.20.0 EXTRAGALACTIC OPTICAL-RADIO LINK RESEARCH AT USNO 1 1 1 1 1 N. ZACHARIAS , M.I. ZACHARIAS , D. BOBOLTZ , A. FEY , R. GAUME , 1 1 1 G.S. HENNESSY , K.J. JOHNSTON , R. OJHA 1 U.S. Naval Observatory 3450 Massachusetts Ave. N.W., Washington DC 20392 e-mail: nz, miz, db, alf, gaume, gsh, kjj, [email protected] ABSTRACT.Over500counterpartsofICRFsourceswereobservedduring24deepCCDobservingruns aspartoftheUSNO CCDAstrographCatalog(UCAC)project,providingadirectlink toTycho-2stars. Forsome sourcesapositionalaccuracyof10mas is achieved. Asample of12extragalacticICRFsources are being observedat the NavalObservatory Flagstaff Station (NOFS) 1.55-metertelescope overseveral years to monitor optical position stability. First high resolution imaging of selected sources are obtained at the Lick 3-meter AO system to correlate source structure with optical-radio centroid offsets. As part of the Space Interferometry Mission (SIM) preparatory science about 240 bright QSO’s are monitored for photometric variability in B,V,R and I. The USNO Robotic Astrometric Telescope (URAT) will be able to combine deep CCD imaging of all ICRF2 target areas and millions of compact galaxies with a stellar, astrometric, all-sky survey of multiple epochs. 1. OPTICAL COUNTERPARTS OF ICRF SOURCES Figure 1: Observed optical-radio position offsets of 199 ICRF optical counterparts. 1 Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE 3. DATES COVERED 2007 2. REPORT TYPE 00-00-2007 to 00-00-2007 4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER Extragalactic Optical-Radio Link Research at USNO 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) 5d. PROJECT NUMBER 5e. TASK NUMBER 5f. WORK UNIT NUMBER 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION U.S. Naval Observatory,3450 Massachusetts Avenue, REPORT NUMBER N.W.,Washington,DC,20392 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR’S ACRONYM(S) 11. SPONSOR/MONITOR’S REPORT NUMBER(S) 12. DISTRIBUTION/AVAILABILITY STATEMENT Approved for public release; distribution unlimited 13. SUPPLEMENTARY NOTES 14. ABSTRACT 15. SUBJECT TERMS 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 18. NUMBER 19a. NAME OF ABSTRACT OF PAGES RESPONSIBLE PERSON a. REPORT b. ABSTRACT c. THIS PAGE Same as 6 unclassified unclassified unclassified Report (SAR) Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std Z39-18 Between1997and2004theCTIO0.9m,KPNO0.9mand2.1mtelescopeswereutilizedin24observing runs to image over 500 optical counterparts of ICRF sources. Contemporaneous to these deep CCD imagingtheUSNOastrographobservedthesamefieldsaspartoftheUCACproject(Zachariasetal.2004) in order to provide a direct tie to the Tycho-2 and thus the Hipparcos reference frame. Reductions have been performed for 236fields (mainly in the southernhemisphere) ofwhich 199 gaveopticalcounterpart position results (Figure 1), see also Zacharias & Zacharias (2005). The total estimated positional error per coordinate for many sources is in the 10 to 20 mas range. However, a higher than expected fraction of sources show an optical-radio position offset larger than 3-sigma of the total error. Whether this is the result of underestimated systematic errors or caused by physics (optical source structure) is yet unknown. 2. OPTICAL ASTROMETRIC STABILITY ON MAS LEVEL Between 2002 and Oct. 2007 484 CCD images were obtained with the Naval Observatory Flagstaff Station(NOFS)1.55mStrandReflectorofaselectionof12ICRFopticalcounterparts(seeTable1). The goal of this differential astrometry investigation is to assess the positional stability over time at optical wavelengthswithsingleexposurestandarderrorsofabout3maspercoordinateand14to77observations per source. The targets were selected to cover a wide range in parameters (ICRF defining, contributing or other sources, quasar or BL-Lac object type, redshift, radio quality, radio structure index). For radio characterizationsee Fey & Charlot (1999). source I t mag z Q SIX 0241+622 C Q 12.2 0.04 18 2 0552+398 C Q 18.0 2.37 96 2 0738+313 D Q 16.1 0.63 61 4 0754+100 D L 15.0 0.66 68 2 0839+187 D Q 16.4 1.27 46 4 0851+202 C L 15.4 0.31 83 2 0912+297 D L 16.4 ? – 1 1656+053 C Q 16.5 0.89 57 3 1830+285 D Q 17.2 0.59 58 3 1937−101 C Q 17.0 3.79 52 3 2059+034 D Q 17.8 1.01 66 2 2201+315 O Q 15.6 0.30 74 3 Table 1: Sources on the optical stability monitoring program at the NOFS 1.55m telescope. I = ICRF object type (D = defining, C = contributing, O = other), t = optical type (Q = QSO, L = BL-Lac), mag = approximate V magnitude, z = redshift, Q = radio quality, stability over time (0 = poor, 100 = excellent), and SIX = radio structure index (1 = very good, 4 = poor). 3. HIGH RESOLUTION OPTICAL IMAGING In September 2007, following an unsuccessful attempt in 2006 (poor seeing), high-resolution (0.3 arcsec) images of 3 extragalactic, compact sources were obtained using the Lick 3-meter adaptive optics (AO) system. The source IRAS 0147+3554 clearly shows an asymmetric image profile due to structure of the underlying spiral galaxy. This is a programin collaboration with E. Gates, Lick Observatories. USNO has also pursued observing opportunities using HST and the LuckyCam at ESO, however,no observing time was been granted yet. It will be very challenging to obtain sufficient observing time to evenimageonceallopticalcounterpartsofthecurrentICRF1withhighresolutionatopticalwavelengths, something which is being achieved regularly at radio frequencies. 2 Figure2: Skydistributionofselected,opticallybright,compactquasarsforopticalvariabilitymonitoring program. Targets have been classified by redshift and visual appearance on digital sky survey (DSS) images. Figure3: Photometricresultsfromasingleobservingrun. TheobservedB-Vcolorindex(lowerleft)and number of observations per source (lower right) is shown together with the standard errors in B and V (top row). 3 4. BRIGHT QUASAR PHOTOMETRY A total of 242 optically bright quasars are on a photometric monitoring program (Figure 2). The sources have been selected, regardless of radio flux, starting with the Veron catalog (Veron et al. 2000), as the optically brightest targets giving a good all-sky distribution and displaying symmetric, compact appearance on digitized Schmidt survey images. Most targets are in the 15 to 17 mag range with some asfaintas19thmagto fillgapsnearthe galacticplane. The opticallyverybrightestquasarsandBL-Lac objects were excluded due to asymmetric structure already visible on the arcsec level. Quasars are being observed with the CTIO and NOFS 1.0-meter telescopes in B, V, R, and I bands. This programis partof the Space InterferometerMission(SIM Planet-quest,see also Unwin etal. 2007) preparatory science within the “Astrophysics of Reference Frame Sources” SIM key science project (PI: K. Johnston). With the lack of high resolution optical images (see above)optical variability information can be used as the next best indicator for “stable” sources to select most appropriate candidates for a future SIM celestial reference frame, which needs about 50 pre-selected sources. Results from this programwill also be used in the work leading to the construction of the future ICRF2 (Ma et al. 2007). Preliminary photometric results for a single observing run are presented in Figure 3. Significant differences in observedmagnitudes w.r.t. the Veron catalog are found. A paper is in presentation for AJ (Zacharias et al. 2008). 5. THE URAT PROJECT The USNO Robotic Astrometric Telescope (URAT) project aims at an all-sky survey to extend the UCAC data to fainter limiting magnitudes and higher positional accuracies. URAT comes in 2 phases. ThefirstwillusetheURAT focalplaneatthe existingUSNOAstrograph“redlens”,whichalsowasused for the UCAC program. Phase 2 requires the construction of a new, dedicated, astrometric telescope of which the primary mirror has been manufactured while currently no funding is identified to complete that telescope. Table 2 gives an overview about the properties of the 2 phases of the URAT program. parameter URAT phase 1 URAT phase 2 telescope redlens astrograph new design focal length 2.00 m 3.60 m aperture 0.20 m 0.85 m field of view 9.00 deg 4.50 deg field diameter 324 mm 283 mm ′′ ′′ pixel scale 0.905 /pix 0.515 /pix sky cov./exposure 27 sq.deg 9.1 sq.deg bandpass 670–760nm brightness range 10–18 mag 13–21 mag catalog accuracy 10 mas 5 mas solve for position, proper motion, parallax begin of survey 2009 not determined Table 2: The above values for pixel scale and sky coverage assume the use of the “4-shooter” camera which is based on 4 CCD chips of 111 million pixel each (STA1600 chip). The catalog accuracy is for well exposed stars; at the limiting magnitude the errors will be about a factor of 3 larger. Details of the optical design of URAT are given by Laux & Zacharias (2005), and Zacharias et al. (2006), while the project itself is described in Zacharias (2005, 2007). Latest progress with the detector development is presented in Zacharias et al. (2007). Figure 4 shows the 10k camera dewar with asingleCCDchipofthekind(95mmby95mm,full-waferdevice)anticipatedforthe“4-shooter”URAT focal plane assembly. Astrometric characterizationof this back-illuminated detector is in progressat the USNO astrographtelescope. The largeskycoveragewith asingle exposurewillallowmultiple skyoverlapsper year. With atleast 2 years of operation on each site (northern and southern hemisphere) not only mean positions but also propermotions andparallaxescanbe derivedfor allstarsfromthe URAT observingprogramitself. The limitingmagnitudewillbedeepenoughtobeabletoaccessopticalcounterpartsofICRFsourcesdirectly. 4 Figure 4: Dewar of 10k camera with STA1600 chip. This monolithic CCD detector has 10,560by 10,560 light sensitive pixels of 9µm size. First light was obtained 2007 Oct 09 at the USNO astrograph. The URAT focal plane assembly will acommodate 4 of these detectors. The URAT programcanprovidehighly accuratereferencestarsforthePanSTARRSandLSST projects. 6. CONCLUSIONS The analysis of ICRF-2 sources at radio frequencies is in “good shape” (e.g. Ojha et al. 2005, Ma 2006)whiletheopticaleffortislaggingbehindincharacterizationofopticalcounterparts(highresolution imaging, variability and geometric stability analysis), and positional accuracy. The Hipparcos Celestial Reference Frame is, 10 yearsafter it became available,still the primary standardof the optical reference frame, with no significant deviations from the ICRF detected. Currentopticalreference frame researchis active in 3 areas: a) building on the Hipparcos and Tycho data and going to fainter stars for a densification of the existing optical reference frame (e.g. UCAC and URAT),b)maintainingthelinkbetweenthecurrentlybrightopticalreferenceframe(Hipparcos)andthe radio-defined ICRF by observing the optically faint counterparts, and c) preparatory work for a future high accuracy optical reference frame (e.g. SIM and Gaia) which is likely to supersede the VLBI based radio reference frame. In all areas USNO is heavily involved. 7. REFERENCES Fey, A. L., Charlot, P., 1999, “VLBA observations of radio reference frame sources. III. Astrometric suitability of an additional 225 sources”,ApJSup 128, pp. 17–83 Laux, U., Zacharias, N., 2005, “URAT optical design options and astrometric performance”, in Proc. “Astrometry in the Age of the Next Generation of Large Telescopes”, Eds: Kenneth Seidelmann and 5 Alice K.B. Monet ASP Conference Series 338, p.106–109 Ma, C., 2006,“Present Status Of The Celestial Reference System And Frame”, IAU GA 2006 (Prague), JD 16, paper 2 in “Nomenclature, Precession and New Models in Fundamental Astronomy” Ma, C. et al. 2007, “ICRF2 ...”, these Proceedings Ojha,R.,Fey,A.L.,Charlot,P.,Jauncey,D.L.,Johnston,K.J.,Reynolds,J.E.,Tzioumis,A.K.,etal. 2005, “VLBI Observations of Southern Hemisphere ICRF Sources. II. Astrometric Suitability Based on Intrinsic Structure”, AJ 130, pp. 2529–2540 Unwin, S. et al. 2007, “SIM science”, PASP in press Veron-Cetty, M.-P., Veron, P., 2003,“A catalogue of quasarsand active nuclei: 11th edition”, A&A 412, pp. 399–403 Zacharias,N.,Urban,S.E.,ZachariasM.I.,WycoffG.L.,HallD.M.,MonetD.G.,RaffertyT.J.,2004, “The UCAC2 release”,AJ 127, pp. 3043–3059 Zacharias M. I., ZachariasN., 2005, “optical counterparts 2.1m”, ASP Conf.Ser. 338, pp. 184–187,Eds. P. Kenneth Seidelmann & Alice K.B. Monet Zacharias, N., 2005, “The URAT project”, in Proc. “Astrometry in the Age of the Next Generation of Large Telescopes”, Eds: Kenneth Seidelmann and Alice K.B. Monet ASP Conference Series 338, p. 98–103 Zacharias, N., Laux, U., Rakich, A., Epps, H., 2006, “URAT: astrometric requirements and design history”, Proceed. SPIE 6267, p.22 Zacharias, N., Dorland, B., Bredthauer, R. et al. 2007, “Realization and application of a 111 million pixel backside-illuminated detector and camera”, SPIE, 6690 paper 8 Zacharias, N. 2007, “Dense optical reference frames: UCAC and URAT”, in Proceed. IAU Symp. 248 (Shanghai), in press Zacharias,N., Ojha, R., Hennessy, G. et al. 2008,“Photometric monitoring of compact, optically bright quasars for SIM and other future celestial reference frames”, in prep. for AJ 6