Welcome to the ASM Light Curves Overview
Quick-look results provided by the ASM/RXTE team
Web page design and software coding by Derek Fox


Please Read Me

This web site provides an overview of ASM results for the convenience of users in preparing proposals and other work. The full and
official site is on the web page of the RXTE Guest Observer Facility (GOF) at Goddard Space Flight Center. That site makes available all ASM data products, including light curves in three energy bands, details of the solutions, and SSC pointing histories.

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.

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:
  1. We use a single elevation correction based on the 2-10 keV flux from the Crab nebula. This elevation correction, however, has a spectral dependence because of increased low-energy absorption in the thermal shields and counter windows. Sources of different spectral shape than the Crab, then, may well have greater scatter.
  2. Sources located in crowded regions or near a bright source may well have additional systematic error.
  3. Other less frequent but substantial systematic errors, not yet quantified, may occur.
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.

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.

Filter Criteria

  1. Reduced chi-square of the fit <1.5, except for Sco X-1 <8.0
  2. Number of sources in the field of view <16
  3. Earth angle >75 degrees
  4. Exposure time >30 seconds
  5. Long-axis angle -41.5< theta <46 degrees
  6. Short-axis angle -5< phi <5 degrees
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:
  1. MJD of the observation (JD - 2,400,000.5)
  2. SSC unit number
  3. Dwell sequence number
  4. Dwell number
  5. Reduced chi-square of the fit
  6. Number of sources in the field of view
  7. Earth angle (degrees)
  8. Fitted ASM unit count rate (counts/second; Crab is ~75)
  9. Estimated error (counts/second)
  10. Exposure time (seconds)
  11. Long-axis angle theta (degrees)
  12. Short-axis angle phi (degrees)
  13. A channel background count rate parameter (counts/s)
The "One-Day Averages" ASCII data files contain five columns:
  1. MJD of the observation (JD - 2,400,000.5)
  2. Averaged ASM unit count rate for the day (counts/second; Crab is ~75)
  3. RMS estimated error (counts/second)
  4. RMS deviation of the points from the one-day mean (counts/second)
  5. Number of dwells averaged

The "Power Spectrum" ASCII data files contain two columns:

  1. Frequency (in 1/hours)
  2. Power/<Power> for that frequency
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.


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.