Co-discovery of an exoplanet transiting XO-1

Early September 2004, I was the first amateur astronomer to detect a transit of exoplanet TrES-1b, after its discovery announcement by professional astronomers. My observations attracted quite some attention (see e.g., Sky & Telescope). One week after my TrES-1 observations, I was contacted by astonomer Dr. Peter McCullough from the Space Telescope Science Institute STScI, who is the principal investigator of the NASA-funded XO project, a photometric wide-field search for Jovian planets transiting bright stars. Peter invited me to join the project for photometric follow up observations of suspected targets (see below).

The Extended Team of the STScI XO Project

The XO Mark I observatory is located at Haleakala (Hawaii), and hosts twin 200mm Canon EF200 lenses, coupled with Apogee Ap8p CCD detectors, and mounted on a Paramount ME equatorial mount. Each detector covers a 7.2 degrees field on the sky. It monitors tens of thousands bright stars (V < 12) twice every 10 minutes on clear nights, for over 2 months per season of visibility for each particular star. It operates since September 2003. 

Following my initial contacts with Peter McCullough, a team of 4 amateur astronomers was established, with as a main mission to assist with the photometric follow up of candidates identified in the XO data. Peter denoted this team the XO Extended Team (E.T.). Members are Bruce L. Gary, Peter J. Howell, Ron Bissinger and myself. We all have previous experiences with CCD photometry of exoplanets.

Candidate 240q31.634

In the months following my enrollment in the E.T. team, I started monitoring several XO targets of opportunity (TOO), that were selected by Peter McCullough and his colleagues. Many of them turned out to be false positives, or regular (unknown) variables. One of the TOO's had code name 240q31.634, and was intensively observed by our E.T. team in June and July 2005. It quickly revealed a 'transit-like' signature (steep ingress and egress, flanking a relatively wide flat bottom), and evidently became one of the prime targets of our observing campaigns.

Unfiltered observations of XO-1, obtained at CBA Belgium Observatory on 2005, Jul 12/13,
using a 0.35-m f/6.3 telescope and ST-7XME CCD camera.
The light curve shows part of a transit (ingress and flat bottom).
Each dot in the light curve is an average of 3 observations. Error bars depict the standard deviation.

On the basis of the E.T. light curves of 2005, Peter and his colleagues acquired spectra of 240q31.634, followed by additional photometry using a 1.8-m telescope at Lowell Observatory, and using the E.T. telescopes. Combining all available data, the XO team concluded that 240q31.634 indeed was a star hosting a transiting extrasolar planet, and the star was designated XO-1. The discovery of exoplanet XO-1b finally was announced in the Astrophysical Journal. It clearly is the direct result of a productive international collaboration between professional and amateur astronomers. It is only the second reported discovery of an exoplanet from a telephoto-based wide-field survey, and the tenth planet discovered using the transit method.

Below are multi-color (V and R) observations of XO-1, that I obtained on 2006, Apr 06/07 at CBA Belgium Observatory, by simultaneously using two 0.35-m f/6.3 telescopes, each equipped with a ST-7XME CCD detector. The similarity between the V and R light curve is obvious, except for the egress part. It currently is unclear if this difference is to be explained by a (so far) unknown physical characteristic of XO-1, or if it's some kind of observational artefact.

Observations of XO-1 obtained in V (black dots) and R (red dots) at CBA Belgium Observatory
on 2006, Apr 06/07, using two 0.35-m f/6.3 telescopes and ST-7XME CCD detectors.


XO-1b properties

XO-1 is a Sun-like G1V star in Corona Borealis, also known as GSC 02041-01657. It's an 11-th magnitude star at RA = 16h02m11.83s and Decl = +28°10'11.27" (J2000.0). Of the ten stars currently known to host extrasolar transiting planets, XO-1 is the most similar to the Sun in its physical properties. It has a radius of 1.0 +/- 0.08 solar radius, and a mass of 1.0 +/- 0.03 solar mass. The orbital period of the planet XO-1b is 3.941534 +/- 0.000027 days, and the mass is 0.90 +/- 0.07 Jupiter's mass.

Websites of the XO-1b discoverers  (Peter McCullough, STScI) (Johns-Krull, Rice University) (Kenneth Janes, April Pinnick & Paul Howell, Boston University) (Peter McCullough & James Heasley, STScI; Bill Giebink, Les Hieda, Jake Kamibayashi, Daniel O’Gara, and Joey Perreira, Univ. Hawaii staff) (Bruce L. Gary) (Ron Bissinger) (Paul Howell)





Copyright © 2006 - Tonny Vanmunster.