Since it's a Friday night and this source may be a little challenging, I thought I'd not make a campaign alert quite yet. I'd rather not interrupt Elizabeth or the Sequence team's night.
Aquila X-1 (V1333 Aql) is the prototypical recurrent neutron star low-mass X-ray binary. It undergoes outbursts about every 9 months to a year. When this occurs, the accretion disk that surrounds the neutron star drains quickly onto the neutron star. Although there is another star 0.5" away and a donor (a K4+/-2) star that feeds the accretion disk, when outbursts occur, the accretion disk (and perhaps to some extent a relativistic jet) gets so bright that it dominates the optical and UV emission. This is what makes this a very variable star.
The star was found this week to be beginning a new outburst (ATel #13953; ATel #13961). But, it's not too bright. Some light curves of past observations by Los Cumbres Observatory show its behaviour. Right now the source is probably V~18 / i'~17 and will likely brighten significantly, maybe by ~3 mag.
Los Cumbes Observatory observations in V and i' will continue, although I am not sure how often they will occur. We also are getting V, B, U, (and some bluer filters) with Swift Ultraviolet/Optical Telescope) no denser than one observation per day. Beware, the A_V extinction is ~ 2, so while bluer filters get closer to the action, our view of them is getting increasingly bloacked.
So where can you come in? There's been some interesting work connecting the UV/Optical to the X-ray emission, the latter of which traces the inner accretion disk. Since my colleagues in the Netherlands and United Arab Emirates on this are likely asleep, I have not run the idea of an AAVSO campaign with them. Thus, I can't guarantee that we will be able to use AAVSO data in a paper. However, my gut suggests it would be useful. Fortunately the star peaks currently at an airmass of ~ 1.2 during the night (closer to the night before than the morning. And it's an equatorial star (19:11:16.05 +00:35:05.8) so both hemispheres can join the fun. I can see two types of interesting observations, depending on your facility's capacities and your own interests.
- Measure the star's photometry in multiple filters, preferably at least once per night, 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.
- Measure the star's photometry in one filter multiple times per night (as rapidly as your facility can do so given the star's brightness at the time). This need not be done by lots of people, but having a few of these in different filters allows us to quantify the effect of smaller scaler variability for when we make non-simultaneous colours.
I will be also alerting Elizabeth, so I hope we'll see an official campaign.
Gregory Sivakoff, PhD
Associate Professor (Observational Astrophysics), University of Alberta
AAVSO Board Member
The 151-163 comps have now been updated for V1333 Aql + the sequence has now been extended to ~ 18.9V.
Good Observing & Ad Astra
Tim Crawford, Sequence Team
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 under the first post in this thread) - 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
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.
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.
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.
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
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!
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!
We have caught the source beginning a decay, likely into quiescence.
- AAVSO light curve: https://drive.google.com/file/d/1zmJ-s9sR7sToxeAns4EMdQNVOBUy2xjX
- X-ray light curve: https://drive.google.com/file/d/1z_PHI8TJhZ4-4Eg-hbS-jZ5BKQwENWxI
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!
I have changed my observing schedule and will take only images of Aql X-1 in I filter as long as its airmass is smaller than about 2. I will keep this for a couple of nights and see how things evolve.
I have added from last nights observations two images to reduce uncertainty. I have also seen that I have overlap with Geoffrey Stone's data which fluctiate heavily. Maybe one need caution as there is a bright star close by to V1333 Aql. I tried to avoid it by selecting a smaller aperture for the variable and a larger for the donut between variable aperture and background aperture. Maybe Geoffrey needs to look at his images about this star as I do not see the fluctuations he has observed.
I have had issues with the filter wheel and we found out tha the I and R filter were swapped. I have reanalysed all images for V1333 Aql and re-submitted them with the correct filter (R in this case). I have kept to the R filter after I have found this out (only one night I have used an I filter).
Observations from last night at detection limit. I had to add two images and increse the aperture to 4 pixel. Will stop further observations.
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 (firstname.lastname@example.org) 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!