The ascii-formatted data available here are provided primarily for convenience. While MIT and the GOF 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.
We ask that when results from these analyses are used, they be
referenced as "quick-look results provided by the ASM/RXTE team."
This team includes all those working on the ASM at MIT and at the
Goddard Space Flight Center SOF and GOF.
About ASM Light Curves
Each raw data point represents the fitted source flux from one 90
second dwell. Data for the light curves are taken from the last 100
days of observations, using all three Scanning Shadow Cameras, and are
quoted as nominal 2-10 keV rates in ASM counts per second, where the
Crab nebula flux is about 75 ASM c/s (when at the center of an SSC
field of view, with 8 anodes operational). The points plotted in red
are "three-sigma" detections (but read
"About the Uncertainties", below).
Each "one-day average" data point represents the one-day average of the fitted source fluxes from a number (typically 5-10) of individual ASM dwells. The error value used in plotting the one-day averages is the quadrature average of the estimated errors on the individual dwells from that day (note that for sources expected to show little flux variation on shorter time scales, the RMS of the individual flux measurements -- fourth column of the data files -- may provide a better measure). With these estimated uncertainties, the points plotted in red are "three-sigma" detections.
The ASM light curves are updated on a near-weekly basis. The data
have been filtered to include only those points that are less likely
to give a spurious intensity; filter criteria are summarized at the
bottom of this page. For the latest information on the status of the
instrument and analysis techniques, please see the
ASM Status
Reports at the
RXTE MIT
Home Page.
This problem is generic to any pair of sources that are close together
on the sky. It is mitigated when there are a number of strong sources
in the field of view of an SSC that together allow the analysis S/W to
obtain with high probability the correct ASM rotation angle. When the
problem does occur, it is particularly noticeable when one of the
sources is very strong and the other is not.
As of late 2008 January, we are developing materials to give specific
indicators of the situations where the above problem may be an issue.
We urge users of ASM data to check with us in cases that are suspect.
The "Power Spectrum" ASCII data files contain two columns:
About the Uncertainties
The light curve of the Crab nebula gives a measure of the rms
error (5%) of which 3% is attributed to systematics. The error
bars for other sources, then, consist of counting statistics
convolved with a 3% systematic error. This will be an
underestimate of the error in many cases for the following
reasons:
For these reasons, the results should be treated with caution.
TOO alarms should not be generated until repeated observations
confirm the sighting with different SSCs and different scan
directions.
Important Caveat
A significant problem with a small number of ASM source intensities in
the light curve of a Galactic X-ray source has recently come to our
attention (2008 January). The source in question is just under one
degree of arc away from one of the bright GX sources. On rare
occasions, i.e., in roughly one out of 1000 dwells, the ASM analysis
software incorrectly attributed the flux from the GX source to the
source in question. The possibility of this is a consequence of the
"slip and slide" nature of the ASM data analysis algorithm. "Slip and
slide" refers to the procedure in the analysis algorithm in which
multiple camera orientations are assumed; for each orientation, a
complete coded-aperture analysis is performed and the orientation that
gives the smallest value of the chi-square goodness-of-fit statistic
is taken to be the correct orientation. This procedure is necessary
because there are circumstances when the measured value of the Drive
Assembly rotation angle which is obtained from potentiometer outputs
may be inaccurate. The problem noted above occurs when the best fit
is obtained for an incorrect Drive Assembly rotation angle where the
nearby source appears to be at the position in the field of view of
the given Scanning Shadow Camera (SSC) that is in actuality the
position of the bright source.
Filter Criteria
For the last two items, note that the full SSC field of view is 110
degrees (theta) by 12 degrees (phi).
Data Files
The "Data by Dwell" ASCII files contain thirteen columns:
The "One-Day Averages" ASCII data files contain five columns:
Power density spectra are created by averaging the light curves into
0.05 day (1 hour 12 min) bins, subtracting the mean value of all
inhabited bins from each inhabited bin, and taking an FFT. The
resulting power in each frequency bin is divided by the average power
over the whole frequency range. Note that the frequency is plotted in
inverse hours.
Acknowledgments
Data reduced and compiled weekly by the ASM team:
Hale
Bradt,
Deepto Chakrabarty,
Wei Cui,
Alan Levine,
Ed Morgan,
Ron Remillard,
Garrett Jernigan,
Bob Shirey, and
Don Smith.
Power density spectra produced by Don Smith.