eROSITA-DE:DataRelease1:FWC

Filter Wheel Closed (FWC) data for DR1

Description

The instrumental background of each eROSITA telescope module (TM) is monitored regularly by taking exposures with the filter wheel being put in CLOSED position. The filter-wheel closed (FWC) data are especially useful for users interested in the faint and diffuse X-ray emissions in eROSITA data. In Data Release 1 (DR1), the eROSITA German Consortium (eROSITA-DE) compiles a list of FWC observations from the Calibration and Performance Verification (Cal-PV) phase to the end of eROSITA All-Sky Survey 1 (eRASS1). The cumulative exposure time (only the good time intervals, GTIs, as in the data) of each TM is shown in the figure below.

Note:

(1) An eROday is 4 hours in length, which is the scan duration of one great circle in the eRASSs.

(2) In this plot, eRASS0 refers to the eROSITA Cal-PV phase.

(3) The significantly longer FWC exposures for TM5 and TM7 in eRASS0 are due to light leak investigations.

(4) The horizontal dotted line indicates the TM8 exposure time. The intersection of this line with a constituent TM of TM8 corresponds to the time limit beyond which the events recorded in this constituent TM no longer contribute to the TM8 data.

Data Products

The FWC data were processed with eSASS processing version 010, in line with the other astrophysical data in DR1. A standard flag filtering of flag=0xE000F000 was applied, and all valid patterns pattern=15 (singles, doubles, triples, and quadruples) were selected.

There is a known source of contamination from the filter wheel which raises the background level following a filter wheel rotation. The level and duration of this enhancement are TM-dependent but generally < 10 minutes. These periods have been removed, along with FWC observations with a count rate deviating from 90-170 counts/min in the 0.5-10 keV band. A more detailed description of the cleaning and stability of the FWC data can be found in Appendix B of Yeung, M., et al. (2023), as well as a general description of the eROSITA background in Freyberg, M., et al. (2020).

The data products, including Event lists, spectra and spectral models for each TM, can be found in the table below. Besides the seven TMs in eROSITA, TM8 refers to the combination of TM1, 2, 3, 4 and 6, namely the non-light-leak cameras (see here for file naming scheme). The spectral model in the latter is convenient to users if the small variations in the FWC spectral shape are not critical in their analysis. Please note that we imposed a criterion that all constituent TMs should have the same exposure time, following TM4, which has the least exposure time in total. Events of other TMs that were recorded after reaching the exposure time of TM4 are not included in the TM8 data. You can find a summary of the start and end dates of the TM8 constituent TMs at the bottom of this page. The end dates of this table correspond to the intersections mentioned in Note (4) of the figure above. The excluded events due to this time cut can be found in the Event lists of the individual TMs.

TM5 and 7 are affected by the light leak. Their spectral models are only fitted down to 0.5 keV (this does not mean E>0.5 keV is safe from the light leak), but the events with lower energies can be found in the Event lists. The spectra and the spectral models of all TMs correspond to a sky area of 0.8235 deg² (inscribed circle of CCD imaging area). The spectral models can be readily read into XSpec with a standard RMF from the CALDB (see here for further details). When modelling the instrumental background in eROSITA spectra (do not use an ARF for this component), it is possible to use the BACKSCAL header keyword in the srctool-output spectra to scale the FWC background appropriately.

Note: The following spectra are plotted using the RMF in the eSASS4DR1 CALDB. These are the same RMF you will get when using srctool to extract your spectra.

TM	Exposure Time (s)	Event list	Spectrum	Model
1	210770	TM1 EventList	TM1 spectrum	TM1 model
2	162231	TM2 EventList	TM2 spectrum	TM2 model
3	161129	TM3 EventList	TM3 spectrum	TM3 model
4	157624	TM4 EventList	TM4 spectrum	TM4 model
5	467597	TM5 EventList	TM5 spectrum	TM5 model
6	180193	TM6 EventList	TM6 spectrum	TM6 model
7	766368	TM7 EventList	TM7 spectrum	TM7 model
8	157624	TM8 EventList	TM8 spectrum	TM8 model