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

Abstract

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 Ðrst 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 Ðrst Very Large Array (VLA) images (September 16.027 UT), being long with an axis ratio of at least 10 : 1. The total Ñux D0A .25 density decayed by a factor of D4 over the Ðrst day, and by September 17.94 UT the radio emission was conÐned 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 day~1) correspond to v/c D 1.0È (0A .4È1A .1 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 day~1, assuming d \ 0.5 kpc, are v D 0.85c and D0A .4 i D 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.

GRB 070610: A Curious Galactic Transient

“…The radio flux of V4641 Sgr declined very steeply initially; from 360 mJy at 8.3 GHz to 30 mJy at 8.46 GHz in one day (Hjellming et al 2000). According to Orosz et al 2001, the distance to V4641 Sgr is 7.4Y 12.3 kpc and the apparent expansion velocity is >9.5c (assuming the lowest proper motion estimate from Hjellming et al 2000)making V4641 Sgr the most relativistic of Galactic sources. This suggests that perhaps the key difference between fast X-ray novae and the regular X-ray novae is the speed at which the ejecta is emitted.…”

“…Regular X-ray novae are essentially black hole binaries undergoing the equivalent of dwarf novae i.e., instabilities in the disk. During the major burst of 1999, V4641 Sgr exhibited radio emission and relativistic motion (Hjellming et al 2000). The radio flux of V4641 Sgr declined very steeply initially; from 360 mJy at 8.3 GHz to 30 mJy at 8.46 GHz in one day (Hjellming et al 2000).…”

“…During the major burst of 1999, V4641 Sgr exhibited radio emission and relativistic motion (Hjellming et al 2000). The radio flux of V4641 Sgr declined very steeply initially; from 360 mJy at 8.3 GHz to 30 mJy at 8.46 GHz in one day (Hjellming et al 2000). According to Orosz et al 2001, the distance to V4641 Sgr is 7.4Y 12.3 kpc and the apparent expansion velocity is >9.5c (assuming the lowest proper motion estimate from Hjellming et al 2000)making V4641 Sgr the most relativistic of Galactic sources.…”

Flaring variability of Microquasars

“…We had to conclude that the thermal electron density, responsible for the low frequency absorption, grows up during grow-up of the relativistic electron density. The later stage of the spectral and temporal evolution could be fitted by the modified finite segments model by Marti et al [17] or Hjellming [18] and Hjellming et al [19]. Indeed in Fig.3 the radio spectra of the July flare evolved from the fourth day (MJD53942) as usual adiabatically expanding relativistic jets moving with v jet ∼ 0.74c, and thermal electron component has: T e = 10 4 K, n th = 2 10 4 cm −3 , magnetic field B 0 = 0.07 Gs, and energy index γ = −1.85.…”

A Relativistic F[CLC]e[/CLC] Kα Emission Line in the Intermediate-Luminosity [ITAL]B[CLC]eppo[/CLC]SAX[/ITAL] Spectrum of the Galactic Microquasar V4641 Sgr

“…In particular, advectiondominated accretion flow (ADAF) models for transient systems (e.g., Esin, McClintock, & Narayan 1997) Orosz et al 2001). Hjellming et al (2000) mond ≤ 12.31 itored the outburst in radio and found jets expanding at apparently superluminal velocities; V4641 Sgr is therefore termed a "microquasar." Significant low-luminosity activity was observed by BeppoSAX and Rossi X-Ray Timing Explorer (RXTE) Galactic bulge scans (Swank & Markwardt 2001) earlier in 1999, however, well before the better known outburst.…”