MBCAA Observatory

EX Hya

Observed: 19, 23, 24, 29 May, 6 June 2004

Michel Bonnardeau
13 May 2005
Revised 13 Feb 2011 (new apertures, new comp value, extra-losses filter, new spin ephemeris)

Abstract

Light curves for this cataclysmic star, classified as both IP and NL, are presented and compared with the ephemeris.

Introduction

EX Hya is a cataclysmic star, that is a binary system with an accreting white dwarf. The orbital period is 1.63 hour. The white dwarf is magnetic and this modulates the accretion with the spin period of 1.12 hour (almost equal to the orbital period). The system also has outbursts. It is then classified as both an Intermediate Polar and a Nova Like.

According to Hellier and Sproats (1992) the ephemeris for the orbital eclipse is:
HJD = 2,437,699.94179 + 0.068233846*E
(the orbital rotation has also a tiny modulation (residuals less than 0.0005 day) with a period 17.5 yr).

And according to Mauche et al (2009) the ephemeris for the maxima of the spin modulation is:
HJD = 2,437,699.8917 + 0.046546484*E -7.3*10^-13*E^2 + 2.2*10^-19*E^3
that is, the white dwarf is spinning up.

Observations

The observations were carried out with a 203 mm f/6.3 SC telescope, a Rc filter and a SBIG ST7-E camera (KAF401E CCD). 204 usable images were obtained, each with an exposure duration of 60 seconds.

The comparison star is GSC 6709-0321 (AAVSO 000-BBT-327) with a Rc magnitude of 10.886, computed from the CMC14 r' magnitude, the 2MASS J, H, Ks magnitudes and using the transformation formulas of Bilir et al (2007).

The check star is GSC 6709-0006 (AAVSO 000-BBT-331). It is observed with the magnitude
11.848 +/- 0.024 +/- 0.031
where the first +/- is the average 1-sigma statistical uncertainty, and the second +/- is the standard deviation.

There is an uncertainty of +/-80s on the timing. The air mass was very high (>3). However the sky was clear, an Rc filter was used to minimize the atmospheric effect, and EX Hya is fairly bright, so the data should be trustworthy.

On 6 Jun 2004 observations were carried out with a C filter and 30s exposures, not shown here.

The light curves:

Red: EX Hya, Blue: the check star offset by +2mag. The error bars are +/- the 1-sigma statistical uncertainties. The eclipse is computed from the Hellier & Sproats (1992) ephemeris at 145.423.

The extra-losses during the session.