Outburst of the Galactic microquasar V4641 Sgr!

CCD field view

[vsnet-campaign-v4641sgr 1]

V4641 Sgr outburst!!!

Rod Stubbings reports that the microquasar V4641 Sgr = SAX J1819.3-2525 (formerly called GM Sgr) is in outburst as indicated by the following observations.

V4641 Sgr    020517.569    13.4
V4641 Sgr    020519.699    11.5
             020519.704    11.5
             020519.716    11.5
             020519.723    11.5
The object underwent a giant outburst in 1999 September (Stubbings and Pearce (1999) IAUC 7253), preceded by optical brightening (Kato et al. (1999) IBVS 4777) which was first detected by T. Watanabe.

[Excerpt from Uemura et al. (2002) PASJ 54, 95]

Microquasars are a class of X-ray novae with relativistic radio jets. Their X-ray and optical emissions are thought to be from an accretion disk around a neutron star or a black hole, however their relationship with the jet production mechanism is poorly understood. Here we report on the unprecedented discovery of a giant optical outburst in the X-ray nova V4641 Sagittarii (= SAX J1819.3-2525 = XTE J1819-254), which enabled subsequent X-ray observations accurately pinpointing an intense X-ray flare reaching 12 Crab and establishing it as a microquasar at a small distance. The decay with an e-folding time of 0.2 day is the shortest ever observed for any X-ray novae, implying a new category. Observations revealed the first ever exemplification of the detailed optical behavior prior to the X-ray maximum, and moreover, the totally unexpected 7 hour precedence of the optical maximum to the X-ray maximum. The seemingly anti-correlated light curves in X-ray and optical can be best understood as a short episode of supercritical accretion producing a relativistic jet.

X-ray novae are binary systems which exhibit luminous X-ray and optical outburst which lasts for a few tens of days. They uniquely provide the most compelling evidence for the existence of steller mass black holes using radial velocity studies, giving mass functions exceeding the maximum mass of a stable neutron star (~3 Msolar). Microquasars are X-ray novae with superluminal jets whose mechanism is poorly unknown while a number of observational results and models have been discussed.

Supercritical accretion disks have recently been discussed for black hole candidates shining at the Eddington luminosity which frequently show jets. The accretion disks theoretically become geometrically and optically thick when the mass accrete over a critical rate, on the other hand, the geometrically thin disks are applied for the subcritical accretion rate. We can consider that supercritical accretion occurs in persistent jet sources, for example, active galactic nuclei or SS433 whose mass accretion rate is observationally suggested to be near critical. On the other hand, during any transient jet source outbursts, we had detected no implication that supercritical accretion occurs. However, a new atypical microquasar, V4641 Sgr showed an evidence of this for the first time.

20th February 1999, the Dutch-Italian X-ray astronomy satellite (BeppoSAX) detected a faint flaring X-ray transient. There was a known variable star V4641 Sgr [Goranskij (1978, 1990) discovered an eruptive variable star in the close vicinity of the nominal position of Luyten's variable (HV 4048). Goranskij's object took over the GCVS nomenclature, GM Sgr, of HV 4048. However, it was established that they are two independent variables after the giant outburst on 1999 September, and then, the object which is the optical counterpart of SAX J1819.3-2525 is named as V4641 Sgr. When the small flare on February 1999 was reported, the object was thereby called GM Sgr, however we only use V4641 Sgr for simplicity in this paper], whose visual magnitude in quiescence is 13.8, within the error box at the position of this transient. Ironically the object had been overlooked until 8th August 1999 when V4641 Sgr brightened to 12.9 mag.


Following the first small outburst on 8th August (JD 2451399), the brightness of V4641 Sgr modulated between 12 and 14 mag for about one month, and then, on 9th September (JD 2451430), rose to 11.7 mag. We detected the quasi-periodic modulation of an amplitude of ~1 mag with a period of 2.5 days for six days from this second outburst, and then, V4641 Sgr suddenly flared to 8.8 mag on 15th September (JD 2451436). The optical flux then began an exponential decay with the e-folding time of 0.2 day and returned to the pre-outburst level within 12 hours after the maximum. This demonstrates that this high energy event lasted only 1 day. The magnitude returned to the quiescent level within one week, and then, no other activity has been detected.


The object was further studied in detail, and it is now established to be a close binary containing a black hole (Orosz et al. (2001) ApJ 555, 489). Radio jets (with superluminal motion ~9 c?) was detected (Hjellming et al. (2000) ApJ 544, 977), indicating that highly relativistic ejection was taking place at the time of optical/Xray flare-up.

If the current outburst follow the course of the 1999 outburst, the source may brighten up to a binocular object in near future (tonight? in several day? or more? .. no one knows!). This outburst provides excellent opprotunity to follow full evolution of the expected flare-up in all wavelengths. In optical, time-resolved photometry and spectrocopy are highly recommended. Prompt ToO X-ray observations, as well as radio interferometry, are also very wanted. In order to record this rare event, we have set up a new list


   Those who wish to subscribe to this list should send an e-mail with
a line

   SUBSCRIBE vsnet-campaign-v4641sgr

   to vsnet-adm@kusastro.kyoto-u.ac.jp

   For the details of the 1999 event, see also


   Good luck!

Taichi Kato
VSNET Collaboration team

Naked-Eye Visibility of the Twinkling Black Hole

The outburst of the Galactic microqusar V4641 Sgr follows a very complex course. The object violently varies with various time-scales. At the time of this writing, the object undergoes its third outburst during the present activity phase. This course of the outburst strongly resembles that observed in the 1999 September event (http://www.kusastro.kyoto-u.ac.jp/vsnet/Xray/gmsgr.html), which suggests that the present activity may be a precursor to a giant event in near future. Please keep the cloest watch to see what is ongoing!

Further to this, the object shows "naked-eye visibility of flickering of a black hole". A comment from Mike Linnolt (Hawaii) follows:

[vsnet-campaign-v4641sgr 8]

I watched this object over a half hour period and saw it varied chaotically from mag 12.5 to 12.8 with a "period" of just a few seconds! I have never seen such a behavior in a variable star before.

[vsnet-alert 7340]

I observed this amazing object again this evening, and noticed it has brightened significantly over past 24 hours. In addition, the chaotic ultra-short variations in brightness seems to have increased its range to >0.5 mag compared to ~0.4 yesterday. The periodicity remains on the order of a second to a few seconds, but the mean brightness tends toward mag 12.0, on the higher end of the range 12.5-11.8: which I observed in 15mins of continuous visual monitoring between 20020522.4097 and 20020522.4201

The following observations are just representative points, the actual variations occur at an unprecedented rate for even 4 decimal CV reporting.

SGRV4641 20020522.4132 11.8: LMK SGRV4641 20020522.4133 12.3 LMK

Note that visual observation or video recording is the only way to monitor the full range of the brightness variations. CCD would be limited by the integration times and thus yield only a mean value, unless one has sufficient aperture to do sub-second integrations.


The light variation was also recorded with CCD photometry by Berto Monard and other contributors:

[vsnet-campaign-v4641sgr 9]

We have received time-series CCD photometry on May 20-21 from Berto Monard. The light curve is very complex and very short-term fluctuations up to 0.4 mag is observed (the light curve even resembles a X-ray light curve of some black hole binaries!).

(Berto Monard's observation)

[vsnet-campaign-v4641sgr 13]

V4641 Sgr is not in a monotonic fading, but still very active! As already reported by B. Monard in [vsnet-alert 7339], by M. Linnolt in [vsnet-alert 7340], and by A. Pearce in [vsnet-obs 39944], the object has been showing short-term modulations.

Furthermore, the combined data set shows that the object is now oscillating between 11.5 - 13 mag with a time-scale of 1 day. After the first maximum observed by R. Stubbings on May 19, the object once faded. Then, B. Monard's observations detected a rising phase to the second maximum on May 21.

Our CCD observations at Kyoto last night shows a fading from the second maximum. It was 12.68mag (average) at that time. The short-term modulations seem to be less prominent during our observations. A. Pearce's observations on May 22 shows that the object again brightened, indicating the third maximum. He reports that short-term modulations again appear.

Durations of each oscillation is short, no longer than 1 day. Multi-longitude observations are essential to study the correlation between short-term modulations and larger-scale oscillations.


The activity has been detected in X-rays:

[vsnet-campaign-v4641sgr 11]

According to IAUC 7906 (Markwardt and Swank), V4641 Sgr is also reported to be active in X-ray. X-ray flux from RXTE PCA monitoring (mCrab): 4.5 +/- 0.3 (May 17.6), 10.0 +/- 0.5 (May 20.9).


We are conducting an extensive campaign on this source. Those who are able to obtain high-speed (i.e. seconds to several tens of seconds) CCD photometry are most welcomed to join the present campaign to fully record the current exotic activity of this black hole binary. Other modalities of observations are also highly encouraged.


(Eclipse? observation by Berto Monard)

[vsnet-campaign-v4641sgr 20]

Dear Colleagues,

We have received two new data sets from Berto Monard (see vsnet-alert 7342). On May 21/22, the system underwent a deep fading between May 22.0 and 22.1 UT. This timing agrees with the expected inferior conjunction (ephemeris from Orosz et al. 2001; updated by Uemura et al. 2002). During this fading, strong flickering activity virtually disappeared! Such behavior could be reasonably explained if the (compact) flickering body was eclipsed by the secondary (first case in microquasars, if confirmed). There was an indication that flickers rapidly emerged when the system brightened from this fading. During the declining branch, flickering was less apparent than on the brightening branch.

On May 22/23, the strong ultra-short-period flickering were always present in high-speed photometry (5-s integratrations), as on May 20-21.

Expected times of inferior conjunctions (likely eclipses), in geocentric UT. Times are approximate (to 1 hr or so) owing to the unavoidable uncertainty in the epoch and period.

  20020519 06:39:14
  20020522 02:15:53
  20020524 21:52:33
  20020527 17:29:15
  20020530 13:05:57
If this interpretation is correct, the object is expected to remain active until late May 24. Continuous coverage is most urgently needed. More detailed analysis, together with newly available data, will be later presented by Uemura-san. The Kyoto team is planning to make high-speed (1s) photometry at Ouda tonight, depending on the conditions. You will hear more from Uemura-san about the progress of the observation and data analysis.
Taichi Kato
VSNET Collaboration team

Violent variation again on May 23

(Observation by Berto Monard)

(Observation by Tom Richards, later on May 23)

[vsnet-campaign-v4641sgr 21]

From behind the eyepiece V4641 Sgr seems to flicker at a very fast rate, as was mentioned by other observers. The attached light function shows that there is obvious activity at the minute level too.

CCD observations for the night 21-22 May show V4641 Sgr on a steady return to a calm state. Just at the end (unfortunately dawn came so early..) there was a strong sign of resuming activity. This lightcurve has been sent to Kato-san.

Last night (22-23 May) we had heavy cloud and a downpour with thunder activities (unusual for the time of the year). However the sky did open after 3 o'clock and fast photometry (under humid conditions) showed quite some detail in the activity at the 10 minute level. This was a very rewarding run (and test trial on fast photometry) at the Bronberg Observatory.

As mentioned above, I hereby attach the sub 3h run of this morning, 23 May. A plot of this lightcurve is a pleasure to the eye!


Berto Monard
Bronberg Observatory

Mike Linnolt's report and comment

[ vsnet-campaign-v4641sgr 26]

The rapid, chaotic flickering with a period of a second or so continues. I observed the object from 20020523.4555 to 20020523.4611 and saw it vary between 13.0 and 12.1, same comps as usual - 131, 128, 126, 123 from aavso d chart. At 20020523.4569 I believe I saw a momentary flash to mag 10.7:

[vsnet-campaign-v4641sgr 28]

Beware of drawing conclusions from "short term" variations in this star using CCD. The fluctuations are so rapid that even fast photometry with 1sec exposures and 2.5sec integrations wont catch them faithfully. Visual observations show the true behavior here. To compare favorably, CCD would need somewhere around 0.1-0.2sec integration times, and continuous monitoring without gaps. Essentially you need video recording equipment.

I would recommend someone just hook up a DV camera or camcorder to a good size observatory class instrument and record several minutes of the stars behavior for a true reference of its variability.

Mike Linnolt (LMK)
Honolulu, HI

Short-term variation recorded at Ouda Station

Variation on May 24

Active again!

[ vsnet-campaign-v4641sgr 78]

We are now having a run on V4641 Sgr at Ouda and found that the object is brighter at Rc=12.6mag. It is not clear whether this brightening is a temporary 1-mag hump or sign of a new giant outburst, but it is certain that the object entered a new active phase! This brightening trend has been also reported by R. Stubbings in [vsnet-obs 40119].

Makoto Uemura

Rapid optical flare in July

(Rapid optical flare (1 mag) observed by Tanabe-san, Okayama, U. Sci.)

Radio observation (communicated from Michael Rupen)

V4641 Sgr: continued strong & variable radio emission

Dear colleagues,

I have reduced part of our 23may02 VLA run on V4641 Sgr, spanning 4.5 hours with a time resolution of 3.3s, time-sharing between 1.425, 4.86, 8.46, 15, 22, and 43 GHz. At 8.46 GHz the flux smoothly declined from ~170 to 110 mJy in the first 90 minutes, but rose equally smoothly from about 80 to about 140 mJy in the last 11 minutes. There is no evidence for extremely rapid fluctuations, or for any dramatic extension to the source (0.72x0.46 asec beam, rms a few tenths of a mJy [varies with time]). A preliminary version of this light curve is available at


The only other frequency I've reduced so far is 43 GHz. The flux density here ranges from ~15 to ~130 mJy, with ~30% [smooth] variations on timescales of a couple minutes. Again there are no obvious extensions (beam 0.14 x 0.09 arcsec), but I've not tried very hard to bring them up yet. The spectral index between 8.5 and 43 GHz can only be measured at a couple times (due to that frequency time sharing I mentioned); on the first occasion it's around alpha~ -0.4 (S_nu going as nu^alpha), while on the second it's basically flat (alpha~ 0). This is consistent with multiple flares from a small source, quite likely a relativistic jet as we saw in Sep99.

In sum, it's actively flaring on timescales of minutes to hours, though I don't see any evidence (with admittedly poor SNR at the high frequencies!) for anything like the few-second oscillations reported in the optical. Even so, good temporal coverage at high time resolution is very helpful.

Note that this is all very different from either of the two previous radio flares. In Sep99 we had a very rapid radio burst with evidence for extremely rapid (0.4 arcsec/day) expansion, followed by a couple weeks of slow, smooth decay; in Jul00 we had an optically-thick and quite stable radio source, until it rather suddenly disappeared. This time around it's much less stable.

Unfortunately it does not look like we'll be getting VLBA time for at least a few days, as we too have been hit by the Chandra Sgr A* campaign. Similarly there is not a great deal of VLA time available just now. The next definite observations will be on the 31st, from 0935 to 1135 UT. Simultaneous optical/X-ray observations would be lovely :)

Good hunting --

                Michael Rupen, Amy Mioduszewsk, & Vivek Dhawan

ATCA observations of V4641 Sgr

The Australia Telescope Compact Array made radio observations of V4641 Sgr on 24 and (briefly) 25 May, at 20, 13 6 and 3cm. On 24th, the source had an inverted spectrum, showed substantial variation, and approached 1 Jy at 3cm. On 25th, the source was much weaker, being about 70mJy.

The light curves and radio data, and more details, are available from


Bob Sault

RXTE observations


I have composed a web page with Jean Swank about the most recent RXTE PCA X-ray observations of V4641 Sgr. The source is indeed exhibiting large amplitude X-ray variations which are comparable in timescale to the optical modulations. This is much stronger variation than the source exhibited in June of 1999, whose light curve is also shown.

Please find it at the following address:



Craig Markwardt ( & Jean Swank )

Letter by Uemura et al.

Get a preprint!

Light curve (requires Java 1.1)

Light curve

VSNET data search


VSOLJ news (in Japanese)

Link to VSOLJ News on V4641 Sgr (2002 event)

Link to VSOLJ News on V4641 Sgr (1999 event)

Comparison with "load average" curves of a computer...

Can you distinguish this from the light curve of V4641 Sgr? :-)

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