Astrophotography by Anthony Ayiomamitis

Differential Photometry - Cygnus

A variable star, as its name suggests, is a star whose magnitude varies intrinsically, in contrast to eclipsing binaries whose magnitude varies as a result of one star in the binary system eclipsing the other. True variables are one of five types, namely Mira stars, semiregular stars, cepheids, eruptive variables and, finally, cataclysmic variables. Minimum to maximum magnitude can range from days to many months with some variables displaying irregular periods.

A popular method for the study of variable stars, particularly short-term variables, is by the use of the technique known as "differential photometry". Rather than measure the (variable) magnitude of a variable star on an absolute scale, measurements are made over time relative to one or more non-variable star(s) and these differences are then plotted so as to study and illustrate the relative or differential change in magnitude. Due to the very large number of variables stars, the field of differential photometry represents one of the key fields in astronomy whereby the amateur astronomer can make a meaningful and long-lasting contribution to both science and astronomy.

More recently, the search for extrasolar planets (over 750 discovered so far) has identified yet another interesting application for the practice of differential photometry whereby the minute drops in magnitude of a star hosting an exoplanet are studied. Further details for the interested party are available here.

Note: Cygnus X-1 has been regarded for a long time as the best and most probable candidate for a black hole. However, this claim was surpassed with the 1992 discovery and consequent study of the x-ray emissions originating from V404 Cygni. Known for four previous ourbursts (1938, 1956, 1978 and 1989), V404 Cygni lies a mere 10 light-years away and, similar to Cygnus X-1, also lies in the constellation of Cygnus. V404 Cygni is a main sequence K-class star with a magnitude of 11.5 (at maximum) and mass of about 0.6 solar masses which is locked by another "star" with a mass of between 8 and 15 solar masses in a binary formation and with a period of 6.64 days. Since such a mass precludes any type of white dwarf (or neutron star), the only remaining alternative is a black hole and which at one time was a member of this binary system and prior to its collapse. Similar to Cygnus X-1, V404 Cygni was discovered in the fall of 1992 thanks to its x-ray emissions. The variable nature of V404 Cyg was discovered by A. A. Wachmann in 1938. Further details from the International Variable Star Index are available here whereas an AAVSO finder chart is available here.

Note: The fifth documented outburst of V404 Cyg was announced on June 18, 2015 and the results below indicate the strong ongoing activity five days later.

Image Details
Light Curve for V404 Cyg
Imaging Details
Variable Star:
V404 Cyg

Other Designation:
GS 2023+338

RA / Dec:
20h 24m 04s /
+33° 52' 02"


Magnitude:
11.5 - 18.48 (p)

Period:
6.64714 days

Variability:
LMXB/XN

Comparison Star:
GSC 2693:1051

Check Star:
GSC 2693:1473


Date:
Jun 23-24, 2015
22:51 - 04:07 UT+3


Location:
Athens, Greece

Equipment:
AP 305/f3.8 Riccardi-Honders
AP 1200GTO/CP3 GEM
SBIG ST-10XME
SBIG CFW10
SBIG LRGB filters


Integrations:
Lum :  266 x 60 sec
Dark :  010 x 60 sec
Flat :  ~ 24,200 ADU
Binning :  1x1

Temperatures:
Ambient : + 23.0 ° C
CCD Chip : - 12.5 ° C

Software:
CCDSoft V5.00.201
AIP4Win V2.4.0


Processing:
Reduction
Differential Photometry