Light Curves and Radio Structure of the 1999 September Transient Event in V4641 Sagittarii (=XTE J1819-254=SAX J1819.3-2525)

R.M. Hjellming, M.P. Rupen, R.W. Hunstead, D. Campbell-Wilson, A.J. Mioduszewski, B.M. Gaensler, D.A. Smith, R.J. Sault, R.P. Fender, R.E. Spencer, C.J. De La Force, A.M.S. Richards, S.T. Garrington, S.A. Trushkin, F.D. Ghigo, E.B. Waltman, M. McCollough

Research output: Contribution to journalArticlepeer-review


We report on radio observations of the 1999 September event of the X-ray transient V4641 Sgr (=XTE J1819-254=SAX J1819.3-2525). This event was extremely rapid in its rise and decay across radio, optical, and X-ray wavelengths; the X-rays rose to 12 crab within 8 hr and faded to below 0.1 crab in less than 2 hr. Radio observations were made with seven telescopes during the first day following the onset of the strong X-ray event, revealing a strong radio source that was detected for 3 further weeks by the more sensitive telescopes. The radio source was resolved even in the first Very Large Array (VLA) images (September 16.027 UT), being ~0farcs25 long with an axis ratio of at least 10 : 1. The total flux density decayed by a factor of ~4 over the first day, and by September 17.94 UT the radio emission was confined to a slowly decaying, marginally resolved remnant located at one side of the early elongated emission. The H I absorption spectrum gives a minimum kinematic distance of about 400 pc; various other arguments suggest that the true distance is not much greater than this.

The inferred proper motions for the early extended emission (0farcs4-1farcs1 day-1) correspond to v/c ~ 1.0-3.2 (d/0.5 kpc), and this together with the radio morphology argues that this is a relativistic jet source like GRS 1915+105 and GRO J1655-40. The proper motion of the late-time remnant is at least 100 times smaller. One simple interpretation posits the ejection of a single short-lived jet segment, followed by a more slowly decaying, optically thin jet segment ejection. These two components can explain both the multifrequency radio light curves and the radio images. The most likely parameters for the fast-jet system with net-averaged proper motion of ~0farcs4 day-1, assuming d = 0.5 kpc, are v ~ 0.85c and i ~ 63°, where i is the inclination to the line of sight. The corresponding apparent velocities are 1.4c and 0.6c for the approaching and receding jets, making V4641 Sgr the closest superluminal jet source known.
Original languageEnglish
Pages (from-to)977-992
Number of pages16
Issue number2
Publication statusPublished - 1 Dec 2000


  • Black Hole Physics
  • Gamma Rays: Bursts
  • Radio Continuum: Stars
  • X-Rays: Bursts
  • X-Rays: Stars


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