MIT X-Ray Timing Explorer Project

From RP
Jump to: navigation, search

ASM Light Curves Overview

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: 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. Sources located in crowded regions or near a bright source may well have additional systematic error. 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.

Access the Data

Full Data

As mentioned in the introduction, this is a partial dataset for light purposes. To access the full dataset, visit Goddard Space Flight Center RXTE Guest Observer website: