New and Improved ASM Light Curves: The Continuing Story

RXTE GOF	New and Improved ASM Light Curves	RXTE FAQ

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visit the RXTE ASM X-ray Weather Map

For the latest results for a particular source
visit the ASM Source Catalog

For usage notes on the ASM Products
visit the ASM Data Productspage

For a comprehensive 'how-to' on data analysis techniques
visit to the ASM Recipe

Below you will find our newest infomation about the asm production process. You can also check out previous announcements related to the asm products.

January 24, 2002

NOTE: A slow calibration drift in SSC3, starting about MJD 52200, shows up as a flux increase in many sources (see, for example, the light curve of the Crab). ASM light curve users interested in behavior that is affected by this problem are urged to evaluate data from SSC3 separately, and exclude it if necessary.

December 1, 2000

For the past several months, the FITS versions of the ASM light curves have not been updated in the RXTE archive. Several issues combined in such a way that the GOF decided to wait until the next redelivery of the entire ASM light curve archive to restart updating the archived light curves.

That time has come! The ASM team at MIT has completed a new recalibration of the entire light curve data base, fixed a bug in the color channel information, and implemented a new and improved method of background subtraction which has resulted in cleaner light curves with a higher density of points.

Some things to note when analyzing the latest and greatest ASM data:

Recent Week-long RXTE Attitude Problem Affected ASM Data
As you probably know, the RXTE attitude control system was confused between MJD 51793.63 and 51800.65. The ASM results during this time interval are unreliable, since the nominal camera positions provided by the spacecraft were in error by an amount too large for reliable recovery within the ASM software. As a result there is a gap in every ASM light curve corresponding to these dates.
The ASM team is investigating ways of retrieving good data for the bad attitude time interval. (The ASM alone can only estimate the correct pointing positions when there are bright sources in the field of view, so the recovery rate is now uncertain.)
ASM Energy Channel Bug Has Been Corrected
A known bug which caused non-uniform numbers of ASM energy channels {A, B, C} in FITS files had been tracked down to dropouts (most in channel A) due to filtering on the chi square value, or due to elimination of results with S/N < -4.0. This problem is now fixed, and all of the ASM FITS files should now have uniform numbers of {sum, A, B, C} results.
Comment on Adjustments to ASM Channel Boundaries
Consistent with the channel-boundary changes of MJD 51548.625 (Jan 5, 2000), there are different but unique channel boundaries per SSC / band. Users should understand that the ASM channel boundaries were changed to make it easier to normalize (internal to ASM analysis software) the results so as to RETAIN the target ranges {1.5-3.0 ; 3-5 ; and 5-12 keV }. This was NOT a quantized gain change, but merely an adjustment to the evolving gain values in SSCs 1 and 3. Therefore, from the users' standpoint, these changes are NOT like the PCA gain shifts, where the User must be aware of the gain epoch and choose external calibration files to properly interpret the results. For the ASM, Users need only to recognize A from B from C, and ignore the fact e.g. that "A" changes its channel markings in two cameras on MJD 51548.625.
Our normalizations to SSC c/s have time-dependent factors chosen to hold constant the Crab count rates per band and the Crab hardness ratio (HR) values throughout the Mission. Where a given source has a spectrum very different from the Crab, there may be secular drifts in the HR value or a jump at the date of ASM gain change. Since we can only force the camera normalization to one spectrum (and the Crab is obviously the best reference), Users have two choices:
(1) if the signal-to-noise is sufficient, select ONLY SSC 2 (on the 1,2,3 scale used in the FITS files), which has NO gain evolution (and hence no shifts in channel boundaries). SSC 2-based HR curves of e.g. GRS1915+105 are appreciably better than the nominal 3-camera HR curves. (2) develop an intelligent ad-hoc HR correction tailored to a given source.
Updating the entire GOF ASM archive.
Periodically, the ASM team re-issues the GOF FITS files made from the entire ASM ascii database. The occasion for this is usually the end of their calibration and re-normalization efforts, upon a re-analysis of some number of weeks that may have shown degraded results. We have just been delivered the seventy version of the entire database. Starting with the *next* version, ASM FITS files will include header keywords that identify the version of the calibration used, and the date up to which the recalibrated solution is known.
A Temporary Difference Between FITS files in the GOF and ASCII light curves available through MIT.
The ASM team has added a 13th column to their archive of ascii light curves on the MIT web site. This new column will be incorporated into the next version of the FITS ASM Light Curves. The new column is the fitted value of the diffuse X-ray background in the A channel (softest), for the given dwell/SSC. This parameter has been found to be the best diagnostic of scattered solar X-rays (off Earth atmosphere or collimator), and can be used to flag suspicious points:
- ANY dwells with xbg_A > 40.0 will be eliminated from the ascii light curves (too high!).
- For faint sources, MIT recommends using xbg_A < 10.
Normal values are distributed 4.6 +- 2.1 c/s, with a typical uncertainty per dwell of 0.4 c/s.

Also of interest: ASM Team Lead Ron Remillard Describes a significant improvement in ASM Production Processing:

"I have been looking into data yield and source confusion, and conclude there is significant value in expanding the ASM production processing from 2 levels ("pass1" and "pass2") to 4 levels of processing. Each level is defined by the ASM catalog that drives the analysis. Pass 2 is the "minimum" catalog (all sources above 10 mCrab including current transients) and pass1 is the whole catalog.

"This is the first report on this topic (more to follow).

"The problem with the galactic center is that our threshold there is elevated by all the bright sources, while there are a bunch of "faintish" sources that appear to be above 10 mCrab, but less than 30 mCrab. They cause a lot of dropouts by increasing source confusion. It is highly questionable whether they MUST be included in the catalog, when analyzing data, e.g. for 14 brighter ones. For the sake of 14 really bright sources near the GC, I have created a "prime" processing catalog that gets rid of these faint ones. (prime[week] = pass2[week] - faint_GC) To de-confuse dwells further, I have also merged gcx-1 and e17407-2942 into one - by averaging the positions and adding a 6 arcmin blurring function.

"The purpose of "prime" is ONLY to deliver higher yields for the 14 brighter cases in the GC.

"Comparing old and new values, there does not appear to be any significant systematic problems in using the "prime" ASM catalog. This includes the rapid burster (x1730-333) in its "off" states.

"The increase in yield is substantial; see Table below.

"Prime" statistics:

(sampling IOC - week 239 ($1 < 51781.0))

. --amount ASM data-- .

name prime current Increase factor

scox1 25592 21884 1.169

x1700-377 19278 15565 1.239

gx349+2 18998 15257 1.245

x1702-429 19267 16342 1.179

gx9+9 18031 15407 1.170

gx354-0 17385 13521 1.286

x1730-333 17236 13135 1.312

ks1731-260 14864 9891 1.503

gcx-1 ** 10387 **

gx3+1 14571 9514 1.532

gx5-1 15753 10695 1.473

gx9+1 17353 13089 1.326

gx13+1 19686 16412 1.200

n6624 18137 14366 1.263

gx17+2 20904 18346 1.139

total[14] 257055 203424 1.264

** merged with e17407-2942 into a blurred "fake-gcx".

The results of this "prime" processing have now been included in the latest release of the ASM Light Curves.

Other Important Developments:

March 21, 2000: MIT has applied the new channel boundaries to all ASM sources. Mission-long light curves are now available which take this transition into account.

March 03, 2000: Channel boundaries were redefined for the ASM during week 205. New misison long light curves for all sources which account for this change, are expected in the near future.

June 20, 1999: The definitive products have now been updated with data accumulated through June 08, 1999.

With this update, the definitive products have also been revised back to the beginning of the mission using the calibration which the ASM team at MIT began applying in the spring of 1998 (see March 16, 1998 note below). Data after March 6, 1998 (Mission day 1525) are unchanged, but data previous to that are now in complete agreement with the ASCII data posted on the MIT ASM web pages.

Dec 1, 1998: We've summarized the new sources added to the ASM products on a separate web page. See New Sources in the ASM Products Archive for a summary of the sources added to the archive since the last whole-scale revision of the products in 1997.

March 16, 1998: MIT has applied their most recent calibration to the ASCII data for all the ASM sources back to the beginning of the mission. The ASCII light curves are available via the MIT ASM web pages. This correction has not yet been applied to the FITS formatted data, which are available via this page (see below). Hence, for times before MJD 50878 (= Mission Day 1525 = 1998 Mar 6), the MIT ASCII data and the FITS data currently differ. In the light curves, the differences are generally 1-2 %. In the colors, they can be 10 % in HR1 (1.3-3.0 keV / 3.0-5.0 keV) or HR2 (3.0-5.0 keV / 5.0-12.0 keV) The drifts are most significant in the energy channels of SSC1, where the lowest band drifts to lower flux and the highest band drifts to higher flux in the current FITS files.

The correcting of the FITS files for this latest calibration is underway.

August 2, 1997: The definitive products have now been updated back to the beginning of the mission using the improved ASM instrument model. This model, which has been used for the weekly updates since January 24, 1997, uses time dependent calibrations to track the anodes' response to the coded mask shadows as the SSCs evolve during the RXTE mission. In addition, several subtle geometric features of the detector window and strongback are also modelled for the first time. This new model has now been applied to the data previous to Jan 24, 1997 and new light curves and color files have been constructed. (The old model was static and simpler; it is unknown why the SSCs are evolving, with changes in SSC3 larger than SSC2, which in turn are larger than those for SSC1.)

The light curves and color files in the definitive products thus now result from a consistent model applied throughout the span of the mission. Note, however, that only the results for data previous to Jan. 24, 1997 have changed. The results for data taken after Jan 24, 1997 remain unchanged. The pointing files covering the time from January 5, 1996 to January 24, 1997 have also been revised using this new model.

If you have a question about RXTE, please send email to one of our help desks.

This page is maintained by the RXTE GOF and was last modified on Sunday, 27-Jan-2002 11:51:15 EST.