OMC: The Optical Monitoring Camera onboard INTEGRAL
J.M. Mas-Hesse1,11, A. Giménez2, J.L. Culhane3, C. Jamar4, B. McBreen5, J. Torra6, R. Hudec7, J. Fabregat8, E. Meurs9,
J.P. Swings10, A. Domingo11, E. Díaz12, E. de Miguel12, P. Cabo12, M.D. Caballero11, D. Rísquez11
and the OMC team at INTA, MSSL, CSL, UCD, DOD, AIO, UB
1Centro
de Astrobiología (CSIC-INTA), 2Research and Scientific Support Department (ESA ESTEC), 3Mullard Space Science Laboratory (U.C. London),
4Centre Spatial de Liège, 5Department of Experimental Physics (U.C. Dublin), 6Universitat de Barcelona-IEEC, 7Astronomical Institute Ondrejov,
8Universidad de Valencia, 9Dunsink Observatory Dublin, 10Universitè de Liège, 11LAEFF-INTA
The Optical Monitoring Camera (OMC) observes the optical emission from the prime
targets of the high energy instruments onboard INTEGRAL. The OMC offers the first
opportunity to make photometric observations of long duration in the optical band
simultaneously with those at X and γ-rays. OMC has the same field of view as the fullycoded FOV of the X-ray Monitor JEM-X, and is coaligned with the central part of the
larger fields of view of the Spectrometer and Imager. Variability patterns ranging from
minutes or hours, to months and years, will be monitored. For bright sources, fast optical
monitoring at intervals down to 3 s are possible. OMC will produce an Output Catalogue
with calibrated optical curves for thousands of objects of very different types.
OMC principal characteristics
Field of view
Aperture
Focal length
Optical throughput
System point spread function
CCD pixels
Angular pixel size
CCD quantum efficiency
Time resolution
Typical integration times
Wavelength range
Limiting magnitude
5°×5°
50 mm
153.7 mm (f/3.1)
> 70 % at 550 nm
Gaussian with FWHM ≈ 1.4 pix
1056 x 2061 (1024 x 1024 image area)
17”.6 x 17”.6
88 % at 550 nm
> 3s
10 – 100 s
V filter (centered at 550 nm)
< 17 (V)
(10×100 s, 3σ)
The photometric accuracy of the OMC spans from ∆V = 0.005 (V=9) to
∆V = 0.15 (V=16), for isolated stars with low stellar background.
Contamination by nearby stars (at less than ~100”) and/or a high
background of stellar origin (especially on the Galactic Plane) will affect
the accuracy that can be achieved, and requires an optimized photomety
extraction procedure.
The OMC Flight Model during integration. Detail of the CCD, support
structure and cold finger.
The OMC Input Catalogue
OMC has capability to monitor around 100 targets per pointing, in addition to the
prime high-energy sources. The selection of the targets to be monitored is done
automatically on ground:
• ISOC plans the pointings required for the different observations, determining the
spacecraft attitude.
• Once the expected attitude is known, an automatic OMC Pointing Software tool
extracts from the OMC Input Catalogue the positions of the targets potentially
variable in the optical.
• The coordinates of the CCD windows containing these targets is uplinked as a
Telecommand.
• OMC checks the actual pointing of the spacecraft and recenters the windows, if
required.
• The windows of interest are extracted from the complete CCD image and sent to
ground.
The OMC data are photometrically calibrated by
comparison with a large set of reference stars,
which are observed continuously. Note the
linearity of the detector over a factor 40 in flux.
The OMC Input Catalogue contains presently:
OMC image of the Large Magellanic Cloud region. In the
inset, detail illustrating the Point Spread Function of
individual stars.
• Astrometric and photometric reference stars.
• γ-ray sources from the EGRET, COS-B, Macomb & Gehrels (1999) catalogues.
• The last version of the High Energy Catalogue maintained at INTEGRAL Science
Data Center.
• X-ray sources from the ROSAT Bright and Faint Sources catalogues
• All known galactic and extragalactic variable and suspected variable objects from
the SIMBAD, GCVS, NSV and Véron-Cetty & Véron catalogues.
The OMC Input Catalogue is updated continuously with all new gamma-ray sources
being discovered by INTEGRAL.
OMC recenters autonomously on board the position of the pixel windows of the targets of interest. The corrections applied have been mostly
<1 pixel, showing the good pointing accuracy of
INTEGRAL.
OMC operations statistics
γ-ray sources known
before INTEGRAL.
• Up to revolution 122, OMC observed 56106 different objects.
• Of them, 43168 were scientific targets.
• More than 10000 photometric reference stars allow for a
continuous photometric calibration of the observations.
Galactic and extragalactic
variable stars.
Extragalactic, potentially variable,
non-stellar objects (AGNs, radio
galaxies, HII galaxies,...)
• Number of scientific objects with more than 50
photometric points:
N. of objects
N. of points
7
42
5000 2000
283
500
640 5040
200
50
Sample V band light curves obtained by OMC
Cyg X1
Binary, non-eclipsing system
ROSAT X-ray sources.
OMC CCD temperature
histogram for revolutions
50 to 122. The CCD
temperature has been
always around –80 C.
V689 Cyg
Algol type eclipsing binary
FX Aqr
RR Lyr variable star