Possible nova in Centaurus, PNV J13544700-5909080, 03 Dec 2013 - Page 7

Terry B · #**121** 29-12-2013, 11:49 AM

I have finally achieved a spectra of this nova. Getting up at 3am isn't one of my favorite things to do.

It is very bright. Simultaneous photometry yielded V= 4.27
It is easily naked eye from my observatory.
Because it is is so bright I had to use very short exposures to prevent saturation. I took 26x 3 sec exposures and have processed them in Isis.
I have also flux calibrated the result.
It still has PCyg features.

Terry

Jon (Jonathan) · #**122** 29-12-2013, 12:05 PM

Nice stuff Terry.

I found a couple of articles about novae that rebrighten, suggesting that the P Cygni profiles recur when the brightness peaks. Links are in a long post of mine from a couple of days ago.

I had V=4.2 in my photometry of Saturday morning. Looks like this third "maximum" may have peaked - but who can tell with this thing?

Merlin66 (Ken) · #**123** 29-12-2013, 12:08 PM

Great stuff!!
Well done - good to see you back on stream, even at 3am!
Certainly shows the current detail........

Shark Bait (Stu) · #**124** 29-12-2013, 12:16 PM

Quote:

Originally Posted by Terry B

I have finally achieved a spectra of this nova. Getting up at 3am isn't one of my favorite things to do.

It is very bright. Simultaneous photometry yielded V= 4.27
It is easily naked eye from my observatory.
Because it is is so bright I had to use very short exposures to prevent saturation. I took 26x 3 sec exposures and have processed them in Isis.
I have also flux calibrated the result.
It still has PCyg features.

Terry

Quote:

Originally Posted by Jon

Nice stuff Terry.

I found a couple of articles about novae that rebrighten, suggesting that the P Cygni profiles recur when the brightness peaks. Links are in a long post of mine from a couple of days ago.

I had V=4.2 in my photometry of Saturday morning. Looks like this third "maximum" may have peaked - but who can tell with this thing?

Aside from human error, is there normally a means to explain the difference between photometry results and careful visual magnitude assesments?

0400 to 0435 hrs AEST Sun 29-12-2013 (1800 to 1835 hrs UT Sat 28-12-2013)

This morning I observed Nova Cen 2013 through 7x50 Binoculars and I placed it between Mag 4.0 to 4.1 - Nova Cen 2013 was still visible along with Mu1 Crucis (Mag 4.0) at 0435 hrs local time when the fainter stars were washed out and lost from sight.

I tried something different to get an accurate gauge on the magnitude of Nova Cen 2013. I waited for the morning twilight to wash out the sky and let the stars used for comparison fade from sight. The nova was definetely brighter than Lamda Crucis (at Mag 4.59) and HIP70069 (at Mag 4.28).

malclocke (Malc) · #**125** 29-12-2013, 12:51 PM

Welcome back Terry

I'm just going over my results from the 11th and 14th and trying to do a better reduction to resubmit to ARAS. You'll notice my spectra have a ~ in the validation column at http://www.astrosurf.com/aras/Aras_D...a-Cen-2013.htm - this is shorthand for 'these are crap!'

Just thought I'd share a comparison between my results (black) and Jonathan's (red) from the 11th. Overall not a bad match I think, apart from mine getting a bit lost in the blue end.

Jon (Jonathan) · #**126** 29-12-2013, 01:34 PM

A good fit, Malc. You're getting more detail than me on 11th. My collimation was a bit out - I redid it before the 16th and got much better detail. Plus, I could take longer exposures as the nova wasn't as bright.

Jon (Jonathan) · #**127** 29-12-2013, 01:56 PM

Quote:

Originally Posted by Shark Bait

Aside from human error, is there normally a means to explain the difference between photometry results and careful visual magnitude assesments?

Good question, Stu. When Terry and I talk about "V Mag" we are talking about an image taken through a Johnson V filter. This is essentially a green filter with a passband centred on about 5300 A - the dead centre of the human visual range. So for average stars the measured V mag will be quite close to the estimated Visual mag. Terry and I also took B (Blue) measurements, which were around 4.4. So this nova has a B-V value of 0.2 at the moment. B-V is a shorthand and quantitative measure of the colour of a star: typically B-V around 0 is a fairly blue star, while B-V of 1 or more us a fairly red star.

The problem is, this nova is neither average nor typical! A lot of the light is coming from the Ha line in the red part of the spectrum, and a fair bit from the Hb and other lines in the blue. These are both outside the main passband of the V filter. So I'd expect Visual estimates to be a notch or two brighter than the V measurements. But it is very hard for even experienced visual observers to get closer than 0.1-0.2.

The AAVSO gas an excellent manual for visual observers. http://www.aavso.org/visual-observing-manual. I hope you are reporting your observations to them!

Cheers

Jonathan

Shark Bait (Stu) · #**128** 29-12-2013, 02:10 PM

Quote:

Originally Posted by Jon

The problem is, this nova is neither average nor typical! A lot of the light is coming from the Ha line in the red part of the spectrum, and a fair bit from the Hb and other lines in the blue. These are both outside the main passband of the V filter. So I'd expect Visual estimates to be a notch or two brighter than the V measurements. But it is very hard for even experienced visual observers to get closer than 0.1-0.2.

The AAVSO has an excellent manual for visual observers. http://www.aavso.org/visual-observing-manual. I hope you are reporting your observations to them!

Cheers

Jonathan

Thanks again Jon. I was a little concerned that my best efforts to visually pin down Nova Cen 2013's magnitude were still different to the figures you were all getting.

To be honest, sending my visual estimates to the AAVSO did not occur to me. I am surprised that visual is considered a valid measurement when photometry and spectroscopy can remove the subjective element of the data collection. I will follow up on the link that you provided and will consider sending my observations.

Regards,
Stu.

Jon (Jonathan) · #**129** 29-12-2013, 08:15 PM

Quote:

Originally Posted by Shark Bait

Thanks again Jon. I was a little concerned that my best efforts to visually pin down Nova Cen 2013's magnitude were still different to the figures you were all getting.

To be honest, sending my visual estimates to the AAVSO did not occur to me. I am surprised that visual is considered a valid measurement when photometry and spectroscopy can remove the subjective element of the data collection. I will follow up on the link that you provided and will consider sending my observations.

Regards,
Stu.

Stu, is it weren't for the visual observers, we wouldn't even have a light curve for this thing! For the record, the last visual record uploaded, a few hours ago from South Africa, was by Peter Wedepohl, an experienced observer with over 2000 logged observations going back to 1998. He had it at 4.1, same as you :-)

Photometry is not as an exact science as it may seem. My last measurements in the same session spanned 4.18-4.27, and each of these was a stack of 5 exposures. The problem is that the object is so bright. I'm having to take no more than 5s subs through my little 80mm guidescope - using my main scope would saturate the star in an exposure of less than a second. Even with the guidescope, I can only take 5s exposures, which mean that seeing, scintillation and even the camera shutter can all distort the recorded brightness, and that the signal to noise ratio is low.

Second, photometry works by comparing the measured target's pixel intensity with that of comparison stars of known magnitude, ideally similar magnitude (and colour!) to the target and in the same field. For this object we have to use two 7th magnitude comp stars, which is stretching the capability of the method to be really accurate - I have to make sure the nova doesn't saturate or get close to the non-linear response threshold of my camera, but that means the comp stars are faint, subject to scintillation, and have barely-acceptable SNR.

I hope that makes sense! The short version is - for an object this bright, frequent visual observations are invaluable and can close to photometry in accuracy if done well.

Now, when it fades to mag 12, that will be another story ;-)

Terry B · #**130** 29-12-2013, 09:31 PM

A comparison of flux calibrated spectra of V1369 and Nova Del. The V mag isn't exactly the same. It shows how much further evolved V1369 is at a similar magnitude compared to Nova Del. This nova is much brighter.
http://i566.photobucket.com/albums/s...ps8fdc5697.png

Terry B · #**131** 29-12-2013, 09:37 PM

I was wondering what star you guys are using to correct instrument response (if you are doing so).
I experimented with 2 stars last night but decided on HD118978. It is close by the nova so at a similar airmass. It is a B9III star with clear H lines.
It would be sensible for all observers to use the same star for instrument response.
Any other suggestions?

Jon (Jonathan) · #**132** 29-12-2013, 10:19 PM

Quote:

Originally Posted by Terry B

A comparison of flux calibrated spectra of V1369 and Nova Del. The V mag isn't exactly the same. It shows how much further evolved V1369 is at a similar magnitude compared to Nova Del. This nova is much brighter.
http://i566.photobucket.com/albums/s...ps8fdc5697.png

Indeed. 18/8 was two days after Nova Del's maximum. The emission lines present on 14/8 had all but disappeared, and had yet to reappear. The best spectral comparison date with Nova Cen would be 7 or 8 Dec.

We are currently 23 days after maximum for nova cen. It has fallen from 3.5 to 4.1 in that time. The equivalent for nova del was a fall from 4.2 to 7.3 23 days after maximum.

Del was quite a fast nova. But Cen is not only brighter; it's much slower.

Merlin66 (Ken) · #**133** 30-12-2013, 02:35 AM

Bumma - clouded out again!
Only managed to get a sequence around Hb before they moved in....
I'll process and upload later.
Anyone else have better success???

Merlin66 (Ken) · #**134** 30-12-2013, 08:49 AM

Well, well, well..........
Just started to process the spectra from last night (around Hb) and the Hb, Fe profiles have completely changed!!
I'm glad I managed a second series of exposures (20 x 30s) - they confirm the changes....H gamma also looks brighter??
Hope to post later....
The Na region was almost a wash out due to the cloud....there may be something...let's see.

Jon (Jonathan) · #**135** 30-12-2013, 09:40 AM

I wish I knew what was going on with these re-brightenings. I've been reading all I can. It's clear that there is evidence of spectral changes during these "maximums" in other novae. There seems to be very little available spectra of these sort of novae!

It looks as though we are at the peak brightness for the third "maximum" and that brightness is declining now. Does this correspond with the changes you observed in the Hb and Fe lines, Ken? I wonder ...

Merlin66 (Ken) · #**136** 30-12-2013, 10:23 AM

The attached image shows what looks like secondary (?) P Cygni dips as well as the "saddle tops".
I'll get a copy to Steve Shore for comment......

Jon (Jonathan) · #**137** 30-12-2013, 10:39 AM

Fantastic! Do share what Steve says.

The saddles are also slightly different from two days ago - now the blue end is stronger than the red.

These double P Cygni profiles maybe suggest two "shells" of ejecta, moving outward and different velocities?

Merlin66 (Ken) · #**138** 30-12-2013, 11:16 AM

Certainly will do!
I'm having some "problems" preparing the BeSS fits header (I'm currently working in BASS Project - trying to get it to work well with slit spectra)
Hopefully it will all come together later today and I'll upload the data I have to Francois and the ARAS team.
I'm definitely not smart enough - even after reading Gaposchkin and Steve's notes to figure out what is happening.
I think we're getting some very interesting data, I hope it helps.

malclocke (Malc) · #**139** 30-12-2013, 12:15 PM

Quote:

Originally Posted by Terry B

I was wondering what star you guys are using to correct instrument response (if you are doing so).
I experimented with 2 stars last night but decided on HD118978. It is close by the nova so at a similar airmass. It is a B9III star with clear H lines.
It would be sensible for all observers to use the same star for instrument response.
Any other suggestions?

Hi Terry,

This is a good question and I've been reluctant to make any suggestions due to lack of experience. I looked at François T's spreadsheet of recommended stars and picked Alpha Cir as it had a very similar altitude at my observation times and was quite bright (3.2). It does have spectral peculiarities though. The spreadsheet has a selection of A and B class stars with low E(B-V) values which I believe makes them good candidates for calibration.

It seems to me that getting a similar airmass for the calibration star is crucial here as the nova is so low. Given how close Terry's star is to the nova in angular terms it may be the best choice as a standard.

For the record, here is a selection of stars from the spreadsheet, and the approximate local time when they each is at a similar altitude to the nova. They all have a fairly large angular separation from the nova.

Code:

HD113902 00:00 5.7 B8V
HD115823 01:30 5.4 B6V
HD117150 03:00 5.0 A1V
HD118991 03:30 5.0 B8Vn

Terry B · #**140** 30-12-2013, 12:16 PM

Very interesting Ken.
It was cloudy here last night and I have to work this week during the day so 3am is a no go til next weekend.