MBCAA Observatory

1RXS J180834.7+101041: a new cataclysmic variable with a nearby RRAB variable

Observed: 2, 3, 5 Aug 2008

Michel Bonnardeau
3 Aug 2008
Revised (another comp) and updated (more data) 15 Aug 2008

Abstract

Time-series for this recently discovered cataclysmic variable and for a nearby RR Lyrae variable are presented.

Introduction

1RXS J180834.7+101041 (1RXJ1808+10 for short) is an X-ray source (ROSAT). In the optical range, Denisenko et al (2008) discovered eclipses with a period of 0.070037d=1.68h and they consider the object to be a cataclysmic.

Spectroscopic observations by Bikmaev & Sakhibullin (2008) showed that 1RXJ1808+10 is a UGSU or SU UMa type cataclysmic (a dwarf nova that may shows superhumps).

On the AAVSO Discussion Group, D. Denisenko (11 Aug 2008) explained that the nearby star USNO-A2 0975-10569358 is also a variable, an RRAB RR-Lyrae with an amplitude of 0.5mag and a period of 0.26384d=6.33h.

Observations

I observed 1RXJ1808+10 with a 203mm SC telescope, a Clear filter and a SBIG ST7E camera (KAF401 CCD). Each exposure was 200s long. I obtained 58 usable images.

For the differential photometry, I use USNO-A2 0975-10571346 as the comparison star with an assumed unfiltered magnitude of 14.5. For the check star I use USNO-A2 0975-10561427; I measure a magnitude of 14.245 with an average 1-sigma statistical uncertainty of 0.028 and a standard deviation of 0.035. An image is:

1RXJ1808+10 and the nearby variable have the red ticks, the comparison star the yellow square, the check star the blue ticks.

The resulting light curve:

Red: 1RXJ1808+10, Black: the nearby variable shifted by +2.0 mag, Blue: the check star shifted by +1.9mag. The error bars are +/- 1-sigma statistical uncertainties.
The sky was good but with low clouds passing by and the first images were lost.

Analysis

Denisenko et al (2008) observed an eclipse at 21h00UT on 22 May 2008, that is at JD=2,454,609.37500. Using this and the period as an ephemeris, I interpret the dips in my light curves as:

Dip (THJD) Eclipse number
since 2,454,609.37500

681.446 1029

682.355 1042

684.386 1071

Denisenko (AAVSO discussion 11 Aug 2008) observed eclipses 0.8 mag deep with a fwhm of about 3 mn. My observations do not show eclipses that deep, but this is because of my low time resolution.

The nearby variable was observed by ASAS from 2002 to 2004 (Pojmanski(2002)). A phase plot (arbitrary origin) of the ASAS V data (52 measurements) along with my data is:

Red: ASAS data (V filter), Blue: my observations (unfiltered).