V4641 Sgr is an X-ray binary system containing a stellar-mass black hole and an intermediate-mass secondary.
On September 15, 1999, I visually detected a dramatic optical outburst of the X-ray binary black hole star V4641 Sgr to magnitude 8.8. I raced inside and sent an alert message around the world. The outburst was confirmed by Kyoto University Japan. Taichi Kato recalled that this object had been associated with variable X-ray emissions by scientists working with the Dutch-Italian BeppoSAX spacecraft. Kato forwarded the message to the All-Sky Monitor (ASM) team using the Rossi X-ray Timing Explorer (RXTE) satellite.
Dr Donald Smith of the Massachusetts Institute of Technology…….”Combining the data from all the instruments, we saw four of the most dramatic rapid X-ray intensity changes ever seen from one star,” Smith said. “This behaviour is new. We’ve never seen anything like it.”
CCD images from Kyoto University Japan show the field of V4641 Sgr in quiescence on September 9 (right) and in outburst on September 15 (left).
Excerpts from the National Radio Astronomy Observatory press release January 14, 2000
[ The black hole in the constellation Sagittarius, along with a normal star dubbed V4641 Sgr, formed a violent system that briefly flooded part of our Milky Way Galaxy with X-rays and ejected subatomic particles moving at nearly the speed of light one day last September. At the peak of its X-ray output, V4641 Sgr was the brightest X-ray emitter in the sky.
Black holes harbour enormous gravitational force that can literally rip the gas away from a nearby star. This transfer of gas is visible in many forms of radiation. Both orbiting X-ray telescopes and ground-based radio and optical telescopes saw the outburst of V4641 Sgr.
The ASM surveys the entire sky about once every two hours, and Smith found that the most recent observation of V4641 showed it as a bright X-ray emitter. Subsequent observations showed the rapid rise and fall of the object’s X-ray brightness. A few hours later, it flared again.
Within 24 hours, the National Science Foundation’s Very Large Array (VLA) radio telescope in New Mexico was observing V4641 Sgr. “We could immediately see that it had structure — it was big,” Hjellming said. The first VLA observation showed an object three times longer than the distance from the Sun to Pluto. “What we were seeing was the jets, and we could tell they were moving so fast that they already had expanded to a considerable size,” he said.
The VLA observations showed that the object’s jet was moving at nine-tenths the speed of light. Other radio telescopes observing the object were NRAO’s Green Bank Interferometer in West Virginia; the Australia Telescope Compact Array; the Molonglo Observatory Synthesis Telescope, also in Australia; the MERLIN array in Britain; the Ratan 600-meter radio telescope in Russia; and radio telescopes at the Owens Valley Radio Observatory in California. The radio observations also provided the distance measurement for the binary system.
The RXTE Proportional Counter Array (PCA), a very large X-ray telescope, was rapidly reoriented to observe V4641 Sgr about 4.5 hours after the brightest flare. The PCA recorded a fourth event lasting 20 minutes to reach an intensity of one-sixth that of the brightest flare.
The PCA data reveal a complex substructure, with luminosity changes by a factor of four within one second and by a factor of 500 within minutes. No further high-energy emission from V4641 Sgr has been observed with any satellite since the end of the flare seen by the PCA. “Combining the data from all three instruments, we saw four of the most dramatic rapid X-ray intensity changes ever seen from one star,” Smith said. “This behaviour is new. We’ve never seen anything like it.”]
These three radio images, made from observations at the Very Large Array in Socorro, New Mexico, show the activity of V 4641’s jets on Sept. 16 and 17, 1999. The first image shows a jet shooting outward about 125 astronomical units — three times the distance from the sun to Pluto. It moves at about 90 per cent the speed of light on a path slightly inclined towards Earth. Just 30 minutes later, the jet had faded noticeably (centre image). By the following day, the intensity had dropped 40 times (right), but it remained for about three weeks until its signal faded into the background. Credit: Robert Hjellming, National Radio Astronomy Observatory
“To this day, I am still the only person to visually see and record the outburst at magnitude 8.8. This emphasizes the scientific value of visual observations in variable star astronomy!”
This picture was produced for a press release by Jerry Orosz and Erik Kuulkers.
A visual light curve of V4641 Sgr
The light curve below details my visual observations from October 1999. Note the activity between magnitude 11.1 and 13.0 before the dramatic outburst on September 15, 1999, to magnitude 8.8. Four more outbursts have been observed but only peaking to 11th magnitude, with the last one in 2005.
There was also some activity to a magnitude of 12.7 in June 2007. Combining with the Swift data (ATel #1135), the object showed active phases recurring with time scales of 1-2 d at least between May 20 and May 25 up until June 5.
On January 11, 2014, V4641 Sgr was reported to be active in X-rays ( ATel # 5803). Follow-up optical observations were obtained on January 28, 2014 (ATel # 5836).
Updated light curve to November 2018. V4641 Sgr has been optically quiet since February 2014.
The historical light curve of the black hole binary V4641 Sgr
This historical half-century light curve of V4641 Sgr is based on the Moscow and Sonneberg plate archives.
On September 10, 2010, I observed a very brief brightening to magnitude 12.4 over 30 minutes duration, which was ended by cloud cover. Rechecking the field 2 hours later, V4641 Sgr was back to normal. V4641 Sgr was monitored by Swift/XRT and RXTE/PCA, as noted in the Astronomers Telegram below. (ATel #2892)
Swift/XRT and RXTE/PCA detects an increasing X-ray flux of V4641 Sgr
on September 30 2010; 18:34 UT
[ We have monitored the Galactic microquasar V4641 Sgr in a weak outburst (ATel #2785 and #2832) with Swift twice a week for about a month. The recent Swift/XRT data indicates that the 2-10 keV flux has been increasing from about 1-3 mCrab to about 9 mCrab. The observed fluxes were 1.0e-10, 9.1e-11, 1.5e-10, 1.6e-10, 2.1e-10, and 2.0e-10 erg cm^-2 s^-1 in the 2-10 keV range on September 9, 12-13, 18, 21, 26. and 29, respectively. This trend is consistent with the RXTE/PCA Galactic Center Monitoring data. Overall, the Swift/XRT and RXTE/PCA data suggest an increase in X-ray activity but with significant variability on timescales of a few days. ]
I observed V4641 Sgr on January 31, 2014. As seen in the light curveV4641 Sgr is active visually at magnitude 12.7, the first sign of activity since September 2010. (ATel # 5836)
Optical and near-infrared follow-up observations of V4641 Sgr
on January 30 2014; 23:39 UT
[We performed optical imaging of the black hole X-ray binary, V4641 Sgr, which is now active in X-rays (ATel #5803). Our results suggest that the object is now in the active state in the optical, as well as in X-rays. It is not in a major outburst, such as those observed in 1999, 2002, 2003, and 2004 (Uemura+02,04, and 05).]
The 2005 outburst
The light curve below shows my detailed visual observations from the 2005 outburst. During JD 2453549.962 to JD 2453549.990 (93 minutes), dramatic variations and flares up to one magnitude were recorded.
The 2002 outburst
The black hole X-ray binary V4641 Sgr experienced an outburst in May 2002, which was detected at X-ray, optical, and radio wavelengths. The outburst lasted only 6 days, but the object remained active for several months.
On July 7, 2002, JD 2452463.165–2452463.167, V4641 Sgr exhibited a large-amplitude optical flash. During this flash, the object brightened to the maximum of 12.4mag within 30 s, then returned to the pre-flash level within 90 s. CCD images were taken from Kyoto University, Japan before and during this flash.
CCD images of V4641 Sgr just before (left) and during (right) the giant optical flash. The images were observed at JD 2452463.16542 (left) and 2452463.16577. The time separation from the left to right images is 30 s.
Articles on V4641 Sgr
Variable Star of the Season, V4641 Sgr, an X-ray Nova and stellar Black Hole candidate. The American Association of Variable Star Observers (AAVSO). Link
RXTE Discoveries site, Meet Our Neighbour, a most Curious Black Hole – February 2000 Link
Co-authored papers on V4641 Sgr
Pre-outburst Activity of V4641 Sgr = SAX J1819.3-2525: Possible Existence of 2.5-Day Period 1999IBVS.4777….1K
The 1999 Optical Outburst of the Fast X-Ray Nova, V4641 Sagittarii 2002PASJ…54..95U
Rapid Optical Fluctuations in the Black Hole Binary V4641 Sagittarii 2002PASJ…54L..79U
Outburst and Post-Outburst Active Phase of the Black Hole X-Ray Binary V4641 Sagittarii in 2002 2004PASJ..56S..89K
Optical Observation of the 2003 Outburst of a Black Hole X-Ray Binary, V4641 Sagittarii 2004PASJ…56..823U
Outburst of a Black Hole X-ray Binary V4641 Sgr in 2004 July 2005IBVS.5626….1U