Filter Wheel Closed (FWC) data

Description

In order to facilitate the science analysis based on eROSITA data products, the eROSITA calibration team scheduled repeated observations with the filter-wheel set in CLOSED position. These provide scientists interested in X-ray instrumentation as well as faint or extended X-ray emission with reliable information on the eROSITA instrumental background. The observation list comprises a number of long exposures acquired during the Cal-PV phase to characterise the background level, its spectral shape and stability, as well as periodic shorter exposures obtained during the survey to monitor the long term stability.

As part of the Early Data Release (EDR), the eROSITA German Consortium (eROSITA-DE) makes available version 1.0 of its compilation of filter wheel closed observations. It consists of all clean exposures acquired in processing mode PMENV2 (a description of processing modes is available here) from the eROSITA launch up to October 2020 and reduced with configuration c946 of the eSASS pipeline (virtually identical to the c001 used for EDR observations).

For each telescope module (TM), three types of datasets are provided:

• Event lists:
  In eSASS compatible format, they contain all recorded events without any filter on energy, patterns or flags. This allows to later match the event selection to the exact one performed on the observations to be analysed. We however caution the prospective user that pattern and flag selection must be applied before use for a meaningful interpretation (for instance using the eSASS evtool command). An additional fits extension (OBSGTI) was appended to the event lists, which contains a numeric identifier as well as the time interval corresponding to each individual FWC exposure.
• Spectra:
  OGIP format spectrum files extracted with the eSASS srctool command, within the eROSITA field-of-view mask. Bad events were rejected based on the flag selection flag=0xC00F7F30 and all valid patterns were included (pattern=15). Each spectrum is already coupled with a default RMF file (but no ARF since this background component is not subject to the effective area of the telescopes). The AREASCAL and BACKSCAL keywords of the spectra were both set to the geometric detector area of ~0.83 deg².
• Models:
  Best fit models for the extracted spectra, in a format suitable for use with the Xspec software. Due to our AREASCAL settings, the model normalisations correspond to a detector area of 1 deg². For TMs with an on-chip filter, the models are valid over the energy range 0.3-10.0 keV while due to the light leak the range was reduced to 1.0-10.0 keV for TM5 and TM7. The baseline models for the eROSITA background were derived from the current v1.0 of the FWC dataset. However, older versions of the models are also provided for completeness, which were used for some of the EDR publications, namely: v0.8 relevant for observations of A3158 (Whelan et al. 2021), A3407 (Iljenkarevic et al. 2021) and A3391/95 (Reiprich et al. 2021, Veronica et al. 2021), as well as v0.9 used for the analysis of A3266 (Sanders et al. 2021) and the eFEDS survey (Liu et al. 2021).

Data products

Module	Number of observations	Livetime [ks]	Event-list	Spectrum	Spectral model	Figure (v1.0)
TM1	48	243.6	EventList	BackgrSpec	v0.8
					v0.9
					v1.0
TM2	48	185.1	EventList	BackgrSpec	v0.8
					v0.9
					v1.0
TM3	48	194.0	EventList	BackgrSpec	v0.8
					v0.9
					v1.0
TM4	57	191.4	EventList	BackgrSpec	v0.8
					v0.9
					v1.0
TM5	49	160.1	EventList	BackgrSpec	v0.8
					v0.9
					v1.0
TM6	53	196.8	EventList	BackgrSpec	v0.8
					v0.9
					v1.0
TM7	43	150.7	EventList	BackgrSpec	v0.8
					v0.9
					v1.0