XMM-Newton Users Handbook

3.6.1 EPIC Radiation Monitor Subsystem (ERMS)

The ERMS registers the total count rate and also basic spectral information on the background radiation impinging on XMM-Newton and its science instruments. This is done not only during science observations, but during the entire lifetime of the mission.

Depending on the mode in which the ERMS is operated, it updates background radiation information once in 512 seconds (slow mode) or 4 seconds (fast mode). The ERMS is normally working in slow mode.

The ERMS data consist of three measurements per readout, two spectra from the High Energy Detectors (HEDs) and one from the Low Energy Detector (LED). 256 spectral bins each will be available in the LED and HED. The HEDs are sensitive to electrons above 200 keV and protons above 10 MeV, while the LED is sensitive to electrons with energies above 50 keV.

The two high energy spectra are the “HE detector spectrum”, with events from either one or both HE detectors, and the “coincidence HE spectrum”, in which only events detected by both HEDs will be recorded.

The data from the ERMS comprise the following quantities:

• one header keyword (warning flag),

• seven broad-band count rates, three from the LED, four from the HEDs,

• LE detector spectrum (256 channels),

• HE detector spectrum (256 channels),

• HE coincidence spectrum.

In this way, the ERMS monitors the total radiation dose that XMM-Newton is exposed to. When the level of radiation intensity exceeds a certain threshold, the ERMS issues a warning to the ground for the Spacecraft Operator to safe the instruments.

ERMS data are not delivered to the PI as part of the XMM-Newton science data but can be obtained from the SOC on request. Note that these data are not properly calibrated and therefore are not suitable for general scientific use.