HST campaign on TCP J21040470+4631129 - Alert Notice 714

American Association of Variable Star Observers (AAVSO)
Thu, 08/06/2020 - 18:33

AAVSO Alert Notice 714 announces an observing campaign on the WZ Sge-type variable TCP J21040470+4631129. Please see the notice for details and observing instructions.

Many thanks, and Good observing,

Elizabeth O. Waagen, AAVSO HQ

2nd attempt

Here just a bit of background...

We were planning to obtain ultraviolet spectroscopy of this  unique CV (you may think we say that about each and every CV, but this one is truly fascinating!) on April 10th, which was thwarted by the superoutburst that was detected on March 31st. Observing the system in outburst would have, in the best case, shut HST down into a safe mode, which takes a few days to recover. In the worst case - it would fry the detector of the COS instrument. That's why the ground-based monitoring is so important.

Because real-time communication with Hubble is limited, the telescpoe runs automously, executing a week's worth of commands that get up-linked in advance. So how do we deal with CVs going into outburst? The trick is to have two sets of co-ordinates in the observation: one is the CV, the other one a blank spot a few arc-sec away. The default position is the blank field.

On the day of the observations, we communicate the results of the latest observations to STScI - if they are satisfied that it is safe to observe the CV, they up-link a single command to HST which swaps the two pairs of co-ordinates, so the CV is observed. If that command is not sent ... HST stares at a blank piece of sky - which gets you a very expensive spectrum of the sky background at ~340 miles above sea level!

In April, thanks to the alerto to the superoutburst, we immediately got in touch with STScI. They were just about to up-link the program for the next week, but, with one hour left to go, managed to pull the observations of TCP J21040470+4631129 from the schedule! So we avoided wasting precious Hubble time, and got a 2nd chance.

Fingers crossed the star co-operates this time!

Clear skies,






American Association of Variable Star Observers (AAVSO)
"Fainter than" guidance?

Recent entries in the AAVSO database include several "fainter than" observations for this star, with thresholds ranging from "fainter than 15" up to "fainter than 12.5."  A friend of mine raised the question of just how bright a "fainter than" observation could be and still be adequate to protect the Hubble?  For instance, I'm confident that a "fainter than 8.0" observation would not be helpful, but I don't really know the answer to his question.  Any guidance on that would be appreciated, and would also be helpful for any future alerts involving the Hubble.

To allow the HST observations

To allow the HST observations to go ahead, STScI requires  an actual detection of the target at the quiescent level within 24h prior to scheduled time of the HST observations. However, fainter than reports that are close to the quiescent level (a few tenth of a magnitude, i.e. "V<15" for TCP J21040470+4631129), are still very useful in case that there are gaps in the detections, or some discrepant magnitudes among the detections in the last few days before the HST observations.


Collected data on TCP J21040470+4631129

Stella Kafka was our presenter on The Astro Imaging Channel last night. At the end of the session I told her that my imaging setup at the Sierra Remote Observatories was sometimes (including Sunday night) available to collect data for an AAVSO project. Your project was suggested.

Last night I collected six hours of subs for that target using the suggested G filter. Each sub was 300 sec long. The data is calibrated (not registered), in FITS format and waiting for instructions as to what to do next. Please advise.


American Association of Variable Star Observers (AAVSO)
Hello Eric,

Hello Eric,

Thank you for observing TCP J21040470+4631129 for this campaign! Your images need to be reduced to magnitudes for the target and the magnitudes and their dates/times (and other information) submitted to the AAVSO International Database. I will email you privately to connect you with the necessary resources.

Best wishes,

Elizabeth Waagen, AAVSO HQ


American Association of Variable Star Observers (AAVSO)
HST date/time set for TCP-J21040470+4631129 observations

The HST observations of TCP-J21040470+4631129 (AAVSO Alert Notice 714) have been scheduled for

Sep 12, 2020 07:16:37 to 13:07:28 UT

For the HST observations to take place, it is essential to have a positive observation 24 hours before this time showing the system in quiescence, so the night of September 10-11 UT is crucial.

Please continue nightly V snapshots through September 8, then more intensive monitoring September 9 through 12, then nightly snapshots through September 15. Please report observations promptly, especially from September 9 through 12.

Many thanks, and Good observing,


Entering the critical last week

Dear All,

thanks so much for all the observations of TCP J21040470+4631129, its been very reassuring to see it nice and calm.


I am a little nervous, for the following reason: the timespan between the first and the second-super-outburst were ~105d, and between the second and third one ~111d. Since then, another ~156d have passed, with no more sign of activity. Has this star exhausted its "fuel", i.e. material that has been stored up in the accretion disk over who-knows-how-long (we don't know how long the outburst recurrence time is ... decades, or more?), or is it merely taking a bit more time to build up steam towards another super-outburst? The behaviour of this system is unique so far, in having had an initial mega-outburst(*), followed by a few echo-outbursts, and then three super-outbursts, so we don't really know what to expect.

Thus, please keep an extra close eye on our TCP J2104+4631 throughout the next week, to make sure the COS spectrograph remains safe!

We will have to report a detection of the system at its quiescent magnitude to STScI on noon (EDT) Friday Sept. 11th. Therefore observations Thursday night, pushing as far into Friday morning as visibility allows are particularly important (US observers will have an advantage here). And please report the data as soon as you get them.

Thanks so much for supporting the HST observations of this CV!


(*) what is the right terminology for the different types of outbursts that TCP J2104+4631 displayed? I just made up the term "mega-outburst", to set it apart from the three super-outbursts.






American Association of Variable Star Observers (AAVSO)
Tonight critical for TCP J21040470+4631129 coverage

Tonight (2020 Sep 10-11) is critical for coverage of TCP J21040470+4631129 - please see Boris' post above; quoting from it: "We will have to report a detection of the system at its quiescent magnitude to STScI on noon (EDT) Friday Sept. 11th. Therefore observations Thursday night, pushing as far into Friday morning as visibility allows are particularly important (US observers will have an advantage here). And please report the data as soon as you get them."

Also, please continue your coverage through at least Sep 12 (see the Alert Notice).

Many thanks and Good observing,


Decision point: 9am EDT

We have had an update from STScI that they need to anticipate the decision whether they consider it safe to observe TCP J21040470+4631129 with Hubble by three hours. That means the need to have the latest measurements of the brightness of the system by 9am EDT on Friday, Sept. 11th.

Thanks for your help with this, and clear skies!


We are go for tomorrow's COS observations

STScI approved the COS observations for tomorrow. What happens next is that the flight engineers at NASA Goddard will send a tiny command to Hubble to swap the two sets of co-ordinates stored in the on-board observing program, so that TCP-J21040470+4631129 will go into the aperture of COS, rather than the blank sky position that was the safety default.

Thanks so much for all your observations, without which this HST program would not have been possible. 

Success - almost!

First of all, thanks so much for your detailed observations of TCP J21040470+4631129 that were the key to get the HST observations through the final approval stage.

The HST ultraviolet observations of this CV went ahead, and the spectra we got reveal a fantastic amount of detail: the white dwarf totally dominates at ultraviolet wavelengths, and is still rather hot, in fact, hotter than any similar system a year into cooling from the outburst.  That is consistent with the outburst having been tremendously large and long, and we will be able to work, among a number of things, out how much energy was dumped into the white dwarf during the outburst. What is clear is that it will take the star many years to cool back into quiescence.

But, as mentioned, in the subject, there is a "but": during two of the six exposures, the COS shutter remained closed. It took a few days to figure out the reason - and to our relief, it wasn't the star misbehaving, or a mistake in the observing sequence: Hubble struggled for some time to acquire the guide stars it needs for the precision pointing, and for safety reasons the COS shutter remains closed in that case.

We're still establishing how much the loss of data affects the goals of this program, and there is a possiblity that the failed observations may be repeated. For that reason, it would be very helpful to continue  monitoring of TCP J21040470+4631129, though at a less intense level, maybe a few snapshots per week.

I'll get back in touch in a week or two.
















Repeat of the HST observations may happen in November

Just a quick update: STScI reviewed the malfunction of Hubble, and approved a full repeat of the COS observations of TCP J21040470+4631129. They are now working hard to get it into the schedule for November, when the visibility from the ground is still favourable. I'll be in touch again as soon as I have more news.

In the meantime, please give the star a look every few days so that we keep a good baseline of its activity.

Clear skies!


Next COS observations: 21st of November

Dear All,

STScI is planning to re-observe TCP J21040470+4631129 on November 21st. The procedure would be pretty much the same as last time, we need to provide monitoring of the baseline brightness in the weeks leading up to that day, and then a detection at the  (hopefully) quiescent magnitude within 24h before the COS observations.

Fingers crossed that Hubble and the weather will both work well!

All the best,




This is the week!

Dear All,

we are now in the critical week, with the HST/COS observations of TCP J21040470+4631129 scheduled for Saturday, Nov 21, 05:18-11:09 UT. 

STScI needs the last update on the brightness of the CV by Friday, November 20 by 11:00 AM EST (UT-5) to decide if they consider it safe for the COS observaitions to go ahead - failing this,  HST will stare at a blank piece of sky for six hours...

Please try to give  TCP J21040470+4631129 a closer look between now and Saturday, it would be ideal if we get a few snapshots of its brightness (V-band or clear) from a range of longitudes.

Thanks so much for your support of this campaign & clear skies!




AAVSO web site transition on Thursday

Dear All,

the AAVSO web site will undergo some maintenance on Thursday afternoon (EST), which will affect all services. Just in case that there is some glitch, please copy any observation obtained tomorrow by e-mail to boris.gaensicke@gmail.com.

Thanks & clear skies!


We are go!

STScI gave to go-ahead, so HST should slew to V3101Cyg in less than seven hours.

And again, all of this is only possible because of all the support you gave us. A big "thank you" from all our team!




Dear All,

mission accomplished: the second attempt of obtaining phase-resolved ultraviolet spectroscopy of V3101 Cyg (I still have to get used to the new name) was successful, HST carried out all observations exactly as planned this morning.

Thanks again for your support of this project!