Probable SN in NGC 6946

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Probable SN in NGC 6946

Postby James Foster » Sun May 14, 2017 2:06 pm

James R. Foster
Lhires III (2400 & 1800 ln/mm Grat) Spectroscope,
LISA Visual & IR Kit Spectroscope,
Alpy 600, and Star Analyzer 200
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Re: Probable SN in NGC 6946

Postby etienne bertrand » Sun May 14, 2017 10:01 pm

Il me semble bien être sur l'objet (après vérification), mais je n'y comprends pas grand chose, le spectre ne m'a pas l'air de coller avec une SN... ?
Acquisition commencée assez basse à 20° et en ce moment à 30°.
Image

Spectre de 2017SNeaw typeII avec des poses de 1200s.
Image
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Re: Probable SN in NGC 6946

Postby Paolo Berardi » Mon May 15, 2017 7:52 am

Nice spectrum Etienne! In the early stages SNe spectra tend to be featureless, so the classification is not easy. You could check with Gelato (https://gelato.tng.iac.es/ , sign in to obtain an account if you don't have one), uploading your fit spectrum.

With ATel # 10376 the SN is classified as type IIP, -7 days from the maximum light. There is also a link to the profile on TNS. Astronomers derived an expansion velocity of ~ 10600 Km/s from the P-Cygni H-alpha minimum (around 633 nm).

http://www.astronomerstelegram.org/?read=10376

The new SN should be "easier" in the coming days because it should become brighter and the profile is expected to enhance its spectral features.

Paolo

PS: thanks to James (and Robin on Yahoo mailing lists) for the alert announcement
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Re: Probable SN in NGC 6946

Postby etienne bertrand » Mon May 15, 2017 12:15 pm

Hello Paolo,

Like you say, thanks to James and Robin for they informations about this supernova it was very interesting to see this !

In my case with the first spectra of 900s, Gelato said to me that was a SN type II but with unreliable classification. THe 1 result in the list was Type IIP with 3.9 days but i'm not a expert and it was the first times that i saw a spectrum of this classification I so i was not sure about my result...
A little later in the night i saw the confirmate result in the website of weizann by a Chinese team of observers, so congratulation to this team for their classification !
No regret because I had no account to deposit the spectra on Weizmann and i was not sure about my result.
It was a excited experience !
So the spectro Alpy is very good, good results even 20 or 30° of altitude !
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Re: Probable SN in NGC 6946

Postby Paolo Berardi » Mon May 15, 2017 1:42 pm

Hi Etienne, it is definitely a very interesting experience!

Consider that also the excellent high SNR spectrum by BAO-2.16m Cassegrain on the TNS returns an "unreliable classification" with Gelato. You can try to download the profile from TNS, rename as DAT extension and convert it to FIT with ISIS.

Without any parameters, Gelato find the type IIP in fifth place on the table of best fitting templates, preceded by several IIn types.

Adding the reddening E(B-V) parameter (~0.2, ref ATel #10377), type IIP fitting move to first place (1999gi) because the continua have a similar slope. The presence of some dust along line of sight can be deducted from the narrow Na D absorption line (~590 nm), well visible in your spectrum.

So, we should try with various parameters and learn to interpret the results of the fitting, especially observing the related plot with spectra comparison. Low SNR is definitely our enemy because can hide some "macro-features" that even an amateur would be able to recognise.

I don't know if SNID, the other tool for SN classification, provides the same performances.

Paolo
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Re: Probable SN in NGC 6946

Postby etienne bertrand » Mon May 15, 2017 4:05 pm

Thanks Paolo for yours coments.

How do you calculate the vitesse of expansion of this supernova ? Is it possible with the spectra low resolution ?
The galxy ngc6946 have a rtdfshift z = 0.00015

The spectra with 4 x 1200s.
http://p4.storage.canalblog.com/43/59/799945/115934753.fit
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Re: Probable SN in NGC 6946

Postby Paolo Berardi » Mon May 15, 2017 6:43 pm

Thanks for sharing your observation Etienne. You should also send the profile to ARAS db (to Christian, François?).

Concerning the ejecta velocity estimation, the wavelength of P-Cygni absorption minimum is commonly used. It is difficult for me (I'm not expert) to identify the exact point even on the professional TSN profile of SN 2017eaw (note in the two ATels 10376 and 10377 are derived different values from the H-alpha line: 10600 and 14300 km/s). The broadening of the emission component is also related to the expansion speed but I don't know how accurate the measure could be.

Unlike this, other bright SN I observed in the past showed very clearly P-Cygni profiles (H-alpha, Si II, etc, depends on the type). This is surely connected also with the age of the SNs (featureless spectra in early stage), we'll see the development.

An old example, SN type II, very low resolution spectrum with Star Analyser 100 (note the broad and deep H-alpha absorption):

Image

I think the recession speed of NGC 6946 is negligible (40 Km/s) but the main problem is to find the correct absorption wavelength on the profile. Atel # 10377 talks about a shallow, narrow P-Cygni absorption that probably is lost in the noise of your spectrum (at least, I am unable to discern).

Paolo
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Re: Probable SN in NGC 6946

Postby Francois Teyssier » Mon May 15, 2017 7:23 pm

Beau travail, Etienne

Je te transmets les félicitations de Steve et la confirmation des propos de Paolo : pour une SN II à cette étape (stage), il n'est pas étonnant d'avoir un continuum très lisse

It's a SN II so that's not entirely surprising, and there's also stellar background, but this was a GREAT attempt. Please thank Etienne for this effort!!!

Ca vaut la peine d'essayer de prendre d'autres spectres dans le jours à venir.

Spectres SN :https://ned.ipac.caltech.edu/level5/March03/Filippenko/frames.html

Pour ce qui est du redshift, ce n'est pas du spectre de la supernova que tu le déduis. C'est au contraire le redshift de la galaxie qui permet de corriger la vitesse des raies (lorsqu'elles apparaissent).

Tu peux m'envoyer le fit pour inclusion dans la prochaine lettre

Bonne continuation,

François
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Re: Probable SN in NGC 6946

Postby etienne bertrand » Mon May 15, 2017 8:31 pm

Bonsoir François,
Merci beaucoup pour ton message, et ton liens sur les supernovae.
Remercies aussi chaleureusement Steve pour tout ce qu'il fait, son commentaire est la meilleure des considérations et cela fait vraiment très plaisir.
Dès qu'il fait beau je m'y remets pour suivre cette supernova !
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Re: Probable SN in NGC 6946

Postby etienne bertrand » Mon May 15, 2017 8:57 pm

Paolo thanks for your descritpion,
if i understand in SN type II there is hydrogene absorption, so if i zoom my spectra before the bump of hydrogene h-alpha region the minimum is at 6280.5A
Image
With doppler calcul il find a z = -0.04301517 so i find a velocity : v = -12985 km/s
What do you this about this ?

(exellent your spectra of M51, with staranalyzer is very clean !)
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