V1333 Aql outburst - AAVSO Alert Notice 718

American Association of Variable Star Observers (AAVSO)
Wed, 08/26/2020 - 16:00

AAVSO Alert Notice 718 announces an observing campaign on the outbursting X-ray binary V1333 Aql (Aquila X-1). Please see the notice for details and observing instructions.

To stay informed about this campaign, subscribe to this forum thread (option 1 under Subscribe below) -  see feedback from the PI,  comments and questions from observers, and notes from AAVSO HQ. Add a post yourself  -  join in the discussion about this interesting target!

Many thanks, and Good observing,

Elizabeth O. Waagen, AAVSO HQ

American Association of Variable Star Observers (AAVSO)
Observation Update

Hi All,

My apologies for the delay in getting back to you all.

First, thanks to DFS, SDM, NROA, and WKL for already getting data into the AAVSO on this source.

Optical Update

As of August 28, you all have shown the source to be at about V~16.5-16.7, R~15.8-16.5 (I'm not sure if the big drop was real, but given there is also a drop in V at the same time, it may well be) and B rising from 18.7 to 17.7.

I have not heard word back from the LCO team yet to see what they are seeing.

Ultraviolet Update

I apologize, but I have not heard word back from my UVOT person as to what they are seeing. But by eye, we are detecting the source in multiple filters.

X-ray Update

We've been monitoring the source with the Swift X-ray Telescope (XRT) and its Burst Alert Telescope. The former only gives us data when scheduled observations occur, while the latter monitors large swaths of the sky. The source is still in the early stage of an outburst, with X-ray luminosities around 1-4 e36 erg/s. It can get a hundred times brighter! Evolution has been slower than expected, and of course we had some bad luck. Scheduled Swift observations do not always occur as higher priority observations like Gamma Ray Bursts can shuffle what actually gets observed. Due to that, we did not get XRT observations on the 26 and 27. And when we did get XRT observations on the 28, the telescope did not slew onto target for the instrument mode we were using (another arcminute and we would have been great). But, for the first time today, the daily average of the BAT data is beginning to pick up the source!

Aql X-1 XRT Swift count rate /L_X (1-10) from the MCL / BAT count rate on that day / mCrab (from XRT)
08-20 16:21 12.4 1.4e36 0.0019+/-0.0017 13
08-21 19:31 27.7 2.3e36 0.0027+/-0.0016 29
08-22 22:43 43.0 4.9e36 0.0028+/-0.0020 45
08-23 22:35 28.8 3.3e36 0.0017+/-0.0022 30
08-24 19:10 27.5 3.2e36 0.0022+/-0.0022 29
08-25 03:19 33.9 3.9e36 0.0027+/-0.0019 35
08-26 XX:XX XX.X X.Xe36 0.0035+/-0.0028 XX
08-27 XX:XX XX.X X.Xe36 0.0034+/-0.0033 XX
08-28 12:13 XX.X X.Xe36 0.0082+/-0.0031 XX


Take the following with a grain of salt (because I may be predisposed to the answer). But given AAVSO measurements, the public LCO light curve, the recent Swift/BAT data and the historic Swift/BAT data, I expect that the source will get much brighter in optical.


Since there are two forums, I suggest all messages get posted in both


Clear Skies!



American Association of Variable Star Observers (AAVSO)
Bright source expected!

Hi All,

First off, keep up the great work (AAVSO lightcurve). I'm particularly thrilled with looking at Geoffrey Stone (SGEA)'s R band data as it shows a lot of variability --- 2 mag!! I'd have to check the literature to see if we've seen such extreme flickering on rapid time-scales for this source before.

And for all of you interested folks out there, the source is likely to be reaching peak brightness in the next week.

While we got some unlucky happenstances over the past week with the Swift X-ray observatory, the source was notably brighter the last few days (I'll try to keep the X-ray light curves updated). The source is now within a factor of about 2 of its peak in X-rays during the last few outbursts I have studied, although it never got bright at the hardest X-rays measured by the Swift BAT instrument.

So I think that the next week will be quite critical. After that, I'm likely to suggest skipping the rapid variability stuff on a daily basis. This will be picked up again when the source decline begins.

Thanks for all of your hard work!

Clear Skies,


American Association of Variable Star Observers (AAVSO)
The Plateau Stage & You

Hi All,


Thanks for your continued hard work. The outburst of Aql X-1 is in a plateau stage, from which we expect it to slowly decline. X-ray and matched extent optical lightcurves are presented offsite. Your data show the source is at approximately V~17-16, R ~16.5-15.5, and I~16-15.

We have begun to request observations from the Swift X-ray Observatory (getting both X-ray and Ultraviolet data) for observtions every 3-4 days. The next observations are nominally scheduled for some time on September 22 and September 26. These are the days when rapid cadence observations in one filter and observations across your filters are most useful.

But, because we need to know when this source goes into its rapid decline. I ask that if you can keep a daily eye on this source in one constant filter, that would be extra helpful. Given that many of you have seen 1 (to even 2) mag variability over a few hours, at least 4 snapshots over the night when the source is visible would be super useful.

Finally, if you have any questions about the campagin, both logistical and scientific, please feel free to ask!

Clear Skies,

American Association of Variable Star Observers (AAVSO)
Aql X-1 begin its decay

Dear All,

We have caught the source beginning a decay, likely into quiescence.

The soure underwent a superburst, where there is unstable thermonuclear burning of a carbon-rich layer on the neutron star surface, on October 7. To my knowledge, that should not be associated with the decay (as the decay should come more from the accretion disk than the the surface of the NS), but the timing is interesting.

If the decay continues, I estimate the source may reach quiescence (V~19.3 / I~18.3 mag Vega), where the nearby star dominates the emission from the accretion disk, in two weeks time. Given that the science for this campaign is focused on the UV-to-X-ray and Optical-to-X-ray flux ratios, the decay is a great time to (re-)concentrate our efforts! There are two main programmatic choices from which I ask you to make.

  • Measure the star's photometry in multiple filters, preferably cycling at least twice per night (e.g., B V R I B V R I ...., akin to what Geoffrey - SGEA - and a few others have been doing) , until the source goes back into quiescence or you get several nights of non-detections. This is probably the most useful as it helps us track how the colours of the source change over the outburst. I ask for at least two cycles because we have seen that the source shows ~0.6 mag ish variability.
  • Josch (HMB), Can I ask that you measure the star's photometry in I filter as rapidly as your facility can do so given the star's brightness at the time, for as long as you like. This work has allowed us to quantify the effect of smaller scaler variability for when we make non-simultaneous colours.

Thanks for your hard work and I wish you all clear skies!


American Association of Variable Star Observers (AAVSO)
Outburst appears to be finished

As of last week, my colleagues that use the Las Cumbres Global Observatory (and first noted the source was going into outburst at the state of this campaign) let me know that "the latest points are from [Oct 27]  and they are within ~0.5 magnitude of the quiescent level in V-band, and already consistent with the quiescent level in i'-band." Combined with Josch's reaching the detection limit of his scope, I think that this is likely a good time to end the observational part of the campaign.

I've been exceptionally swamped this fall. Between preparation for the AAVSO Annual meeting and a much needed vacation, I will be unable to start talking about analysis until December.

Elizabeth, is it kosher for me to ask people to send me (sivakoff@ualberta.ca) their email addresses so that I may incorporate them into the discussions with the "professional" astronomers?

Thank you all for your great efforts here. This data set will be quite rich!