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The angular size of the lensed star could be inferred from the Gaia parallax, the apparent magnitude and the spectral classification.
I can’t offhand think of any way this could be determined spectroscopically. Do you have a suggestion? I am guessing that might be one of the parameters in the light curve fit but I don’t know the details.
Cheers
Robin
Here is a similar event back in 2006 which went from mag 11.5 to mag 7.5. There were several papers on this eg this one with members of the amateur community (BAA/CBA etc) as co-authors
https://ui.adsabs.harvard.edu/abs/2008ApJ…677.1268G/abstract
which deduced the lensing object was likely a low mass star or brown dwarf. Their paper also suggested that it could be a a productive technique for discovering exoplanets.
(I believe my first (crude slitless) spectrum was the earliest of this object which alerted the community that this was something unusual though as far as I know that was not mentioned in any of the subsequent papers). I remember there was considerable scepticism at the time as lensing of such a bright target was considered unlikely but there have been a few more bright ones since then.
Cheers
Robin
Yes the spectrum will be a blend but the lensing object must be very much fainter (or spectroscopically similar) as there is no evidence of any change in the spectrum even though the total brightness (driven by a change in the brightness of the lensed component) changed by a factor of ~25 (3.5 mag) I am not an expert on lensing but I think once we have a complete model we may have a distance and mass for the lensing component. That would also give an idea of how far back in time they would have been resolvable. Perhaps historical deep images would show something? (Of course not if it is a black hole though)
Cheers
Robin
look out for a ASI120M, they came with a wide angle lens eg currently
https://www.astrobuysell.com/uk/propview.php?view=180950
but better in mono (and the price of this one is too high, they were only about that new)
Cheers
Robin
As well as my classifying spectrum mentioned in VSSC 190, I now have four spectra following this event as it faded down to ~16.3 g. (No change within the uncertainties as expected)
The spectra are in the BAA spectroscopic database but I will also put something on my BAA observations page sometime in the next few days with a few notes about them.
Cheers
Robin
According to this tweet from the ASSAS-SN team it may have topped out early at just mag 12.9
Robin
Eyeballing it from the image it looks slightly brighter than the star above it on the top edge of the frame which is V mag 13.7 (APASS)
Putting the 2SPOT spectrum through the SNID supernova identification program estimates that it was ~11 days from maximum then so possibly maximum around 25th? Plenty of time to go yet.
Hi Roger,
Spectra can only rule out or support the microlensing hypothesis, not confirm it. (Any changes in the spectrum would have indicated a potential alternative cause of the brightness change) but yes I am happy for my spectra to be included in a paper/article for the JBAA and I can supply a paragraph of explanation. (I will leave it to others to judge its merit as I have little knowledge of the subject)
Cheers
Robin
Hi Roger,
I saw a comment suggesting a Be star on the TNS discovery entry where it is AT2021ziw. I uploaded my spectrum there to eliminate that suggestion
https://www.wis-tns.org/object/2021ziw
but at the time the gaia team uploaded their discovery there it had only brightened by around 0.4 mag so I would have said the possibilities were pretty wide open at that point. That’s why a spectrum is useful.
Not sure why a brightening Be star would be singled out as a likely possibility, (there must be many mechanisms that can produce such an upward drift in brightness) but my first spectrum clearly eliminated that possibility and that given it looked like a pretty average K class star, probably many other alternatives. The shape and symmetry of the light curve now pretty much seals it as microlensing and my second spectrum supports that. (Only a microlensing event or eclipsing binary would be expected to leave the spectrum unchanged through a significant change in brightness)
I see there is now another spectrum in TNS, taken using the 3.8m Seimei telescope. Although higher resolution the features are the same as in my spectra. (There is a bit of difference in continuum slope, possibly down to my choice of reference star.)
Cheers
Robin
It has now faded ~1.3 mag since the previous spectrum and there has been no significant change, supporting the microlensing hypothesis.
Cheers
Robin
Now confirmed spectroscopically as Ia, by ePESTO and by the amateur 2SPOT team using their remote ALPY600 spectrograph in Chile (Image and spectrum on ARAS forum)
The spectrum does not look obviously reddened so it may be heading for being one of the brightest SN for some time
Cheers
Robin
It is slightly strange that they decided to use a subset of all the available spectra (There are currently 104 in the BAA, 392 in the ARAS and 113 in the AAVSO databases, most of which would meet the resolution criterion though many are likely duplicates) I wonder what the selection criterion was.
On it now with the ALPY200. I make it roughly mag 13.5 unfiltered off the spectrograph guider so significantly brighter than at quiescence.
EDIT: I see that is consistent with the 13.64 V (APASS) at 16:20 today in the Gaia follow up data
They used to be even smaller I think (AstroTech 65mm/70mm refractors) They also had at least one with a Star Analyser 200 in the filter wheel which Arne Henden and I sorted. Unfortunately that does not work well with the new Takahashi E-180 astrographs which at f2.8 are too fast for the SA200
Raising the public profile through funding permanent BAA sponsored exhibits promoting the role of amateurs in science museums and similar venues perhaps? Or perhaps taking a stake in the rebuilding of the Galloway Forest Dark Sky Observatory which (was) burnt down earlier this year?
and we have a member with expertise in this area
A remote spectrograph in the southern hemisphere could be productive and is not something that the existing “telescopes for hire” provide. There is 2SPOT, a proof of concept dedicated remotely operated spectrograph on a telescope farm in Chile run by an amateur group, funded from commercial donations, individual personal investment and crowd funding. (The equipment was shipped out and commissioned on site and is maintained locally)
