E. Momjian, T. Ghosh, C. Salter, & A. Venkataraman (NAIC-Arecibo Observatory) eVLBI with the 305 m Arecibo Radio Telescope ABSTRACT Using the newly acquired Mark 5A data acquisition system, we present the first eVLBI results from the Arecibo Observatory. The Mark 5A system, developed by Haystack Observatory, is a Gbps VLBI data system based on magnetic disk technology. Incorporating primarily low-cost PC-based components, it can record data at rates of up to 1024 Mbps on to an array of inexpensive, removable IDE/ATA disks. Until now, VLBI has been severely inconvenienced because the data had to be recorded onto tape or disk and then shipped for correlation. Consequently, it was impossible to judge the success of the observations until weeks or months after they had been made. The solution, to link the telescopes electronically in real-time, now enables us to cross-correlate the data as it arrives. This technique, naturally called eVLBI, is now possible with high-bandwidth network connectivity having become a reality. INTRODUCTION   Very Long Baseline Interferometry (VLBI) is a form of radio interferometry with no direct links between antennas that are separated by 100s to 1000s of km.   The post-upgrade 305 m Arecibo radio telescope became a part of the VLBI network in late 1990s as part of the HALCA mission of the Japanese VSOP project.   In 2001, Arecibo acquired its hybrid tape-based VLBA-Mark IV (VLBA4) recording system.   In 2004, the Observatory procured its Mark 5A disk-based recording system, enabling the largest telescope in the world to become electronically connected to other radio telescopes in the world in real time via eVLBI observations. Observations   The first eVLBI test observations with the Arecibo radio telescope took place on September 10, The ICRF source, , was observed at 1.6 GHz using an intercontinental VLBI array consisting of Arecibo and the EVN telescopes in Cambridge (UK), Torun (Poland) and Westerbork (Netherlands). The observations resulted in the first transatlantic eVLBI image (Figure 1). Arecibo-Torun is believed to be the longest real-time interferometer baseline yet created.   The above experiment was followed by the first scientific eVLBI observations on September 22, 2004, in which Arecibo, Cambridge, Torun, Westerbork, and Onsala (Sweden) participated. The OH/IR star, IRC10420, the ULIRG IRAS (Figure 2), and the radio galaxy, 3C166 (Figure 3), were observed during the session.   In both observations, the data from all the participating stations were directly transferred to JIVE, and an on-line correlation was performed. The total data rate was 32 Mbps. Figure 3. Figure GHz continuum image of 3C166 at 32 x 19 mas (PA=-37 o ) resolution. The peak flux density is 318 mJy beam -1. The contour levels are at 4, 8, 16, 32, 64, 128 times 1  (1.8 mJy beam -1 ). The reference point (0, 0) is  (J2000.0) = 06h45m24.1s, and  (J2000.0) = 21 o 21 ’ 51.2”. Figure 1. Figure 1. (left) The 1.6 GHz continuum image of at 80 mas resolution. The reference point (0, 0) is  (J2000.0) = 05h30m56.4s, and  (J2000.0) = 13 o 31 ’ 55.15”. (right) Visibility amplitude and phase plots between Arecibo and other EVN stations. Images courtesy of EVN. Figure 2. Figure 2. An Arecibo spectrum of the OH 1667 and 1665 MHz emission from the ULIRG IRAS (z = ). The eVLBI 1.6 GHz continuum data analysis of this ULIRG resulted in an upper limit for the intrinsic brightness temperature on the order of 4.8 x 10 5 K, which is typical of starburst dominated galaxies (Condon et al. 1991). Torun JIVE WSRTOnsala Arecibo Arecibo data path to Jive:   PRISANET (PR) of the Arecibo Obs./UPR.   AMPATH (US) of Florida Intl. Univ.   Abiline (US) of Internet 2.   SURFnet (NL) of SURFnet BV. EVN data path to Jive:   GEANT, the pan-European research network.   SURFnet, the Dutch network. Heliocentric Velocity (km/s) Flux Density (Jy) 1665 MHz 1667 MHz