Scientific Performance Summary

The Joint European Monitor for X-rays (JEM-X) on-board INTEGRAL fulfills three roles:

• It provides complementary data at lower energies for the studies of the gamma-ray sources observed by the two main instruments, IBIS and SPI. Flux changes or spectral variability at the lower energies may provide important elements for the interpretation of the gamma-ray data. In addition, JEM-X has a higher spatial resolution than the gamma-ray instruments. This aids with the identification of sources in crowded fields.
• During the recurrent scans along the galactic plane JEM-X provides rapid alerts for the emergence of new transients or unusual activity in known sources. These sources may be unobservable by the other instruments on INTEGRAL .
• Finally, JEM-X may deliver independent scientific results concerning sources with soft spectra, serendipitously detected in the field of view (FOV) during the normal observations.

JEM-X operates simultaneously with the main gamma-ray instruments IBIS and SPI. It is based on the same principle as the two gamma-ray instruments on INTEGRAL : sky imaging using a coded aperture mask. The performance of JEM-X is summarized in Table .

Table: JEM-X parameters and performance.

Energy range	3 - 35 keV
Energy resolution
Field of view (diameter)	4.8 Fully illuminated
	7.5 Half response
	13.2 Zero response
Angular resolution (FWHM)	3
Relative point source location error	1 (90% confidence radius for a 5 isolated source)
Continuum sensitivity	ph cm s keV @ 6 keV
for a single JEM-X unit	ph cm s keV @ 30 keV
(isolated source on-axis)	for a 3 cont. detection in 10 s, E = 0.5E
Narrow-line sensitivity	ph cm s @ 6 keV
(isolated source on-axis)	ph cm s @ 20 keV
	for a 3 line detection in 10 s
Timing resolution	122 s (relative timing)
	ms (absolute timing)

The energy resolution is slowly changing (degrading) over time.
At the half response angle the sensitivity is reduced by a factor 2 relative to the on-axis sensitivity.

Figure: JEM-X effective area with the mask taken into account. The dashed line shows the effective area before the high voltage reduction and the full curve shows efficiency when taking into account the effect of the electronic low-signal cutoff (approximately).