[Aavso-photometry] AAVSO Special Notice #120: SGR 0501+4516

[Denis Denisenko]

Hello all!

Regarding the variable star in the field of SGR 0501+4516.  It is 
located in a close group of three stars with a similar brightness, and 
is the middle one of three.  It's brightness (and coordinates in USNO) 
are contaminated by the two companions.  The brighter star is located 
just 2.0"W and 2.5"S from the variable.  On the Palomar plates their 
images almost overlap, and even on the 2MASS infrared CCD images you can 
hardly put the nail between 'em.

So I expect some difficulties for the observations with the amateur 
equipment, especially for those observatories located at the low 
elevation above sea level.  Perhaps if you have seeing worse than 2.5" 
or even 2", you won't be able to separate the two stars.  I think Arne 
will correct me, but in this case it can be possible to report the joint 
magnitude of two stars, as it was done in the past for the Novae getting 
fainter than their close neighbor.

For you to better understand what I'm talking about, please take a look 
at Palomar and 2MASS images of the field on my web page 
<http://hea.iki.rssi.ru/~denis/SGR0501+4516.html> devoted to this 
object.  Top three pictures are animations of different Palomar plates 
and have 101"x101" field of view at 2x magnification, i.e. 2 pixels per 
arcsec.  (Hint: if you're tired of animation, hit [Esc] in your browser, 
and pictures will stop blinkin').  The Big Picture is a color-combined 
BRIR Palomar image with 10'x10' FOV and scale 1" per pixel (i.e., no 
zoom).  The very bottom picture is a pseudocolor Infrared image with 
100"x100" field of view, and again at 2x magnification, just like the 
first three.

Note that the third (North-Eastern) component is very red on Palomar 
plates, but rather faint on 2MASS images.  This star is the closest one 
to the Swift X-ray position, but still outside the 90% confidence error 
circle.

Actually the variable and its two components are present in 2MASS 
catalogue, but at the limit of sensitivity.  Only the South-Western star 
has all three magnitudes (J, H and K) accurate to better than 0.1m:

-----------------------------------------------------------
Obj 2MASS Identifier  J mag Error  H mag Error  K mag Error
-----------------------------------------------------------
S/W 05010624+4516246 14.822 0.076 14.062 0.070 13.845 0.064
VAR 05010648+4516274 14.407  N/A  14.435 0.072 13.643  N/A
N/E 05010659+4516311 15.244  N/A  15.579 0.137 14.376  N/A
-----------------------------------------------------------

Note: J and K values for VAR (14.407 and 13.643) and N/E star (15.244 
and 14.376) are computed by VizieR, and are not part of the original 
2MASS data.

Den in Moscow

aavso at aavso.org wrote:

> AAVSO Special Notice #120
>
> SGR 0501+4516
> August 25, 2008
>
> Denissenko (GCN 8114) reports on archival
> Palomar Observatory Sky Survey plates, and finds that a star at
>
> 05:01:06.47 +45:16:28.0 (J2000)
> (also known as USNO-B1 1352-0128683), appears to be variable,
> with B ~ 18 and R ~ 16. This object is 7arcsec from the
> XRT position, and therefore formally outside of the error circle.
> Halpern (GCN 8129) reports that 2 hours of MDM monitoring
> found this star to be constant at R=17.16 for the first hour,
> but then monotonically declining to R=17.35 over the next hour.
>
> At R=17, this variable source is within range of most amateur
> CCD systems. If you have a 30cm or larger telescope, we
> recommend obtaining filtered observations, either V or Rc,
> with at least S/N=20 per image, and monitor the field as long
> per night as possible. For smaller telescopes, go unfiltered.
>
> Report observations of the possible variable star as:
>
> USNO-B1.0 1352-0128683 (AUID 000-BFT-697)