These are ACIS background event files (particle + sky, or particle-only for ACIS-"stowed" (or ACIS "undercover") data) for imaging observations. The format of the filename follows the form, for example, acisi_F_6_bg_evt_20130213.fits - ACIS-I in aimpoint, period F, chips 6 (ACIS-S2), version 2013 Feb 11. Period F is 2009-09-21T12:00:00 to now (early 2013). FP_TEMP = -120C, the background rate is declining from the peak. This dataset includes ACIS-I and ACIS-S observations in VFAINT mode. Files: Period "F": "Blank Sky", ACIS-I aimpoint: ACIS-0: acisi_F_0_bg_evt_20130213.fits ACIS-1: acisi_F_1_bg_evt_20130213.fits ACIS-2: acisi_F_2_bg_evt_20130213.fits ACIS-3: acisi_F_3_bg_evt_20130213.fits ACIS-6: acisi_F_6_bg_evt_20130213.fits ACIS-7: not enough data; extend Period E to cover this "Blank Sky", ACIS-S aimpoint: ACIS-2: aciss_F_2_bg_evt_20130213.fits ACIS-3: aciss_F_3_bg_evt_20130213.fits ACIS-5: aciss_F_5_bg_evt_20130213.fits ACIS-6: aciss_F_6_bg_evt_20130213.fits ACIS-7: aciss_F_7_bg_evt_20130213.fits ACIS-8: aciss_F_8_bg_evt_20130213.fits "Blank Sky", ACIS-S aimpoint, no CTI correction applied (added in CalDB 4.5.7): ACIS-5: aciss_F_5_bg_nocti_evt_20130213.fits ACIS-7: aciss_F_7_bg_nocti_evt_20130213.fits "ACIS Stowed": ACIS-0123567: acis_F_0123567_stowed_evt_20130213.fits ACIS-01236: acis_F_01236_stowed_evt_20130213.fits How these datasets were made: ----------------------------- Each event file is a combination of several observations of relatively empty fields, with the additional conditions of high galactic latitude (|b| > 20 deg) and low soft X-ray emission in the ROSAT All Sky Survey maps (sum of R4 + R5 bands < 200). All observations were performed using the standard timing mode, no subarrays and no gratings. The level 1 event lists were reprocessed applying the appropriate temperature-dependent CTI correction and time dependent gains. The bad pixel list badpix_E_110210noedge was applied using the tool badpixfilter (M. Markevitch tool). A combination of wvdetect (A. Vikhlinin program) and SAOimage ds9 was used to identify potential sources, and sources identifiable by eye were added to the list to be masked out in sky coordinates. In some cases the exclusion regions were adjusted to conform better to the extent of the source (larger or smaller). Observations with large extended sources were not used. After combining all data for each chip into one file, sky coordinates and detector coords (X,Y,DETX,DETY columns) were reset to (0,0). The TIME column was reset to 1 for all events (this column is not removed completely to avoid problems with some analysis software). Events in each combined file were sorted by chip coords to provide anonymize the input data. The event files are corrected for the image holes caused by source exclusion. The correction is performed by creating an "exposure map" in chip coords (by convolving the source exclusion mask with the inverse aspect track for each observation, and stacking them together). Events from remaining chip areas with exposures <100% were randomly multiplied to fill the holes. The final exposure was set below the true full exposure, and a fraction of events from the fully exposed regions were randomly discarded, in order to remove the shallowest depressions and minimize this artificial event multiplication. Each observation was cleaned of ASCA grade 1,5,7 events (after the CTI correction) and the background flare intervals were identified (using lc_clean). The files are cleaned of the cosmic ray afterglow events using the CIAO tool `acis_detect_afterglows'. The chip images were examined chip coordinates and events in with more than ~5 events (FI) or ~8 events (BI) were removed using the 'badpixfilter' tool. The thresholds were based on a Poisson estimate of the expectation of exceeding N events in any pixel given the mean rates and exposure times. The CIAO tool `destreak' was applied to all FI chips; it has the strongest effect on the S4 chip. The combined exposures are written in the file headers as the EXPOSURE, ONTIME and LIVETIME keywords (all equal; the DTCOR keyword is reset to 1 for consistency), in a fake GTI extension that contains a single interval from 0 to EXPOSURE, and as keywords TSTART=0 and TSTOP=exposure, so most standard software should have no problem determining the exposures automatically. The keywords DATE-OBS, DATE-END correspond to the beginning of the earliest observation in the dataset and the end of the latest one. Dataset details: ---------- acisi_F_* (ACIS-I in aimpoint, period F), ver. 20130211: ---------- ccd ontime --- ------ 0 430000 1 430000 2 430000 3 430000 6 600000 obsid ccd ----- -------------- 0 1 2 3 6 -- -- -- -- -- 10456 y y y y y 10455 y y y y y 10454 y y y y y 10453 y y y y y 12014 y y y n y 12091 y y y n y 11874 n n n n y 11792 n y y y y 12306 y y y y y 12308 y y y n y 12936 y y y y y 13394 y y y y y 13392 y y y y y 12189 n n n n y ---------- aciss_F_* (ACIS-S in aimpoint, period F), ver. 20130211: ---------- ccd ontime --- ------ 2 800000 3 800000 5 800000 6 800000 7 800000 8 380000 obsid ccd ----- ----------------- obsid 2 3 5 6 7 8 ----- -- -- -- -- -- 10461 y y y y y . 12003 y y y y y . 11570 y y y y y y 11575 y y y y y y 11016 y y y y y y 11756 y y y y y . 12095 y y y y y y 11828 y y y y y . 11572 y y y y y y 11569 y y y y y y 11574 y y y y y y 11568 y y y y y y 11829 y y y y y . 11830 y y y y y . 11592 y y y y y . 12369 y y y y y y 12374 y y y y y y 12381 y y y y y y ---------- acis_F_*stowed_evt* (ACIS "stowed", or "undercover", period F), ver. 20130211: ---------- ontime 240353 (ACIS 0, 2, 3, 5, 6, 7) obsid 0 1^1 2 3 5 6 7 ----- -- --- -- -- -- -- -- 62810 y y y y y y y 62809 y y y y y y y 62808 y y y y y y y 62804 y y y y y y y 62802 y y y y y y y ----- -- --- -- -- -- -- -- ^1: synthesized from c0 data; not statistically independent from c0