Apparent magnitude and Simbad

[Brutha]

Hi All,

Quick question: at some point in the coming months I'll be having a play with a star analyser, and one of the things I'd like to achieve (eventually!) is to get the spectrum of a distant quasar.

So, I've been playing with Python etc, with the idea of creating a list of viable targets: the first one would seem to be 3C273, which in Sky Safari and Wikipedia is shown as 12.9 apparent magnitude.

However, when I query Simbad, I don't seem to see anything matching it - from what I understand, the V-band should be the closest match, but this gives 14.83, similar to many other fainter quasars.

Does anyone know where the 12.9 might have come from? The full list of measurements in Simbad is shown below - thanks for any ideas!

Cheers

B 13.05 [~] D 2010A&A...518A..10V
V 14.830 [0.022] D 2000A&A...355L..27H
R 14.11 [~] D 2008ApJS..175...97H
G 12.844090 [0.003505] C 2020yCat.1350....0G
J 11.692 [0.023] C 2006AJ....131.1163S
H 10.953 [0.023] C 2006AJ....131.1163S
K 9.937 [0.020] C 2006AJ....131.1163S
u (AB) 13.859 [0.005] C 2009ApJS..182..543A
g (AB) 12.990 [0.002] C 2009ApJS..182..543A
r (AB) 12.871 [0.002] C 2009ApJS..182..543A
i (AB) 12.630 [0.002] C 2009ApJS..182..543A
z (AB) 13.242 [0.006] C 2009ApJS..182..543A

 

[DirkSteele]

I always thought that estimates came from John Isles who once upon a time headed up the BAA Variable Star Section and has something silly like 30,000 reported measurements (so very skillful).  As I recall, he stated it could show variability between around 11.8 and 13.2 but the average is around 12.8-12.9.  Having tracked it down many moons ago, something around 13th magnitude is accurate.

 

 

[Brutha]

20 minutes ago, DirkSteele said:

I always thought that estimates came from John Isles who once upon a time headed up the BAA Variable Star Section and has something silly like 30,000 reported measurements (so very skillful).  As I recall, he stated it could show variability between around 11.8 and 13.2 but the average is around 12.8-12.9.  Having tracked it down many moons ago, something around 13th magnitude is accurate.

 

 

Thanks! Maybe I am optimistic thinking I can just find the number on a database somewhere - more digging needed I think then!

[lunator]

The difference may be to do with the difference between visual and V.

The Human eye can see ~380-700nm

The V band is around 500-580nm

So the object will appear fainter in V as it restricts the light reaching the sensor.

I guess it depends on the amount of each wavelength of the light being emitted. 

Cheers

Ian

[vlaiv]

38 minutes ago, lunator said:

The difference may be to do with the difference between visual and V.

The Human eye can see ~380-700nm

The V band is around 500-580nm

So the object will appear fainter in V as it restricts the light reaching the sensor.

Actually V band is a pretty good match for human eye sensitivity to brightness.

Here is human eye response to sensitivity:

Black line is Photopic and green is Scotopic (daytime / night time vision) sensitivity.

Here is UBV filter repsonse. Note that V filter starts at around 450 nm, raises at around 475nm, peaks at about 520nm and so on and compare that to Photopic sensitivity.

In fact - best match would probably be a cross between Photopic and Scotopic - so not the full night adaptation but some level of night adaptation.

[vlaiv]

On 03/11/2024 at 13:41, Brutha said:

Does anyone know where the 12.9 might have come from? The full list of measurements in Simbad is shown below - thanks for any ideas!

I think that discrepancy maybe comes from type of measurement conducted.

12.9 is V magnitude - but original source lists 8 arc seconds of diameter for measurement, so it could be that this is integral magnitude - all the light from the quaser it self and its host galaxy.

Perhaps quaser itself has 14.83 V magnitude and the rest is simply light from the host galaxy.

 

[lunator]

Hi Vlav

Thanks for this. You are probably right

Extended object magnitudes are always a bit tricky.

Cheers

Ian

[robin_astro]

Like most Quasars 3C273 is varies in brightness as the accretion onto the black hole varies. Here is the light curve in AAVSO. It has been dropping over the past 20 years and is currently V mag 13.3.

quite  a bit fainter than the mag 12.5 when I measured it in Patrick Moore's back garden with a prototype of the Star Analyser back in 2005. (Bob Hawksley and I brought the Star Analyser to market later that year)

http://www.threehillsobservatory.co.uk/astro/spectra_12.htm

According to the reference accompanying the SIMBAD V mag of 14.8, it apparently comes from the Tycho 2 catalogue but it has not been as faint as that for some time.

Robin

 

 

 

Edited November 7 by robin_astro
added SIMBAD reference

[robin_astro]

Here is the visual magnitude AAVSO light curve going back 60 years. The values are pretty close to the photometric V magnitudes. (The V mag 14.8 in SIMBAD is clearly nonsense)

[robin_astro]

3C273 is probably still the brightest QSO though, but there are fainter and much more distant ones reachable with the Star Analyser like this one with gravitational lensing assistance

http://www.threehillsobservatory.co.uk/astro/spectra_22.htm

and still with amateur equipment (my specially modified ALPY200 faint object spectrograph) all the way back to when the universe was just 10% of its current age

https://britastro.org/observations/observation.php?id=20210408_191229_b70f474be2035d42

[lunator]

Hi Robin

Very interesting info. 🙂

It looks like Simbad is the source of mag confusion. 

I had never thought of using my SA on this target but I might consider this now.

Cheers.

Ian