OMC Observer`s Manual - Integral
Contents
1. oo Figure 3 Optical system layout 1 filter assembly housing 2 7 lenses 8 lens barrel 9 14 spacers 15 17 retainers 18 aperture stop Doc No SRE OO AO 00135 INTEGRAL Issue 1 0 Date 9 March 2015 OMC Observer s Manual Page 10 of 25 3 Instrument operations Because of telemetry constraints only 2 2 kbps are allocated to the OMC it is not possible to transmit the entire OMC image to the ground For this reason windows are selected around the proposed main gamma ray target as well as other targets of interest in the same field of view The observers obtain the data pertinent to their target as well as all the other OMC CCD sub windows taken during the observation see also the Overview Policies and Procedures document These additional targets are automatically selected from the OMC Input Catalogue Two observation modes are available to the observer the normal and the fast monitoring modes 3 1 Normal science operations mode In the normal science operations mode the OMC monitors the optical flux of a number of targets including the high energy sources within2. lenses and ensuring their alignment Doc No SRE OO AO 00135 INTEGRAL Issue 1 0 Date 9 March 2015 OMC Observer s Manual Page 9 of 25 2 3 The CCD detector The full well capacity 1s the maximum number of counts measurable per single pixel which in the case of OMC is 120000 cts This parameter critically determines the dynamic range of the detector The Analogue to Digital Converters ADCs used in the OMC have the capability of digitizing the analogue signal coming from the CCD read out ports to 12 bits 1 e they provide a discrete output in up to 4096 digital levels The ADCs are operated with 2 gain values At the standard low gain the full dynamic range of the CCD 0 to 120000 cts per pixel is digitized into 0 to 4095 Digital Levels DN at a linear scale of 30 cts DN At high gain which is currently used only during calibration only the 0 to 20000 cts per pixel range 1s digitized into 0 to 4095 DN with 5 cts DN This allows a more accurate photometry in some cases down to approximately the noise limit of the CCD Finally the CCD is cooled by means of a passive radiator illustrated in Figure 2 to an operational temperature of around 80 C
3. OO AO 00135 INTEGRAL Issue 1 0 Date 9 March 2015 OMC Observer s Manual Page 12 of 25 For this purpose a catalogue has been compiled by the OMC team containing over 540 000 sources It can be down loaded by FTP ftp ftp cab inta csic es pub integral catalogue and a search form is also available http sdc cab inta csic es omc The input catalogue includes e Known optical counterparts of gamma ray sources e Known optical counterparts of X ray sources e X ray sources detected and catalogued by ROSAT e Quasars observable with the OMC e Additional known AGNSs e Known eruptive variable stars including novae and cataclysmics e Variable objects which may require additional optical monitoring e HIPPARCOS reference stars for positioning and photometrical calibration During the mission additional sources of interest are added to the catalogue namely e Newly discovered optical counterparts of high energy sources e Regions of special interest for INTEGRAL science e New supernovae e New eruptive variable stars e Any other Target of Opportunity TOO For every scheduled observation the coordinates of all the targets of interest within the corresponding field of view are extracted from the OMC input catalogue The table of targets of interest is included in the telecommands to be sent to the OMC before any new pointing allowing the observer to identify all downloaded CCD windows 3 4 Gamma ray bursts and tra
4. Unfortunately the first one GRB 050626 was very close to Alpha Crucis a very bright star V 0 8 mag which saturated completely Doc No SRE OO AO 00135 INTEGRAL Issue 1 0 Date 9 March 2015 OMC Observer s Manual Page 13 of 25 the CCD and hit the optical counterpart of the CCD as shown in the image in Figure 6 For the second one XRF 120118A the OMC starting the observations only 45s after the XRF did not detect any coincident new source brighter than V 17 7 mag see Figure 7 Figure 6 The first GRB which occurred in the field of view of the OMC on 2005 July 26 GRB 050626 The red cross marks the position of the GRB within the error circle in green Unfortunately the GRB happened to be close to a very bright 0 8 magnitude in V star Alpha Crucis The bright star completely saturated the CCD causing the white vertical strip and therefore completely contaminating the GRB emission Figure 7 OMC image of the X Ray Flash detected by INTEGRAL on 2012 January 18 XRF 120118A The red cross marks the position of the XRF within the error circle in green OMC did not detect any new source brighter than V 17 7 mag inside the XRF error circle Doc No SRE OO AO 00135 INTEGRAL Issue 1 0 Date 9 March 2015 OMC Observer s Manual Page 14 of 25 4 Instrument performances 4 1 Background and read out noise There are two main sources of background flux for the OMC both related to the rat
5. discovered in M101 on August 24th 2011 Time is in INTEGRAL Barycentric Julian days Credits OMC Team 5 2 Known limitations l The automatic extraction of fluxes and magnitudes produces reliable results only for point like sources If the source coordinates are inaccurate by more than 2 OMC pixels 35 the analysis software will not be able to re centre the target and the derived fluxes and magnitudes using the default analysis parameters will not be correct For extended sources or high energy sources 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 Note that from OSA 6 0 onwards these mosaics of sub windows can be correctly analyzed by using IMA wcsFlag yes default in OSA 10 0 once the coordinates are well defined e g from X ray observations In this case o src get fluxes creates a virtual 11x11 pixel sub window inside the whole area centred at the source position given in the OMC Input Catalogue After that OSA works on this new sub window and Doc No SRE OO AO 00135 INTEGRAL Issue 1 0 Date 9 March 2015 OMC Observer s Manual Page 25 of 25 ignores the previous windows of the mosaic This is an internal software trick these virtual sub windows do not exist as standard sub windows o ima build for example will not create these virtual sub window
6. its FOV other sources of interest stars for photometrical calibration and masked pixels from the CCD to monitor the dark current and bias Variable integration times during a pointing allow the monitoring of both bright and faint sources Operations are performed automatically in the following way e The observing sequence starts by obtaining a series of images of 10 astrometric reference stars spread over the field of view This makes it possible to calculate on board the pointing of the OMC optical axis with an accuracy better than 0 5 pixels 9 e Then a set of photometric stars is observed 10 stars in the field of view with good photometric quality e The CCD centered in a target field is then exposed with the following sequence of integration times 10s 50s 200s After each exposure the full frame is transferred to the occulted part of the chip and the next integration starts An optimum use of the CCD from the point of view of the noise read out and cosmic rays is obtained for integration times of around 200 s so that for the faintest objects several exposures of 200 s are summed during the analysis on ground The number of integrations that can be added depends on the time during which the spacecraft keeps the same pointing without dithering typically 30 minutes The brightest stars will saturate their corresponding pixels for such integration times but the combination of short and long exposures allows one t
7. 11 3 4 Gamma ray bursts and transient SOULCES cccccccccccccccceceeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeees 12 4 Tasmrumnent perl ormanCES oeeie no en eT On oa 14 Al Background and read out NoiSe seienreenereiioin ien En E E ES 14 A e Me a S aa E E N E N T E stor eacteananeses 15 4 3 Limiting bright magnitude cc cccccsecececeeeeeeeeececeeeeeeecceeccecececeeeeeeeeeeeeeeeeeeeeeeeeeeeees 16 dA WPAN ONG ONG A I AC rae cre ce racine re sess AAEE 18 AS TOIS Rr ne E ee eee eee ee ee 19 5 Data PEO CU CIG eh caanccccrncasitcs susnceannsuacadabeinsanccectueatnchananiirosssoidabelntanscamueatineh EEEN E EEE sanini 20 Del OVERVIEW of the scientific ANAL YSIS a satieasecannsxsncasnesaneanassnnranupaencsaerencasnesaeesdareranienceeree 20 Die MO VV AN AAO INS secs cinc clon se paascens E sas acesuse EEE 24 This page was intentionally left blank Doc No SRE OO AO 00135 INTEGRAL Issue 1 0 Date 9 March 2015 OMC Observer s Manual Page 6 of 25 1 Introduction The Optical Monitoring Camera OMC is a wide field optical instrument using a large format CCD detector limited by a relatively low telemetry rate It measures the optical emission from the prime targets of the two gamma ray instruments SPI and IBIS The OMC offers the first opportunity to make observations of long duration in the optical band simultaneously with those at hard X rays and gamma rays Multi band observations are particularly important in high energy ast
8. 1901 03 which generated a mosaic of 5x5 OMC sub windows 16x16 The green cross and circle are respectively the position and positional error of 4U 1901 03 in the IBIS catalogue 3 2 Fast monitoring mode In the normal mode it is not possible to perform a continuous monitoring with a time resolution finer than 10 seconds Therefore when fast variability is expected the fast monitoring mode should be chosen With this mode integrations of 3 seconds are performed at intervals of 4 5 seconds and only the sections of the CCD containing the target of interest are read from the CCD and transmitted This implies that the position of the source is known with an accuracy better than the window size used for fast monitoring 11x11 pixels i e 3 x 3 and that the source is bright enough to be monitored with integration times below 10 s see also Figure 11 On the other hand note that this mode should be selected for any target brighter than V 7 5 and fainter than V 5 0 to avoid saturation of the CCD whatever kind of variability is expected Targets brighter than V 5 0 are too bright for the OMC even with integrations of 3 seconds nothing can be done in that case the source will then saturate the CCD 3 3 The OMC input catalogue As explained above besides the proposed target s the OMC observes astrometric and photometric stars and other targets of scientific interest within its field of view at a given time Doc No SRE
9. INTEGRAL Science Operations Centre Announcement of Opportunity for Observing Proposals OMC Observer s Manual SRE OO AO 00135 Issue 1 0 9 March 2015 Maintained by Guillaume Belanger This page was intentionally left blank Doc No SRE OO AO 00135 INTEGRAL Issue 1 0 Date 9 March 2015 OMC Observer s Manual jp G A M Ht Page iii Contributors to this manual include in alphabetical order M Mas Hesse OMC PI INTA CAB A Domingo Garau OMC team INTA CAB E Kuulkers ISOC ESA ESAC P Kretschmar ISOC ESA ESAC G Belanger ISOC ESA ESAC Doc No SRE OO AO 00135 INTEGRAL Issue 1 0 Date 9 March 2015 OMC Observer s Manual Page iv Table of Contents 1 MtrOCuctiOn nn cccccssseeeeeeeseeeeeeeeeeeeeeeeeeeeeaaaeeeeeeeeeeeeeeeeeeeeeeeeeeeseaauseeeeeeesseeeeeeeeeeeeeeeeeeeeaaaaaas 6 2 WCSCri Ou Ol ihe SH UNG OG sosesc aE N EEE AE ENST 8 DN MO i a ease ca trcsegne ee A E sathaceqnessseednnalnsouarseaeessde 8 Pad 1 6 05 ee re ee eee ee ee eee 8 DD CO are cs csers cae rine enacts E E E S E EE 9 3 Instrument operations ooscsictsdvahivedensesenssectinndewasentedvatonsdawteboesductiantawaraetedoetunsduutesensdontiameuasantsdsstis 10 sL N ral sci enee 0 0 2210 tLoim 1000 6 oye enn eee E EEEO 10 32 sc 01118 GL 0 oe a MON lt a nn een Pe Pree 11 3 3 The OMC input catalogue cccccccesssssssnnneeeeeeeeeeeeeeeeeseseseeesssseeeeeeeeeeeeeeeeeeeeeeeneeegs
10. Off line Scientific Analysis OSA documentation A TELI K Figure 1 OMC Flight model Doc No SRE OO AO 00135 INTEGRAL Issue 1 0 Date 9 March 2015 OMC Observer s Manual Page 7 of 25 Table 1 OMC parameters and scientific performances CCD pixels 2061 x 1056 1024 x 1024 image area 13 x 13 um per pixel CCD Quantum efficiency 88 at 550 nm CCD full well capacity 120000 electrons pixel ADC levels 12 bit signal 4096 levels 30 cts digital level low gain 5 cts digital level high gain Typical integration times 10 s 50s 200 s Wavelength range Johnson V filter centred at 550 nm Limit magnitude 10 x 200 s 30 18 1 my 50 x 200 s 30 18 9 my 100 x 200 s 30 19 3 my Sensitivity to variations 10 x 100 s 30 Amy lt 0 1 for my lt 16 This parameter defines the factor by which the flux from any source outside the FOV is reduced by multiple reflections before reaching the detector surface as background light The stray light from sources at gt 10 from the optical axis is negligible SRE OO AO 00135 INTEGRAL 10 9 March 2015 OMC Observer s Manual 8 of 25 2 Description of the instrument Forebati j orepalitle 2 1 Overall design The OMC consists of an optical system focused onto a CCD detector The optics are refractive with an Cover entrance aperture of 5 cm diameter and a square field of view of about 5 x5 A Johnson V filter allows
11. al background level oos oos o0 See Section 4 4 for the definition of an effective exposure Doc No SRE OO AO 00135 INTEGRAL Issue 1 0 Date 9 March 2015 OMC Observer s Manual Page 19 of 25 4 5 Focusing The focusing capabilities of the OMC system depend very slightly on the lens temperature and the pixel location over the detector The PSF follows a Gaussian distribution whose FWHM remains in the range 1 2 to 1 4 pixels in most cases as shown in Figure 13 OMC FM PSF 0 9 0 8 0 7 0 6 Intensity normalized 0 5 0 4 0 3 0 2 0 1 65 52 39 26 13 0 13 26 39 52 65 x um 1 pix 13 um Figure 13 Point Spread Function of the OMC optical system detector The plot shows a fit to the average PSF measured under different conditions The PSF measured in orbit follows a Gaussian with FWHM 1 3 pixels More than 90 of the energy falls within 3x 3 pixels Doc No SRE OO AO 00135 INTEGRAL Issue 1 0 Date 9 March 2015 OMC Observer s Manual Page 20 of 25 5 Data products 5 1 Overview of the scientific analysis A complete description of the data analysis pipelines and modules as well as the use of the Off line Scientific Analysis OSA software can be found in the OMC Analysis User Manual see http www isdc unige ch integral analysis The scientific analysis of all the INTEGRAL instruments is
12. e cold side of the companion star and the X ray eclipse 9 0 100 80 9 2 o E O s 9 45 ZOE RXTE ASM 3613 3et4 3615 3616 Ah ol alaan oe al el aa r z figs 0 ani T T 2800 3000 3200 3400 3600 MJD 50000 Figure 18 Long term light curves of the Be X ray binary and accreting pulsar 3A0535 262 HD 245770 obtained with the RXTE ASM left Y axis and the INTEGRAL OMC right Y axis The inset shows the OMC and ISGRI light curves during the INTEGRAL Target of Opportunity observations around MJD 53613 53616 Taken from Kretschmar et al 2006 ESA SP 604 p 273 SS Cyg IBISASGRI ASM RXTE and OMC Figure 19 SS Cygni is a cataclysmic variable white dwarf with a low mass donor star with an orbital period of about 0 275 days The OMC light curve bottom panel shows a strong variability in the optical emission roughly 40 times in flux No trends are seen at high energies with the RXTE ASM and INTEGRAL ABISISGRI top panels Taken from Risquez et al 2008 PoS Integral08 129 18 60 keV c s PERE AN SORE EN o 2 oe oe t b i ka L 9 C Saa n a ae 2 10 keV c s V mag 52800 53000 53200 53400 53600 53800 54000 MJD day 2 MAG V3 ma SRE OO AO 00135 INTEGRAL 1 0 9 March 2015 OMC Observer s Manual 24 of 25 SN 201 Ife 4260 4280 4300 4320 4340 BARYTIME d Figure 20 OMC light curve of the supernova SN201 Ife
13. ecting the shots with the longest integration times 6 Due to thermo elastic deformations the alignment of the OMC optical axis with the spacecraft attitude reference after correcting for the known OMC misalignment may diverge by up to 30 2 pixels This is corrected for in the analysis OSA 5 upwards using the photometric reference stars giving an accuracy of lt 2 in most cases
14. her large angular pixel size of 17 5 x 17 57 scattered sunlight zodiacal light and unresolved stellar sources Maximum background conditions correspond to pointings towards the galactic plane with maximum zodiacal light while the minimum background is achieved around the galactic pole with minimum zodiacal light 1 5 Stars per pixel 0 5 V mag Figure 8 Average number of stars per pixel brighter than a given V magnitude at different galactic latitudes Figure 8 shows the average number of stars brighter than a given magnitude contained within a single OMC pixel It can be seen that on average source confusion does not occur for objects brighter than my 17 at any galactic latitude For my 18 0 source confusion can become problematic in some regions very close to the galactic plane It is important to stress that on the galactic plane there are on average more than one star per pixel with my between 17 and 19 The density of stars on the galactic plane indeed determines the limiting magnitude of the instrument At galactic latitudes b gt 30 the problem of source Doc No SRE OO AO 00135 INTEGRAL Issue 1 0 Date 9 March 2015 OMC Observer s Manual Page 15 of 25 confusion becomes negligible except for specific cases in which bright stars are separated by just a few arcsec The background measured in orbit corresponds well to the expected values The read out noise of the OMC as measured on g
15. lux results are collected into a single table The OSA allows the observers to reprocess the OMC data with different parameters as for example the sampling time or more recent calibration files Figures 14 20 show examples of OMC light curves It can be seen that good photometric results can be obtained for a variety of objects even with the presence of a close companion as is the case for Cygnus X 1 Doc No SRE OO AO 00135 INTEGRAL Issue 1 0 Date 9 March 2015 OMC Observer s Manual Page 21 of 25 MCALC 1060 1062 1064 1086 1088 1070 1072 TFIRST d Figure 14 OMC light curve of high mass X ray binary Cygnus X 1 obtained using single exposures of 10 and 30 s The OMC sub window for this source is shown on the upper right corner of the graph Time is in INTEGRAL Julian days Credits OMC Team 12 4 12 6 12 8 MAG_V ww 13 0 thy aa say BE Py si er mt pg 13 4 1308 85 1308 90 1308 95 1309 00 TFIRST day Figure 15 OMC light curve of the low mass X ray binary Sco X 1 It is based on single exposures of 100 s The OMC sub window for this source is shown on the upper right corner of the graph Time is in INTEGRAL Julian days Credits OMC Team Doc No SRE OO AO 00135 INTEGRAL Issue 1 0 Date 9 March 2015 OMC Observer s Manual Page 22 of 25 Her X 1 IBIS ISGRI ASM RXTE and OMC data Figure 16 High energy RXTE ASM and INTEGRAL IBIS ISGRI and OMC light curves of the low ma
16. nd dark current calibration observation is carried out Figure 9 Limiting faint magnitude for a 30 E detection in the V band as a function of Maximum background integration time The best and the worst background cases are shown It is assumed that 10 separate exposures each with the integration time as given in the plot have been combined together in order to increase the signal to noise ratio 0 50 100 150 200 10 x t s Figure 10 As Figure 9 Best case background conditions are assumed galactic 17 pole no zodiacal light The curves O 20 40 60 80 100 120 140 160 180 200 Doc No SRE OO AO 00135 INTEGRAL Issue 1 0 Date 9 March 2015 OMC Observer s Manual Page 16 of 25 correspond to different total numbers of images being combined 4 3 Limiting bright magnitude The full well capacity of the CCD constrains the magnitude of the brightest stars that can be measured without pixel saturation for a given integration time With 10 s integrations the central pixel becomes saturated for objects brighter than my 7 With integrations of 200 s even stars with my 10 will start to saturate the CCD Severe saturation of the CCD might imply losing the information from the surrounding pixels and potentially from the column containing the source but no damage is expected on the detector Figure II shows the predicted number of counts on the CCD as a function of V magnitude for a 10s integration This number corre
17. nsient sources The INTEGRAL Burst Alert System IBAS is located at the INTEGRAL Science Data Centre ISDC near Geneva IBAS searches for gamma ray bursts GRB using SPI ACS triggers and IBIS ISGRI detections and position measurements If IBAS detects a GRB and it is within the OMC FOV a near real time command is sent to the OMC via the INTEGRAL Mission Operations Centre MOC located in Darmstadt Upon reception of this telecommand the OMC stops the observations planned for this pointing and starts to monitor a single window of 91x91 pixels 24 x 24 around the region where the burst has been detected with a fixed integration time of 100 s This trigger mode is active during the rest of the pointing The expected delay between the start of the burst and the start of OMC monitoring 1s less than 1 minute Specifically the OMC monitoring starts less than 15 seconds after the IBAS trigger This makes it possible to obtain slightly delayed but simultaneous optical X ray and gamma ray data of any burst taking place within the OMC FOV The Overview Policies and Procedures document describes how and under which conditions the OMC data are distributed to the observers in the case of a gamma ray burst or a transient Based on CGRO BATSE observations around one GRB per year was expected within the OMC field of view Nevertheless up to now the OMC has had only two GRBs in its field of view on July 26th 2005 and January 18th 2012
18. o increase the magnitude range for a given field e A number of windows of typically 11x11 pixels or 3 x 3 are extracted around each object of interest and transmitted to the ground see e g Figure 4 When using the Proposal Generation Tool PGT see the Overview Policies and Procedures document observers may specify a Monitoring Window Size for their target The maximum allowed value is 30 corresponding to a 30 x 30 square window Any value smaller than 3 will in fact be executed with a 3 x 3 window Values greater than 3 x 3 are executed as a mosaic of smaller windows e g several 3 x 3 windows piled side by side which will have to be recombined on ground Large window sizes can be useful for targets without 11x 11 pixels Figure 4 OMC sub window of Doc No SRE OO AO 00135 INTEGRAL Issue 1 0 Date 9 March 2015 OMC Observer s Manual Page 11 of 25 precise optically measured coordinates see e g Figure 5 See also Section 5 2 e Continuous monitoring of the central target with OMC will only be possible if the dithering pattern selected in the Proposal Generation Tool is none staring or hexagonal see the Observation Tools Software User Manual document In the case of larger dithering patterns like the 5x5 one the target will fall within the OMC field of view only for a fraction of the pointings Figure 5 This image corresponds to the High Mass X ray Binary 4U
19. photometric calibration in a standard system An optical baffle ensures the necessary reduction of scattered sunlight and also of the unwanted stray light coming from non solar sources outside the field of view FOV A deployable cover protected the optics from contamination during ground operations and early operations in orbit It was released during the first steps of the commissioning phase It now forms part of the baffle See Figure 1 for a picture of the flight model and Figure 2 for a diagram of the OMC I F Supports The camera unit is based on a large format CCD 2061x1056 pixels working in frame transfer mode 1024x1024 image area and 1024x1024 storage area not exposed to light This design with a frame transfer time of around 2 ms allows continuous measurements and makes it unnecessary to have a mechanical shutter A LED light source within the optical cavity provides flat field illumination of the CCD to calibrate the relative sensitivities of the pixels Main baflle Lens barrel Focal Plane Assembly 2 2 The optics The optical system as shown in Figure 3 consists of e a6 fold lens system composed of two different types of radiation resistant glass 7 X CCD e a filter assembly the Johnson V filter has been Radiator defined with a combination of 2 different filters Figure 2 A 3 D cut of the OMC e alens barrel giving mechanical support to the 8 8 PP Camera Unit
20. rophysics where variability is typically rapid unpredictable and of large amplitude The main objectives of the OMC can be summarised as follows e To monitor during extended periods of time the optical emission of all high energy targets within its field of view simultaneously with the high energy instruments e To provide simultaneous and calibrated standard V band photometry of the high energy sources to allow comparison of their high energy behavior with previous or future ground based optical measurements e To analyse and locate the optical counterparts of high energy transients detected by the other instruments especially gamma ray transients e To monitor any other optically variable sources serendipitously within the OMC field of view which may require long periods of continuous observations in order to understand their underlying physics variable stars flaring and eruptive objects etc The purpose of this manual is to present all the information about the OMC which is necessary for the preparation of INTEGRAL proposals We refer the interested reader to a sequence of papers on the OMC payload in the A amp A special INTEGRAL issue 2003 Vol 411 L261 L289 This issue also contains various other papers on the first results from in flight observations A more detailed description of the instrument performances can be found in the OMC Analysis Scientific Validation Report http www isdc unige ch integral analysis part of the
21. round is between 1 1 5 DNs pixel digital levels for low gain and between 3 3 5 DNs pixel in high gain corresponding to 30 45 cts and 15 17 cts respectively The read out noise measured in orbit is consistent with these values 4 2 Limiting faint magnitude Assuming a minimum level of background see the definition in the previous section and a combination of 10 exposures of 200 s each the limiting magnitude of the OMC is found to be my 18 1 30 detection level This value corresponds to a limiting sensitivity of the instrument of 2 1 x 10 erg cm s A or alternatively 5 8 x 10 ph cm s A at 550 nm At a maximum background level as defined above the limiting magnitude is my 17 5 Note that these sensitivities can only be achieved for isolated stars for which the background can be properly estimated Figure 9 shows the limiting faint magnitude for both maximum and minimum background as a function of integration time assuming in all cases that 10 images have been combined to increase the signal to noise ratio Figure 10 shows the limiting magnitude in best background conditions and for different combinations of exposures Measurements in orbit show that the OMC is on average about 30 more sensitive than estimated during ground based calibrations However the absolute photometric calibration changes with time and is continuously updated by the OMC team Currently about every 12 revolution a dedicated OMC flat field a
22. s as 11x11 pixel images Note that with IMA_wesFlag no these mosaics of sub windows will not be analyzed correctly as the software treats each box individually Users should also note that even with IMA _wesFlag yes for those new sources which are not yet included in the OMC Input Catalogue these virtual sub windows can not be created because the software extracts the coordinates from the OMC Input Catalogue In addition to this method the observer may extract the optical photometry manually from the corrected images produced by the analysis pipeline As examples Figure 4 shows the single OMC sub window generated for a point source with precise coordinates like Cygnus X 1 while Figure 5 displays a mosaic of 5x5 OMC sub windows generated for the High Mass X ray Binary 4U 1901 03 which has no accurate coordinates 4 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 change slightly for different rotation angles Some of the bright sources slightly saturating one or a few pixels might not be detected as saturated sources As a consequence their derived magnitudes may not be correctly computed The observer should check in 5 Figure 12 whether the source could be saturating the CCD for a given integration time and re analyze the data rej
23. split into a number of steps with similar tasks COR Data Correction At this step the appropriate calibration data dark current bias flat field for the current science window group are selected and the corrected pixel values for the subsequent analysis are calculated GTI Good Time Handling At this step Good Time Intervals GTI for the current Science Window are derived based on housekeeping data and attitude information IMA Source Flux Reconstruction and Image Creation At this step the fluxes of the individual sources are calculated and the source magnitudes are derived The user can also require to build the individual images containing the OMC boxes at this step Observers will receive the results from all of these steps The raw and corrected CCD sub windows for all pre defined sources in the field of view The data are provided in a tabulated format with pixel values as vector entries in a column of the tables CCD corrected windows will include flat field calibration bias and dark current subtraction but not the removal of cosmic rays In addition a series of tables with derived fluxes and magnitudes for all observed sources as a function of time By default photometrical analysis will be performed combining all images obtained within periods of around 10 minutes IMA2 Results Collection The data concerning one observation are distributed between different files and Science Windows At this step the OMC f
24. sponds to the counts expected in the central brightest pixel only Finally Figure 12 gives the integration time at which stars of different magnitudes will start to saturate the CCD 7 Minimum background Figure 11 Expected number of counts on the central brightest pixel as a function of V magnitude for an integration time of 10 s The error bars correspond to lo The plot also includes the expected background flux computed for maximum and minimum conditions Maximum background Saturation Log counts pix iN on Se Saturation time s NO N 100 Nn Nn Doc No SRE OO AO 00135 INTEGRAL Issue 1 0 Date 9 March 2015 OMC Observer s Manual Page 17 of 25 Figure 12 CCD saturation time as a function of V magnitude Note that for integrations of 50 s all stars with my lt 8 6 will saturate the CCD For the shortest OMC integration times fast monitoring mode 3 s the brightest stars that can be observed should be fainter than my gt 5 Doc No SRE OO AO 00135 INTEGRAL Issue 1 0 Date 9 March 2015 OMC Observer s Manual Page 18 of 25 4 4 Photometric accuracy Table 2 shows the expected error expressed in magnitudes of a given measurement for various effective integration times and magnitudes Effective integration time means the total exposure after combining several shots The value of 300 seconds corresponds to the typical effective e
25. ss X ray binary Her X I over a period of about 12 days The orbital period is 1 7 days marked by vertical lines X ray eclipses when the neutron star is hidden by its companion are evident Credits OMC Team g oe Ee ie oN O a ON ma gt 53574 53576 53578 53580 53582 53584 MJD day Her X 1 COMC 2598000079 OMC data Optical V magnitude D JCOMPANION STAR To OBSERVER 0 2 0 0 0 2 0 4 0 6 0 8 1 0 1 2 Phase period 1 700167412 days Figure 17 OMC light curve of the low mass X ray binary Her X I see Figure 16 folded on the 1 7 days orbital period Risquez et al 2008 PoS Integral08 amp 129 The optical variations are due to the tidal distortion of the companion star and to the intense X ray heating of the illuminated face of the companion produced by the neutron star The insets show an artist s impression of the system top and a sketch of the situation at different orbital phases the orbit is almost circular not to scale and omitting the accretion disk bottom e A Orbital phase zero corresponds to maximal radial velocity e B At orbital phase 0 25 one sees the maximum of the optical emission as one is facing the hot side of the X ray heated companion Doc No SRE OO AO 00135 INTEGRAL Issue 1 0 Date 9 March 2015 OMC Observer s Manual Page 23 of 25 e D At orbital phase 0 75 one sees the minimum of the optical emission as one is facing th
26. xposure obtained by the OSA 10 0 analysis software using the default parameters A value around 900 seconds corresponds to the maximum effective exposure one can get in the standard analysis when changing the default parameters These values should be used as a guide they are the best results that can be obtained with the latest version of the analysis software and they are only valid for isolated stars in Staring mode For an entire 5x5 dither pattern 2000s pointing 900 seconds of effective exposure can also be taken as a representative value The values for photometric accuracy have been computed by taking into account the most current knowledge of the OMC instrument One can see in Table 2 that good photometry can be performed in the V band for objects of quite different brightnesses Note that these accuracies can only be obtained for isolated stars for which the background can be properly estimated Furthermore in case of dithering the photometric dispersion is gt 0 02 magnitudes in all cases This value 0 02 is the accuracy of the OMC flat field matrix So if the source is observed in different detector pixels as occurs for a dithered observation the accuracy of the flat field produces an additional scattering on the observed magnitudes of about 0 02 magnitude Table 2 Photometric accuracy in magnitudes for different effective exposures and source magnitudes meme s fie e e effective exposures assuming a typic
Download Pdf Manuals
Related Search