This web page describes some early observations I made of transiting exoplanet XO-2b, which was made public on May 1, 2007. It can serve other amateurs wishing to start their own transit observations of this interesting solar system. 


The observations reported here are not meant to represent the work of the "extended team" (ET) of amateur observers supporting the Space Institute for Science XO Project. It's purpose is to present in one place some materials that will be helpful to any amateur interested in starting their own observations of XO-2 transits. The web page is also intended as a site where I can place additional future observations for the purpose of "data exchange" with the community of amateur observers also engaged in XO-2 observations.

My observations consist of two types: 1) all-sky photometry of the XO-2 star field, and 2) transit light curves on several occasions using different filters. The photometry section can be used to choose same-color stars for use as reference when reducing images with the differential photometry technique.

All-Sky Photometry of XO-2 Star Field

XO-2 is located at RA = 07:48:06, Decl = +50:13:33. With this RA the "observing season" for it is centered on January 20 (i.e., from October 1 to June 1). The color image below shows the XO-2 star field. 

Figure 1. Color image of the XO-2 star field, FOV = 25 x 16 'arc (crop of original 68 x 45 'arc). North-east is at upper-left. 30 "arc south of XO-2 is the binary companion; the two stars are referred to as XO-2N and XO-2S. [Meade LX200GPS, HyperStar prime focus, f/2.0, SBIG ST-8XE CCD, 2007.01.24, RGB total exposure times = 90, 80 and 70 seconds, Hereford Arizona Observatory]

In this image it can be seen that some very blue stars are present nearby. These should NOT be used for reference when doing differential photometry due to differences of extinction with air mass for stars having different colors (explained below).

The next image shows BVRcIc magnitudes for selected stars near XO-2. 

Figure 2. BVRcIc magnitudes (times 100) for stars near XO-2, based on all-sky photometry (2007.01.24)..

Figure 3. B-V star colors for stars in the XO-2 field.

Star #10 is indicated as a variable in the previous two figures. The next figure shows this for one observing date.

Figure 4. Variations of Star#10 with a sinusoidal fit.

This star should be avoided when choosing reference and check stars.

Below is a table of magnitudes and colors for XO-2N and XO-2S.

Figure 5. BVRcIc magnitudes and colors of XO-2N (left) and XO-2S (labeled "Comp"). The next-to-last column shows the number of Landolt stars used in solving telescope photometry constants on the date that the XO-2 star field was calibrated. The last column is Landolt star RMS residuals from the photometry fits.

XO-2S is slightly brighter than XO-2N.

Figure 6. V-R versus B-V scatter diagram for 853 Landolt stars (red dots), stars in the XO-2 field (diamonds), and XO-2N and XO-2S (red and blue symbols, labeled).

Figure 7. V-I versus B-V scatter diagram for 853 Landolt stars (red dots), stars in the XO-2 field (diamonds), and XO-2N and XO-2S (red and blue symbols, labeled).

The relative locations of XO-2N and XO-2S in these two color-color scatter plots shows that XO-2N is slightly redder than XO-2S. The fact that the other stars in the XO-2 field are located within the region of Landolt star colors indicates that the all-sky photometry solution is free of large systematic errors.

Figure 8. Extinction plot for B-band using 153 measurements of Landolt stars on 2007.01.07.

Figure 9. Dependence of my 14-inch telescope system's sensitivity to star color, based on 153 measurements of Landolt stars on 2007.01.07.

The previous two figures illustrate what's involved in calibrating a telescope system for use in all-sky photometry. Several Landolt star fields are observed at several air mass values in order to solve parameter values in an equation that "fits" the observations of standard stars. When this is done for all filters a set of equations, one for each filter, is produced:

The first constant (e.g., 19.30 for B-band) is a zero-shift constant that is generally constant for months at a time (unless dust accumulated on the corrector plate). The next parameter that is solved-for is a zenith extinction coefficient (e.g., 0.25 for B-band). The last parameter contains star color sensitivity information (e.g., 0.27 for B-band). C is star color, defined as V-R-0.31, or its equivalent 0.57 * (B-V) -0.30. The extinction coefficient has seasonal variations, and sometimes trends within a night's observing session.

Transit Light Curves at Several Filter Bands

This section is devoted to transit light curves taken since January, 2007.

Figure 8. B-band light curve of a complete transit made with a 32-inch telescope (the Junk Bond Observatory, a Ritchey-Chritien, f/7.2 without focal reducer, inside a dome, owned by Dave Healy). The observations were made by the author and JBO Director Dave Healy on February 3. The lower panel shows air mass (red) and atmospheric losses (blue) different from those expected for a zenith extinction of 0.25 magnitude per airmass.

Figure 9. Unfiltered observations of a complete transit made with a 14-inch telescope on 2007.02.16. XO-2S was used as the sole reference star for this differential photometry analysis.

Notice that the unfiltered transit depth is smaller than the B-band depth. This is due to limb darkening of the XO-2N star and the fact that the transit chord has an "impact parameter" (ratio of closest distance from star center to star radius) that is <~0.7.

Figure 9. Transit light curves showing the effect of choosing stars of different colors for reference. The observations are unfiltered on 2007.02.16.

This figure illustrates the importance of using only same-color stars for reference.

Transit Schedule

The exoplanet period is 2.6 days and mid-transit times can be calculated from the following:

    HJD = 2454147.74902 + 2.615838 * N

The difference between HJD and JD is less than 7 minutes so this equation will be accurate enough for the purposes of planning observations.

As a starter, the next mid-transit time is 2007.05.03, 02.64 UT. Ingress and egress are 1.34 hours before and after this time.

Related Links
    Astrophysics Abstracts (first public disclosure of XO-2):
    XO-1 observations and observing tutorial:
    My "exoplanet transit systematics" tutorial:
    Bruce's Astrophotos (with links to all my astronomy web pages):


This site opened:  May 2, 2007 Last Update:  May 2, 2007