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1. Introduction
2. OMC Data Server functionality
   • Archive search
   • Results from search
3. Current status of the OMC Data Server
   • OMC data statistics
4. OMC science analysis processing at CAB
   • Best photometric accuracy achievable
   • Calibration status
5. Column description of output FITS files (OMC light curves)
6. PROBLEMS column description
7. Known limitations
8. Other foreseen functionality

Introduction

The Optical Monitoring Camera (OMC) observes the optical emission from the prime targets of the gamma-ray instruments on-board the ESA mission INTEGRAL (launched on Oct. 17, 2002). OMC has also the capability to monitor serendipitously a large number of optically variable sources within its field of view.

Since only a number of OMC sub-windows (typically 100, and in any case less than 228) of 11x11 pixels can be downloaded to Earth, the targets to be monitored by OMC have to be pre-selected on ground. For this purpose, an OMC Input Catalogue were compiled containing:

• Most gamma-ray sources.
• Most X-ray sources.
• Most AGNs within the photometric limits of the OMC.
• Most variable stars (including eruptive variable stars, novae and cataclysmics).
• Several known additional optical variable objects.
• Hipparcos and Tycho reference stars for astrometric and photometric calibration.

The OMC Data Server includes all observations from revolutions 11-2508, 2510-2524 (publicly available on Sep 01, 2023) and public observations up to revolution 2674. Data prior to revolution 951 are from revision 2 of the INTEGRAL Archive. From revolution 951 onwards revision 3 is being used. The Off-line Science Analysis software OSA 7.0 was used to process the OMC data until revolution 890. In revolution 891 the Off-line Science Analysis software was switched to OSA 9.0, which includes an improved PSF calculation with respect to OSA 7.0, and in revolution 1281 was switched to OSA 10.1 improving the centroiding capabilities in open loop slews and in some extreme cases (crowded fields, faint sources...). Concerning the OMC science analysis, OSA 10.1 is equivalent to the latest software (OSA 11.0). Work to update old data to revision 3 and their reprocessing with OSA 10.1 is ongoing.

The OMC Data Server includes completely the OMC Input Catalogue V0005, so the user can check if a given source has been pre-selected as a candidate to be monitored by the OMC. However the main aim of the OMC Data Server is to give a friendly interface to the OMC science results.

OMC Data Server functionality

Archive Search

The OMC catalogue comprises 541802 objects. The query to access the archive is made by means of an HTML fill-in form which permits to perform queries by object name, coordinates, object type, V-magnitude range, date of observation, time binning, centroid method and/or number of points of the light curve. The output data may be ordered by object name, coordinates, magnitude or date and time of observation. Two output formats are available: HTML or ASCII.

The system has a built-in name resolver utility which makes possible to query the archive using any of the object names provided by SIMBAD. The name resolver gives more than three million and a half identifications for the astronomical objects contained in the OMC catalogue. The full list of the names associated to a given object can be obtained by simply clicking on the target name in the output form.

Results from Search

The following utilities are provided in HTML output format to the users with proper access rights:

• Plot utility: A browse plot of a light curve can be generated on-the-fly by clicking on the corresponding link.
• Fits Header Display: Links are provided to display the FITS headers of each requested light curve file.
• Data Retrieval: Light curves may be retrieved individually or in groups. If a single light curve is requested, it is delivered as an uncompressed FITS file. Multiple light curve retrieval generates a packed file in zip format.
• Help-Desk: A Help Desk facility to channel questions and to provide continuous support to users of the archive is provided.

Current status of the OMC Data Server

The access to the OMC Input Catalogue data is open for everyone. It allows to know which objects have been monitored by the OMC, and when. It also provides a cross-correlation window based on the SIMBAD Database.

Security and privacy of the private data are assured in two ways: user authentication (username+password) and encrypted data transfer. There are several types of user profiles, each of them with different data access policies. The restricted interface to access PV phase and Core Programme data is open only for the INTEGRAL teams. To obtain your password please send an e-mail to omc-support@cab.inta-csic.es.

Currently, you can retrieve from the OMC Data Server the processed light curves. To get the original CCD windows for a manual reprocessing you have to use the standard interface at ISDC. We plan to give also this functionality in the future.

OMC Data Server statistics

Total number of observed sources (at least one photometric point):	281128
Number of "scientific" observed sources (at least one photometric point):	185676
Number of scientific sources with more than P photometric points (time binning of 10 minutes):

Num. points (P)	Num. of sources
> 50	105191
> 100	77085
> 200	51333
> 500	23805
> 1000	11069
> 2000	4140
> 3000	2084
> 4000	1130
> 5000	611

Those sources with more than 50 photometric points (time binning of 10 minutes) and with object type available in the Simbad Database, show the following classification:

Object type	Number of sources
Variable Star	7741
Variable Star of Mira Cet type	3310
Variable Star of RR Lyr type	3254
Eclipsing binary of Algol type	2076
Semi-regular pulsating Star	1638
Quasar	1512
Pulsating variable Star	1332
Possible Quasar	849
Pulsars	711
Eclipsing binary	569
Radio Galaxy	530
Emission-line galaxy	438
T Tau-type Star	401
Eclipsing binary of beta Lyr type	394
Classical Cepheid (delta Cep type)	390
X-ray source	317
Eclipsing binary of W UMa type	307
Nova	275
gamma-ray source	274
Flare Star	269
Galaxy	264
Classical Cepheid variable Star	252
Variable Star of Orion Type	245
Emission-line Star	238
Dwarf Nova	221
Seyfert 1 Galaxy	205
Seyfert 2 Galaxy	203
Low Mass X-ray Binary	175
Carbon Star	172
Variable Star of irregular type	161
Emission Object	144
High Mass X-ray Binary	117
Infra-Red source	110
BL Lac - type object	103
Variable Star with rapid variations	101
Active Galaxy Nucleus	100
Star in Cluster	83
HII Galaxy	64
Variable Star of RV Tau type	62
Star	60
Seyfert Galaxy	56
Variable Star of W Vir type	54
Radio-source	51
Nova-like Star	46
Variable Star of alpha2 CVn type	44
Be Star	43
Variable Star of beta Cep type	43
Variable Star of delta Sct type	39
Star suspected of Variability	38
Spectroscopic binary	37
Cataclysmic Variable Star	36
Symbiotic Star	33
Variable of RS CVn type	32
Planetary Nebula	31
SuperNova Remnant	31
Cataclysmic Var. AM Her type	22
Eruptive variable Star	20
Cluster of Galaxies	19
LINER-type Active Galaxy Nucleus	19
Wolf-Rayet Star	19
Variable Star of R CrB type	17
Blue object	17
Variable of BY Dra type	16
UV-emission source	15
S Star	14
X-ray Binary	14
White Dwarf	14
Galaxy in Pair of Galaxies	11
Variable White Dwarf of ZZ Cet type	11
Cataclysmic Var. DQ Her type	11
HII (ionized) region	10
Star in double system	8
Galaxy in Cluster of Galaxies	8
Elliptical variable Star	8
Pair of Galaxies	8
Possible Planetary Nebula	8
Rotationally variable Star	7
Gravitationnaly Lensed Image of a Quasar	6
Double or multiple star	6
Low Surface Brightness Galaxy	5
gamma-ray Burster	5
High proper-motion Star	5
Variable Star of FU Ori type	4
Object of unknown nature	4
SuperNova Remnant Candidate	4
Nebula of unknown nature	3
Star in Nebula	3
SuperNova	3
Group of Galaxies	3
Cluster of Stars	2
Blazar	2
Star in Association	2
Maser	2
Starburst Galaxy	2
Globular Cluster	2
Young Stellar Object	1
Interacting Galaxies	1
Blue compact Galaxy	1
Molecular Cloud	1

OMC science analysis processing at CAB

Around 100 OMC CCD sub-windows, each of 11x11 pixels, are downloaded to Earth about every minute, without any on-board processing. The data are received at the Integral Science Data Center, where they are completely processed in an automatic way. OMC data are then re-processed at CAB and stored in the OMC Data Server. The principal steps of this science analysis processing are the following:

1. The information in the telemetry packages is converted to CCD pixel values.

2. The OMC CCD sub-windows are corrected from bias and dark current. They are then convolved with the corresponding flat-field matrix.

3. OMC CCD sub-windows obtained within periods of around 10 minutes (default value) as well as within the full Science Window are combined to obtain a better signal-to-noise ratio. A Science Window is a pointing (typically around 30 minutes) or a part of it in case of being too long (staring mode). A third solution without any combination of the OMC CCD sub-windows is also given. These three cases correspond to three different processing options.

4. The photometric value for each source of interest is obtained by applying three different extraction masks of 1x1, 3x3 (default value) and 5x5 pixels to the combined images (see column description below). The local background, as measured on a corona around the extraction mask, is subtracted. If Source coordinates (default) is selected as the centroid method in the fill-in form, the position of the extraction mask is re-centred on the source coordinates given in the OMC Input Catalogue, allowing a maximum offset of 10 arcsec (parameter IMA_maxWcsOff in OMC OSA). Instead if Brightest pixel is selected as the centroid method, the position of the extraction mask is re-centered on the brightest pixel within the central 5x5 pixel section. This extraction software can also be run offline to modify the parameters, in order to minimize the effects of background stars when needed.

5. The photometric values are stored as V magnitudes, thus compiling light curves with three time binnings:

   • Shot-by-Shot: one photometric point per shot.
   • 10 minutes: around one photometric point each 10 minutes. Shots with exposure smaller than 20 seconds are rejected to increase the signal-to-noise ratio.
   • ScW-by-ScW: one photometric point per Science Window. Shots with exposure smaller than 60 seconds are rejected to increase the signal-to-noise ratio.

   We must notice that due to the rejection of the shortest shots when using time binnings of 10 minutes and ScW-by-ScW, the brightest sources will not be processed when selecting these time binnings, because they will be saturated in the remaining shots. For these sources, the user should select the time binning of Shot-by-Shot, and then filter the results to keep only the shortest shots with no saturation effects.

In this science analysis, OMC Input Catalogue Version V0005 and the following versions of OMC OSA 7.0, OSA 9.0 and OSA 10.1 (Off-line Science Analysis) components were used:

OSA 7.0 (revolutions 11-890):

omc_science_analysis 5.3.1
  omc_scw_analysis 5.3.1
    o_cor_science 5.3.1
      o_cor_box_fluxes 6.5.1
    o_gti 5.3.1
      gti_create 2.2
      gti_attitude 1.5
      gti_merge 1.6
    o_src_analysis 5.3.1
      o_src_get_fluxes 8.1
      o_src_compute_mag 4.4
  omc_obs_analysis 5.3.1
    o_src_collect 2.4

OSA 9.0 (revolutions 891-1280):

omc_science_analysis 6.0
  omc_scw_analysis 6.0
    o_cor_science 6.0
      o_cor_box_fluxes 6.5.1
    o_gti 6.0
      gti_create 2.3
      gti_attitude 1.5
      gti_merge 1.6
    o_src_analysis 6.0
      o_src_get_fluxes 8.2
      o_src_compute_mag 4.4
  omc_obs_analysis 6.0
    o_src_collect 2.4

OSA 10.1 (revolutions 1281-->):

omc_science_analysis 6.0.2
  omc_scw_analysis 6.0.2
    o_cor_science 6.0.2
      o_cor_box_fluxes 6.5.1
    o_gti 6.0.2
      gti_create 2.3
      gti_attitude 1.5
      gti_merge 1.6
    o_src_analysis 6.0.2
      o_src_get_fluxes 8.3
      o_src_compute_mag 4.4
  omc_obs_analysis 6.0.2
    o_src_collect 2.4

Best photometric accuracy achievable

Expected best photometric accuracy in V magnitudes of the OMC for different V values and various effective integration times. A typical observation totals 300 s of effective integration time.

	V magnitude
Eff. integration (s)	8	10	12	14	16
10	0.007	0.02	0.1	--	--
300	--	0.005	0.01	0.04	0.3
900	--	0.003	0.006	0.026	0.17

The above values do not account for possible systematic biases.

Calibration status

In August 2006 a new set of flatfield matrices (Version 5) as well as photometric calibration files (Version 5) were delivered by the OMC team to ISDC. These new IC (Instrument Characteristic) files improved substantially the old ones (Version 4).

The data currently available at the OMC Data Server were analyzed using the new IC files. In the following table the version number of each IC file used in the analysis is given:

IC Data Structure	Version
OMC.-PHOT-CAL	5
OMC.-FLAT-CAL	5
OMC.-BDPX-CAL	2
OMC.-DARK-CAL	2
OMC.-GOOD-LIM	8
INTL-GOOD-LIM	2

Column description of output fits files (OMC light curves)

REVOL	Revolution number valid for time of data taking
SWID	Science Window identifier from which this row was taken
TFIRST	Time of the first data element in ISDC Julian days
BARYTIME	Barycentric time for the first data element in ISDC Julian days
TELAPSE	Elapsed time of the integration in seconds
EXPOSURE	Effective integration time in seconds
SHOTTYPE	Type of shots used for building integration (1=Photometric, 2=Science)
OMC_ID	OMC catalogue source identifier (IOMC number)
TYPE_TAR	Target type (1=Photometric, 2=Science, 3=Dark Current)
RA_OBJ	Source right ascension in degrees
DEC_OBJ	Source declination in degrees
FLUX_1	Flux in electron/s derived from 1x1 integration boxes
ERFLUX_1	Error estimate for FLUX_1
FLUX_3	Flux in electron/s derived from 3x3 integration boxes
ERFLUX_3	Error estimate for FLUX_3
FLUX_5	Flux in electron/s derived from 5x5 integration boxes
ERFLUX_5	Error estimate for FLUX_5
SKYBACK	Mean flux from sky background in electron/pixel/s
SKYERROR	Error estimate for SKYBACK
SIZE_MAG	Integration box size for deriving MAG_V
MAG_V	Computed V (Johnson) magnitude (default value)
ERRMAG_V	Error estimate for V magnitude
CATMAG_V	Catalog V (Johnson) magnitude
CATERR_V	Catalog error estimate for V magnitude
MAG_V1	Computed V magnitude for the 1x1 pixel area
ERMAG_V1	Error estimate for V magnitude in 1x1 pixel area
MAG_V3	Computed V magnitude for the 3x3 pixel area
ERMAG_V3	Error estimate for V magnitude in 3x3 pixel area
MAG_V5	Computed V magnitude for the 5x5 pixel area
ERMAG_V5	Error estimate for V magnitude in 5x5 pixel area
PROBLEMS	Flag for various problems (0 = no problems)
NOISE_LL	Read-out noise in e- (low gain, left ROP)
NOISE_LR	Read-out noise in e- (low gain, right ROP)
NOISE_HL	Read-out noise in e- (high gain, left ROP)
NOISE_HR	Read-out noise in e- (high gain, right ROP)
CENTRING_X	Derived X-axis offset of the source from the box centre (pixels)
CENTRING_Y	Derived Y-axis offset of the source from the box centre (pixels)
PSF_FWHM	Effective PSF FWHM in pixels
X_TAR	X coordinate of the lower left pixel of the target box (pixels)
Y_TAR	Y coordinate of the lower left pixel of the target box (pixels)
RANK	On-board sequence number of box
RA_FIN	Derived right ascension in degrees
RA_FIN_ERR	Standard error for RA_FIN*cos(DEC_FIN)
DEC_FIN	Derived declination in degrees
DEC_FIN_ERR	Standard error for DEC_FIN

PROBLEMS column description

This is the meaning of PROBLEMS column in output fits files. Problems are stored in an unsigned integer register, and any problem encountered is logically ANDed to the existing register value. Deconstruction of the total into its only possible component values reveal the individual PROBLEMS. Problem values 64 and 2048 are reserved for future improvements.

NAME	VALUE	DESCRIPTION
OMC_PROBLEM_NONE	0	No problems
OMC_PROBLEM_CENTROID_OUT	1	Centroid does not match maxWcsOff radius; The WCS position has been used as centroid
OMC_PROBLEM_EXTRAPOLATED_MAG	2	The magnitude was extrapolated
OMC_PROBLEM_BAD_CENTROID	4	No centroid is available or is inaccurate
OMC_PROBLEM_BAD_PSF	8	Bad PSF. A default value was used
OMC_PROBLEM_ANOMALOUS_PSF	16	The PSF shape is anomalous
OMC_PROBLEM_LOW_FLUX_1	32	Flux of central pixel too low
OMC_PROBLEM_BADPIXEL_SKY	128	Bad pixel found in sky background
OMC_PROBLEM_BADPIXEL_RIM_5	256	Bad pixel found in 5x5 rim
OMC_PROBLEM_BADPIXEL_RIM_3	512	Bad pixel found in 3x3 rim
OMC_PROBLEM_BADPIXEL_RIM_1	1024	Central pixel bad
OMC_PROBLEM_SKY_ERROR	4096	Sky error larger than accepted limit
OMC_PROBLEM_UNKNOWN_MAG	8192	Magnitude could not be calculated
OMC_PROBLEM_EXTND_SRC	16384	Source is extended - flux not valid
OMC_PROBLEM_COORD_OUT	32768	WCS position close to the edge or out of the OMC box; The brightest pixel has been used.

Known limitations

• The automatic extraction of fluxes and magnitudes produces reliable results only for point like sources when compared with the OMC pixel size.

• If the source coordinates are inaccurate by more than 2 OMC pixels (around 35 arcsec), the software analysis will not be able to re-centre the target and the derived fluxes and magnitudes obtained with default analysis parameters will not be correct.

• For extended sources or high energy source counterparts with large uncertainties in their position, the OMC planning assigns multiple adjacent sub-windows to cover the whole area. In that case, multiple boxes are found with different ranks but with the same OMC_ID. From OSA 6.0 onwards these adjacent sub-windows can be correctly analyzed by using the Source coordinates option as centroid method (default in the fill-in form). In this case a virtual 11x11 pixel sub-window is created inside the whole area centred at the source position. After that, OSA works on this new sub-window and ignores the previous sub-windows mosaic. This is an internal software trick, these virtual sub-windows do not exist as standard sub-windows. Note that if Brightest pixel is selected as the centroid method, these adjacent sub-windows will not be analyzed correctly as the software treats each box individually.

• If another star is within a few pixels of the source of interest, it can introduce systematic errors in the derived fluxes and magnitudes. The strength of this effect can be different for different pointings, since the relative position in the sub-windows will slightly change for different rotation angles.

• Since OSA 4.0 the detection of saturated sources has been improved considerably. However, some of the bright sources slightly saturating one or few pixels might not be detected as saturated sources. As a consequence, their derived magnitudes are not correctly computed. The observer should check whether the source might be saturating the CCD for a given integration time, and reanalyse the data rejecting the shots with the longest integration times.

• Due to thermoelastic deformations, the alignment of the OMC optical axis with the S/C attitude reference (after correcting the OMC misalignment) may diverge by up to 30 arcsec (around 2 OMC pix). From OSA 5.0 onwards, the derived coordinates are corrected at the time of computing the WCS support by using the photometric reference stars, giving an accuracy better than 2 arcsec in most cases.

Other foreseen functionalities

A variety of data analysis tools are being developed and will be available in the near future. The aim of these tools is to provide added-value functionalities to the system giving the ability to perform data analysis tasks remotely.

Retrieval of OMC images: The user will be able to get raw and corrected images. In the last case the OMC sub-windows will be corrected for bias, dark current and flatfield.

Time series analysis: The operation of INTEGRAL from a high orbit allows a continuous observation (only interrupted by the radiation belts crossing) for periods of several weeks giving a unique photometric capability that cannot be addressed from ground-based observatories. Our aim is that the OMC data server also provides information in the frequency domain. Given the diversity of the OMC targets (AGNs, X-rays binaries, gamma-ray burst, eclipsing binaries, pulsating variables, ...) and the variability patterns (long/short-term variations, mono/multi-periodicity,...), a detailed study on the techniques to be applied in each case must be performed.

Data mining. Light curve characterization: Even though the OMC data server allows making queries by object type based on the classification provided by SIMBAD, it is clear that this classification can be greatly improved with the use of the OMC data. Given the vast amount of data to be handled, classification procedures based on the visual inspection by experts are not adequate and data mining techniques must be used instead. We are presently working in a neural network system to classify light curves of periodic variable stars. The network will be trained with the HIPPARCOS light curves and, in a first step, it will allow for the automatic classification of eclipsing binary stars with a further extension to other types of variable stars (e.g. pulsating variables). The system is designed to perform a unsupervised topological mapping based on morphological proximity among the light curves.

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Version 3.0 - March 2012     © CAB (INTA-CSIC)

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