MBCAA Observatory

V404 Cyg polarization

Observed: 26, 28 June 2015

Michel Bonnardeau
2 Jul 2015


Time series, through polarization filters, of the binary system with a black hole V404 Cyg, during its June 2015 outburst, are presented.


V404 Cyg is a binary system with a 8-12 solar mass black hole (Wagner et al (1992)). It is orbiting a K0 star (Casares et al (1993)) with a period of 6.4758468 days (Bulavinova et al (2005)) . It is at a distance of 2.39 kpc (Miller-Jones et al (2009)).

V404 Cyg is usually around mag 18-19; in 1938 it had an outburst and was reported as a nova and also in 1938, 1979(?) and 1989 (Richter (1989)). In mid june 2015 it was reported as having a new outburst, the first one since 1989 (Negoro et al (2015) ATEL 7646).


The observations were carried out with a 203 mm f/6.3 SC telescope, a filter wheel and a SBIG ST7E camera (KAF401E CCD, mostly red sensitive). Two polarized filters were used alternatively, one with its axis roughtly parallel to the RA axis, the other parallel to the DEC axis. Each exposure is 60s long.

For the differential photometry, the comparison star is 14CMC202338.9+335633.

Session 26 June 2015

The light curves:

Upper light curves: V404 Cyg, Lower ones: the check star 14CMC202415.0+335141, shifted by +1.8mag. Red: with the XPol filter, Blue: with the YPol filter. The error bars are +/- the 1-sigma statistical uncertainties.

The magnitudes fluctuate strongly from one measurement to the other. The following figures show the differences between one measurement and the one that follows:

With the XPol filter. Red: V404 Cyg, Blue: the check star.

With the YPol filter.

V404 Cyg is observed alternatively with one polarization filter then the other. To compute the magnitude difference between measurements with different filters at a given time, the measurements are then interpolated. Because of the flickering, this leads to fluctuations in these differences that are actually an artefact. To give ride of it, the differences are smoothed by averaging over 5 successive measurements. The results are:

The magnitude differences between the 2 polarization filters. Red: V404 Cygn, Blue: the check star.

The polarization is around 6%, which is similar (slightly less) to the measurements of Panopoulou et al (2015, ATEL 7674) and Blay et al (2015, ATEL 7678). However it is variable and at at THJD 200.58, the polarization is much stronger and reversed. This may be compared with Lipunov et al (2015, ATEL 7696).

Session 28 June 2015

Two days later, V404 Cyg is about 3 mag dimmer:

The magnitudes of the check star are shifted by +4mag.

V404 Cyg is still flickering:

The magnitude differences, smoothed by averaging over 7 measurements:


Blay P. et al (2015) ATEL 7678.

Bulavinova O.S. et al (2005) ASP Conf. Series 330 347.

Casares J. et al (1993) MNRAS 265 834.

Lipunov V. et al (2015) ATEL 7696.

Miller-Jones J.C.A. et al (2009) ApJL 706 L230.

Negoro H. et al (2015) ATEL 7646.

Panopoulou G. et al (2015) ATEL 7674.

Richter G.A. (1989) IBVS 3362.

Wagner R.M. et al (1992) ApJL 401 L97.

Technical notes

Telescope and camera configuration.

Computer and software configuration.

Data processing.

Site map


Copyright notice

MBCAA Observatory. Copyright 2004-2015