The dual-frequency calibration of ionosphere influence in VLBA data processing

1. The dual-frequency calibration of ionosphere influence in VLBA data processing Andrey Chuprikov Igor Guirin Astro Space Center Moscow Russia Scattering and Scintillation in Radio Astronomy

2. The ionized shell around the Earth is caused by the Sun UV radiation The electronic column density is peaked at an height of 350 km above the ground and spreads by several tens of kilometers around this altitude The TEC variation has time scale of several hours and induce a delay of about 1 ns at 8.4 GHz Such delay is 13 times larger at 2.3 GHz due to dispersion Several strategies have been used to calibrate the ionosphere in VLBI data. One of them is technique of Dual Frequency Observations Scattering and Scintillation in Radio Astronomy

3. The ionosphere can be calibrated out with simultaneous observations in two widely separated frequency bands. Usually, these are • X Band (8.4 GHz) • S Band (2.3 GHz) A linear combination of group delays in these bands yields straightforwardly the ionosphere-free delay : Scattering and Scintillation in Radio Astronomy

4. Description of the experiment We present some results of VLBA experiment titled RDV14. Observations had been carried out on 15/04/1999. FITS-file has been created with VLBA correlator and then placed into archive Later, these data have been kindly placed at our disposal by Dr. L. Petrov (Goddard Space Center) Scattering and Scintillation in Radio Astronomy

5. Frequency structure of initial data F0 (MHz) DF (MHz) nCh IF1 2221.22 8 16 IF2 2241.22 8 16 IF3 2331.22 8 16 IF4 2361.22 8 16 IF5 8406.22 8 16 IF6 8476.22 8 16 IF7 8791.22 8 16 IF8 8896.22 8 16 Scattering and Scintillation in Radio Astronomy

6. List of sources observed Source RA (2000) DEC (2000) Flux [Jy] S-band X-band 2.3 GHz 8.4 GHz 1 0003-066 00:06:13.892894 -06:23:35.33530 1.0 2.2 2 0106+013 01:08:38.771074 01:35:00.31714 1.7 2.0 3 0119+115 01:21:41.595043 11:49:50.41313 0.9 1.5 4 0229+131 02:31:45.894054 13:22:54.71627 1.1 1.0 5 0235+164 02:38:38.930108 16:36:59.27471 0.9 1.0 6 0336-019 03:39:30.937787 -01:46:35.80399 2.0 1.8 7 0528+134 05:30:56.416744 13:31:55.14955 2.0 2.3 8 0539-057 05:41:38.083384 -05:41:49.42839 1.0 1.0 9 0552+398 05:55:30.805608 39:48:49.16500 3.2 5.4 10 0718+792 07:26:11.735177 79:11:31.01624 1.0 1.0 11 0727-115 07:30:19.112472 -11:41:12.60048 2.0 2.3 12 0823+033 08:25:50.338355 03:09:24.52014 1.2 1.5 13 0851+202 08:54:48.874925 20:06:30.64088 1.4 1.8 14 0923+392 09:27:03.013916 39:02:20.85195 4.4 11.0 15 0953+254 09:56:49.875361 25:15:16.04977 1.1 1.1 16 0955+476 09:58:19.671648 47:25:07.84250 0.9 1.0 17 1032-199 10:35:02.155274 -20:11:34.35975 1.0 1.0 18 1044+719 10:48:27.619917 71:43:35.93828 1.3 1.0 19 1124-186 11:27:04.392448 -18:57:17.44166 1.4 2.2 20 1308+326 13:10:28.663845 32:20:43.78295 1.7 1.8 21 1418+546 14:19:46.597401 54:23:14.78721 1.0 1.0 22 1606+106 16:08:46.203179 10:29:07.77585 1.3 1.1 23 1638+398 16:40:29.632771 39:46:46.02854 1.2 1.0 24 1739+522 17:40:36.977850 52:11:43.40750 1.2 1.0 25 1741-038 17:43:58.856137 -3:50:04.61668 4.0 4.0 26 1749+096 17:51:32.818573 9:39:00.728510 1.6 3.6 27 1803+784 18:0:45.683918 78:28:04.01851 1.7 1.6 28 1830+285 18:32:50.185631 28:33:35.95530 1.0 1.0 29 1908-201 19:11:09.652893 -20:6:55.10907 3.2 2.8 30 1958-179 20:00:57.090448 -17:48:57.67251 0.8 1.3 31 2128-123 21:31:35.261758 -12:07:04.79588 1.5 2.1 32 2145+067 21:48:05.458679 06:57:38.60422 2.6 7.0 33 2234+282 22:36:22.470860 28:28:57.41329 1.0 1.0 Scattering and Scintillation in Radio Astronomy

7. Data processing The main idea was to use a brightest and compact source titled 0923+392 (No. 14) (alternative titles are J0927+3902 or 4C+39.25) as a FringeFinder Then, due to phase stability of VLBA equipment, it is enough to interpolate the GAINs obtained and to apply them to any other source to reconstruct the phase of visibility function Scattering and Scintillation in Radio Astronomy

8. Data processing The a priori amplitudecalibration is not a problem because there are calibration tables (TY, GC) in initial FITS-file The trick is tocompensatetheionosphere-freedelaybeforeSelf-Calibration with multiplication the Visibility by Scattering and Scintillation in Radio Astronomy

9. Results of data processing DA193 (J0555+3948) without usage of procedure Scattering and Scintillation in Radio Astronomy

10. Results of data processing DA193 (J0555+3948) with usage of procedure Scattering and Scintillation in Radio Astronomy

11. Results of data processing 1741-038 (J1743-0350) without usage of procedure Scattering and Scintillation in Radio Astronomy

12. Results of data processing 1741-038 (J1743-0350) with usage of procedure Scattering and Scintillation in Radio Astronomy

13. References • Fey, A.F., et al., 1996, ApJSS, 105, 299 - 330 • Brisken, W.F., et al., 2000, ApJ, 541, 959 - 962 Scattering and Scintillation in Radio Astronomy

14. Conclusions • Our experience demonstrates, that usage of dual-frequency calibration of VLBA data can essentially improve the source image even if it is a strong calibrator • If procedure has not been performed for any data, the final source image can be shifted from the phase center, and, additionally, image may have the false components in this case • The similar results have been published by other authors (Fey, A.L., 1996), (Brisken, W.F., 2000) Scattering and Scintillation in Radio Astronomy

15. The “Astro Space Locator” Software Please, contact with me : Andrey Chuprikov Astro Space Center, Moscow, Russia achupr@asc.rssi.ru Scattering and Scintillation in Radio Astronomy