Towards solar radio imaging with LOFAR

Transcription

1 Towards solar rado magng wth LOFAR Olaf Wuckntz Towards solar rado magng wth LOFAR my background: long baselne LOFAR long baselne ssues, frnge-fttng frst long-baselne frnges frst long-baselne mages Sun frst mage(s) varablty, moves dynamc spectra magng strateges, specal ssues LOFAR Solar KSP Workshop IV, Potsdam, 9th November 2010 O. Wuckntz Gravtatonal lenses LOFAR resoluton frnge-spacng θ = λ/l resoluton λ/m freq/mhz 1 km 30 km 300 km 1000 km O. Wuckntz O. Wuckntz German LOFAR statons LBA at Effelsberg O. Wuckntz O. Wuckntz LBA detals at Unterwelenbach HBA + LBA n Tautenburg O. Wuckntz O. Wuckntz

2 Not connected yet: Potsdam VLBI methods for LOFAR Long baselnes requre VLBI technques unstable phases, short coherence tmes weak sgnal: have to average n tme and frequency solve for delays τ = 1 φ 2π ν solve for rates r = 1 φ 2π t = ν τ t non-dspersve τ = τ 0 dspersve ( ν0 ) 2 τ = τ 0 ν O. Wuckntz O. Wuckntz Frnge-fttng for LOFAR ether for sngle subbands (BW 200 khz, τ 5 µsec) Very frst long baselne frnges (NL-Ef, Aug 2009) 3C196, one subband, LBA or coherent mult-band (BW 48 MHz, τ 0.02 µsec) beware of multple peaks n delay/rate produce 2-d delay/rate spectra smultaneously ft for four parameters dspersve/nondspersve delays/rates O. Wuckntz Coherently averaged over tme ntervals, then FFTed n frequency, then ncoherently averaged over 1h. O. Wuckntz Frnges n delay/rate space (sngle subband) Delays and phases 5 subbands contnuous mod phase [full turns] freq [MHz] O. Wuckntz O. Wuckntz Mult-band: more senstvty and hgher resoluton delay Frnges to Tautenburg and Unterwelenbach (Jan 2010) O. Wuckntz O. Wuckntz

3 Results of frnge analyss LBA long-baselne map: some detals long baselne frnges found clock offsets n some statons confuson n LBA polarsaton labels strong 8 MHz rpple 63 MHz LBA resonance strong dfferental Faraday rotaton tme for magng! 3C196, LBA, 31 / 160 subbands, / MHz (rpple!) bandwdth 6 MHz / 48 MHz D h on 12/13 Feb NL + 3 DE statons (Effelsberg, Unterwelenbach, Tautenburg) corrected for 1 µsec and 17 µsec constant delays RR and LL from XX/XY/YX/YY usng geometrc model (self-)calbrated and maged LL/RR n dfmap MFS wth/wthout spectral ndex correcton O. Wuckntz O. Wuckntz UV coverage wth long and short baselnes MTRLI (MERLIN) observatons of 3C196 at 408 MHz [ Lonsdale & Morson (1980) ] O. Wuckntz O. Wuckntz LOFAR maps of 3C196 (LBA: MHz) LOFAR LBA vs. MERLIN 408 MHz NL only, beam NL+DE, beam [ Wuckntz + Lonsdale & Morson (1980) ] O. Wuckntz O. Wuckntz Frst nterferometrc Solar observatons ntated by Solar KSP (Mann, Vocks, Bretlng), Anderson, Polatds, Wuckntz 8 ten-mnute scans on 9th June 2010, 9:48 15:50 UT 4 wth LBA and HBA each phase/pontng centre near Sun and calbrator sources LBA dffcult, other sources domnatng: 3C123, CygA, CasA,... stuaton unclear HBA much clearer sgnal, Sun domnatng on short baselnes self-calbraton possble because of compact component only short baselnes used! frst steps n own software Imagng strategy assess data qualty frnge-fttng to determne clock offsets; correcton flaggng convert to crcular polarsaton (f wanted) averagng n tme and frequency wrte UVFITS fle produce dynamc spectra for data selecton magng/calbraton n dfmap (or AIPS) not used external calbrators yet CLEAN / selfcal loop, compact emsson helps! for moves: subtract mean model O. Wuckntz O. Wuckntz

4 The very frst map Dynamc spectra Sgnfcant varablty as functon of tme and frequency! O. Wuckntz O. Wuckntz Varablty: 10 mn n 10 sec steps (1 subband) Varablty: 30 sec n 1 sec steps (8 subbands) O. Wuckntz O. Wuckntz Further thoughts about magng Summary man challenge: varablty n tme and frequency Earth rotaton and wde bands cannot fll uv plane have to select tme and frequency very sparse uv coverage on longer baselnes how to ensure consstent calbraton? n frequency: delays nstead of phases dspersve and non-dspersve, no resdual phase requred n tme: use the (constant) dsk? use external calbrators? long baselne LOFAR works! but no ppelne yet frnge analyss revealed techncal problems Sun can be observed and resolved wth LOFAR! to do full frnge-fttng and calbraton for long baselnes Sun as functon of tme and frequency consstent calbraton requred self-cal or external calbraton? O. Wuckntz O. Wuckntz Addtonal materal The Internatonal LOFAR Telescope (ILT) Internatonal LOFAR statons Mult-band delay fttng (detals) Delay/rate map of 3C196 Expectatons: 3C196 at 5 GHz Very frst LBA long-baselne magng attempts Frst HBA long-baselne magng attempt O. Wuckntz O. Wuckntz

5 do not ft phases drectly Delay fttng only know phase modulo 2π data are nosy equvalent (but better!): maxmse the corrected sgnal measured and orgnal vsblty hope that V 0 (ν) = const and correct for delay fnd maxmum of ths s Fourer transform f τ = const V(ν) = e 2πντ(ν) V 0 (ν) dν e 2πντ(ν) V(ν) O. Wuckntz Mult-band delay fttng delay almost constant wthn subbands apply FFT for all subbands combne the results ncoherently combne the results coherently f (τ ) = dν e 2π(ν ν )τ V(ν) ( F[τ] = e 2πν τ(ν ) f τ(ν ) ) wth coarse FFT on fne grd, nterpolaton τ arbtrary functon of frequency (non-/dspersve) O. Wuckntz Include frnge rates Delay/rate map of feld around 3C196 have to ntegrate n tme to ncrease S/N take nto account rates (tme-dervatves) do not use phase rates but delay rates r = τ dspersve/non-dspersve t f (τ,r ) = dν e 2π(ν ν )τ dt e 2π(t t 0)r V(ν,t) ( F[τ,r] = e 2πν τ(ν ) f τ(ν ),r(ν ) ) VLSS (74 MHz): A 19 Jy B 6 Jy C 17 Jy 3C Jy all phase rates are frequency-dependent O. Wuckntz O. Wuckntz VLSS vs. LOFAR map of feld around 3C196 3C196 on long baselnes: expectatons Cambrdge 5km at 5 GHz [ Pooley & Henbest (1974) ] O. Wuckntz O. Wuckntz Frst long baselne maps of 3C196 HBA observatons of 3C196: some detals 3C196, HBA, 120 / 244 subbands, MHz bandwdth 24 MHz L h, 22nd May NL + 2 DE statons (Effelsberg, Tautenburg) corrected for 8 µsec n superterp,... (self-)calbrated and maged YY n dfmap phase jumps, rates, nconsstent delays (n freq) low S/N n German statons most of the tme magng very tough, detals not relable yet O. Wuckntz O. Wuckntz

6 HBA observatons: uv coverage, drty beam LBA + HBA mages of 3C196 beam sze O. Wuckntz O. Wuckntz LBA + HBA mages of 3C196 wth contours O. Wuckntz