Space Vlbi Observations of Mkn 501

We present here two epochs of Space VLBI Observations at 18 cm of the BL-Lac type object Mkn 501. Thanks to the high resolution of these new data we have found that the inner jet is centrally brightened at its beginning but becomes extended and limb brightened at ∼ 8 mas from the core. Moreover a comparison between the two epochs shows the presence of a proper motion with apparent velocity = 6.7c. Observational data have been used to constrain the jet velocity and orientation. INTRODUCTION AND OBSERVATIONS Markarian 501 (Mkn 501) is a BL-Lac type object at z = 0.034. It is a well studied source in radio, optical and X-ray bands being one of the brightest Bl-Lac object at all wavelengths. It was the second source (after Mkn 421) to be detected at TeV energies. In the radio band it has a total radio power at 408 MHz of 8.9 ×10 W/Hz consistent with the expectation of unified scheme models that BL-Lac type sources are FR I galaxies seen on the line of sight (we use H0 = 50 km sec −1 Mpc). We observed this source in August 4, 1997 during an orbit checkout of the HALCA satellite, with the following observing array: VLBA (observing time 7); Goldstone (4); HALCA with the Green Bank tracking station (3 only). The data were correlated in Socorro, calibrated and reduced with the AIPS package (15 Apr. 1997 version). Only the 1st IF gave useful data. On April 8, 1998 the source was observed again for 10 with the following array: VLBA, Goldstone, Robledo, HALCA with the Green Bank and Madrid tracking stations. The data were correlated in Socorro and reduced with the AIPS package (15 Apr 1998 version). We found good data from all the telescopes and for both IFs. Amplitude calibration was done initially for VLBA telescopes only and after applied to the whole array. All data were globally fringe fitted and then self-calibrated. SOURCE MORPHOLOGY AND JET DYNAMICS At parsec resolution Mkn 501 shows a one-sided jet structure which changes its orientation from ∼ 140 to ∼ 30. In Fig. 1 we show a full resolution map obtained from uniform weighted data, while in Fig. 2 a map obtained with a larger beam is shown. From an analysis of the two maps we note a clear change in the jet structure at about 8 mas from the core. At the beginning the jet is transversally resolved but it is centrally brightened. After an elongated feature the jet brightness becomes limb brightened: the maximum of the surface brightness is clearly on both sides of the parsec scale jet. We interpret this observational result as an indication of a change in the jet physical MKN501 IPOL 1634.0 MHz 10 20 30 40