XMM Users' Handbook

   
OM sensitivity and detection limits

Both Fig. 71 and Tab. 15 provide the expected count rate of the OM, in counts per second, for m_V = 20 mag stars of various types, with the different filters introduced above. These numbers were obtained from SciSim simulations, under the assumption of a perfect, i.e., deadtime-free, detector.

 

Table 15: OM count rates [10^-4 counts/s] as function of spectral type for stars with m_V = 20 mag under the assumption of a zero deadtime detector

Filter	B0	A0	F0	G0	G2	K0	M0
UVW2	5534	722	59.2	9.66	5.54	0.963	0.306
UVM2	6599	931	114	21.4	17.0	1.41	0.150
UVW1	11368	1898	744	449	369	65.5	7.23
U	14945	3962	2596	1952	1676	845	115
B	10035	8384	6070	4806	4094	3675	2075
V	1893	1847	1790	1778	1806	1743	1667
White	75280	22335	13055	10376	9293	7636	5323

  

Figure 71: OM count rates vs. filter selection for stars of different spectral type with m_v = 20 mag.

The numbers listed in Tab. 15 can be used to calculate the expected count rates of stars of given magnitude, C_mag, by the formula

$$C_{mag} = C_{20} \times 10^{0.4\times(20-b_{mag})} \quad ,$$ (3)

where C₂₀ is the count rate of an m_V = 20 mag star, b_mag is the magnitude of the target of interest and C_mag is the count rate of a star with m_V = b_mag.

However, OM deadtime (coincidence losses) must be taken into accout. This can be done by scaling the above numbers of C_mag by applying the following equation:

$$C_{det} = C_{mag} \times (1 + p/2 + p^2/6 + p^3/24) \times e^{-p} \quad ,$$ (4)

with $p = C_{mag} \times \tau$, where C_mag is the count rate of incident photons from eq. 3 and $\tau$ is the frame time of the OM fast-scan CCD. The frame time is a function of the OM science window configuration, as described in § 3.5.3. In this context a conservative value for the OM CCD frametime is 10 ms. For the 20th magnitude stars in Fig. 71 and Tab. 15, OM deadtime is negligible. For the worst case, which is an B0 star observed with the white light filter, the deadtime correction amounts to 3.7%.

In Table 16 estimates of required OM integration times for 23rd magnitude stars are provided. The values listed are the integration times, in seconds, for a 5-$\sigma$ detection.

 

Table 16: Required exposure times for different types of stars with m_V = 23 mag¹

Filter	Spectral type
	B0	A0	G0
U	340	2100	6400
B	750	980	2400
V	4300	4400	4700
White	70	370	1300

Notes to Table 16:
1) In seconds, for a 5-$\sigma$ detection. Numbers obtained adopting a zodiacal light level of 87 S10 units, where S10 is the brightness of one m_V = 10 mag A0 star per square degree.

Table 17 tabulates the limiting magnitudes that can be detected by OM in an integration time of 1000 s at the 5-$\sigma$confidence level.

 

Table 17: Limiting magnitudes for a 5-$\sigma$ detection in 1000 s¹

Filter	Spectral type
	B0	A0	G0	K0
U	23.9	22.5	21.7	20.8
B	23.2	23.0	22.4	22.1
V	21.9	21.8	21.8	21.8
White	25.0	23.7	22.9	22.5

Notes to Table 17:
1) Numbers obtained adopting a zodiacal light level of 87 S10 units.

The expected levels of different external background radiation processes in the optical/UV are tabulated in Table 18. A spectral representation of the diffuse galactic and zodiacal light is contained in SciSim. The intensity of these background sources, which depends on the target coordinates, is a user-selectable parameter in SciSim.

 

Table 18: Levels of different OM background contributors

$\parbox{4cm}{{\bf Background source}}$	$\parbox{5cm}{{\bf Occurrence}}$	$\parbox{4cm}{{\bf Count rate range$^1$ }}$
Diffuse Galactic	all directions	0.081-0.0007
Zodiacal2	longitude $90^\circ\pm20^\circ$	0.212-2.6 $\times10^{-5}$
Earth	Off-axis angle $<30^{\circ~(3)}$	0.219-0
Moon	Off-axis angle $<30^\circ$	0.0016-10-5

Notes to Table 18:
1) In units of [counts s^-1 arcsec^-2].
2) In differential ecliptic coordinates, i.e., the angle between the Sun and the pointing position of XMM. Maximum is for l,b = 70,0 in the white filter, minimum is for l,b = 110,70 in the UVW2 filter.
3) Forbidden by observatory constraints anyway.

In addition to these external sources of background radiation, the instrumental background must be taken into account. However, with a count rate of $<4\times10^{-5}$ counts s^-1 arcsec^-2 the dark count rate of the detector is generally negligible.