well as the ASI Science Data Center in Italy and the
UK Swift Science Data Center. Those sites make available all Swift survey data products,
including 80-channel histograms, spacecraft pointing information and calibration files.

A full description of the BAT transient monitor is now available here: Krimm, H. A. et al., 2013, ApJSS 209, 14.

The FITS and ASCII-formatted data available here are provided primarily for convenience. While the Swift/BAT team
hope our instructions and information are adequate for most users, we are not prepared formally to support users of
these data. We do, however, appreciate feedback.

These pages contain the Swift Burst Alert Telescope (BAT) hard X-ray transient monitor. Light curves are provided
for 979 sources and are updated each time new data from the BAT becomes available. The transmission and processing
delay averages six hours, but depends on the timing of Swift data downloads: for instance there are ~8 orbits each day
in which the Swift satellite does not pass over its primary ground station at Malindi, Kenya.

Galactic and extragalactic sources are selected for inclusion in the catalog if they are either detected by
BAT on a daily basis or have had a significant outburst during the lifetime of
Swift. Other bright known galactic sources are also included in the catalog.

Light curves for most sources go back through 12 February, 2005. Sources that have been added to the catalog after
December 2011 will have shorter light curves. Work is in place to re-analyze all data and provide light curves for
all sources back to 12 February, 2005.

Each source in the table has a link to a page containing two light curves. The top plot (red) is a light curve of the
daily average counts for the source. The baseline (zero counts) is shown as a solid line and the average flux is
shown as a dotted line. The lower plot (blue) is a light curve of the average flux for each complete Swift pointing.
To keep the plots from being too crowded, the time scale for the orbital (Swift pointing) plots is limited to the
past twenty days. At the top of each page is a link from which one can download the FITS light curve file from which
the plot was made or an ASCII version of the light curve. Points with extremely large errors (more than four times
the average statistical error) and large (> 10 sigma) negative fluctuations are not plotted, although all points
are included in the light curves. Points which are not included in the plots for these reasons are given a data
quality flag of 1 (large negative fluctuation) or 2 (large error bar). Good points have a data quality flag of 0.

The points in the daily plots are the weighted average of all observations starting and ending on that calendar day
(from 0 UT to 23:59:59 UT). Since Swift terminates and restarts all observations at midnight UT, data points do not
span the day boundary. For the last point (the current day), the daily average is a running average which averages
all observations up through the most recent data received and processed. Therefore this data point will change
through the day as new data is accumulated. For example if a source has a short, strong outburst early in the day, the
weighted average for the day will start out high, but will be reduced as data from times after the outburst are
averaged in.

Basic analysis:

The BAT flight software produces "scaled maps" as part of its on-board transient monitor. Scaled maps are maps of the
detector array in a single energy band (15-50 keV) and are compressed, or scaled, in such a way as to reduce file
size, but not lose any information. Scaled maps are produced as part of the image confirmation for BAT GRB rate
triggers and on times scales ranging from 64 seconds to a full observation for the on-board image trigger. Only maps on
time scales >= 64 seconds are analyzed in this analysis.

Each scaled map is processed using the standard BAT software tools. Each map is converted to a sky image using
BATFFTIMAGE and then compared to a catalog of known sources using BATCELLDETECT . (For
complete information on these and other BAT analysis tools please see:
Swift Data Analysis site
This program also searches for unknown sources at high significance. A
further step involves cleaning bright sources from the images using BATCLEAN , a process which allows
more faint sources to be detected.

The analysis process also involves significant data verification including rejecting corrupted data and episodes with
missing attitude data or invalid star camera solutions. There are also several corrections applied which are
discussed below.

Explanation of data fields in output catalogs:

Daily results

1 TIME [d] Modified Julian Date
2 RATE [count/cm^2/s] Rate for object
3 ERROR [count/cm^2/s] Total error (statistical plus systematic added in quadrature)
4 YEAR [yr] Year
5 DAY [d] Day of Year
6 STAT_ERR [count/cm^2/s] Statistical error
7 SYS_ERR [count/cm^2/s] Systematic error
8 DATA_FLAG Data quality flag (0= good; 1= large negative fluctuation; 2= large error bar; 3= both)
9 TIMEDEL [s] Total exposure time acculumated in the daily average rate
10 TIMEDEL_CODED [s] Total exposure time scaled by the partial coding fraction for each pointing
11 TIMEDEL_DITHERED [s] Total exposure time when Spacecraft dither flag == 0

Single pointing results

1 TIME [s] Swift Mission Elapsed Time (Seconds since 1 Jan 2001)
2 RATE [count/cm^2/s] Rate for object
3 ERROR [count/cm^2/s] Total error (statistical plus systematic added in quadrature)
4 YEAR [yr] Year
5 DAY [d] Day of Year
1 MJD [d] Modified Julian Date
1 TIMEDEL [s] Exposure of bin
6 STAT_ERR [count/cm^2/s] Statistical error
7 SYS_ERR [count/cm^2/s] Systematic error
7 PCODEFR Partial coding fraction: Fraction of BAT detectors exposed to the source
8 DATA_FLAG Data quality flag (0= good; 1= large negative fluctuation; 2= large error bar; 3= both)
9 DITHER_FLAG Spacecraft dither (0= yes; >0= no)

Corrections and data cuts:

Most systematic errors are corrected in the analysis. Here is a summary of corrections and data cuts applied to the
data set:

Data cuts

  • All periods with missing attitude data or periods during which the attitude solution is not stable including
    periods when the spacecraft is still settling.
  • All periods with errors in the star camera solution.
  • Occultation effects. Since the BAT field of view is so large, it is possible for sources near the edge of the
    field to be occulted by the earth. Periods of earth occultation are removed from analysis. Since the moon and sun
    cover such small fractions of the FOV, moon and sun occultations are not calculated.
  • Any times when less than 10000 detectors are enabled. This is an effective filter against corrupted or
    incomplete data files or times when only a small fraction of the BAT array was operating.
  • Any times with more than 15 hot detectors. When the spacecraft is close to
    the South Atlantic Anomaly (SAA) then the overall rate increases and fluctuations
    in the rates in different detectors are magnified, leading to increased noise in
    the images. This cut is quite effective at removing such episodes from
    consideration. Note that the flight software does not record data during the
    peaks of SAA passages.
  • For each source the analysis script determines the percentage of the BAT array illuminated by the source; this is
    known as the partial coding fraction. If the partial coding fraction for a particular source in a given pointing is
    less than 10%, then that pointing is not included in the light curve for that particular source.

    Corrections

  • Correction for varying numbers of enabled detectors.
  • Geometric effects including the cosine-theta effect and the partial coding effect wherein fewer detectors are
    illuminated when the source is off-axis.
  • Effects of material in the field of view. As the source gets closer to the edge of the field, gamma-rays must
    pass through more material in the mask support structure before reaching the detectors. An empirical correction to the
    flux is applied as a function of off-axis angle. However, because the nature, density and distribution of material
    in the edges of the field of view is not perfectly known, the correction is not perfect.

    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

    In order to understand residual systematics in the distributions of counts from catalog sources, the BAT transient
    monitor catalog includes 106 "blank" points in the sky, randomly distributed across the sky and chosen to be at least
    10 arcminutes from any reported X-ray source. The light curves from these blank sky positions can be seen here .

    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 systematic error was derived from an empirical analysis of the Crab light curve in which it was found that there
    was more scatter in the data points than could be explained by statistical variations alone. Since the data
    presented here are not corrected using the BAT response matrix, these errors are expected to affect the measured flux
    by ~10%. When averaging the Crab data above 0.2 counts/cm2/sec, it was found that the residual scatter in the
    orbit light curve had a standard deviation of 4.6%, and in the daily light curve 3.5%. This value was applied to
    all light curves, but only makes an important contribution to bright sources.

    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