AXA Light Curves
to this web page
Summary of transits
Professional transit LCs
Amateur transit LCs
Comments on Transit LCs
It is now well-establisehd that the flurry of excitement
about a possible second planet in the GJ 436 system as suggested
by Ribas et al (2008a) has subsided dur to additional observations
and analyses. Alonso et al (2008) simulated a range of hypothetical
orbital parameters for "c" and imposed new observational constraints
to show that it is very unlikely that "c" could exist without greater
observational anomalies. At the 2008 IAU meeting in Boston Ribas et
al (2008b) retracted the suggestion that "c" should be inferred on
the basis of known observations. Prior to this retraction the TTV (transit
timing variations) observations on this web page were interpreted as
supporting "c" but at the suggestion of McLaughlin (private communication,
January 2008) the TTV plot was viewed as merely evidence for the need
of a period adjustment. As more observations were added to this web page
it became clear that this interpretation was the correct one. For awhile,
at least, there were grounds for observing GJ 436b intensively in order
to refine the TTV plot; this served to "educate" the amateur community
on a role we could play in exoplanet studies.
The star is red and
all nearby reference stars are bluer, so all LCs will
have a large "air mass curvature" systematic error (such that
the star appears brighter at higher air mass). This will be
especially pronounced for unfiltered observations. It's best to
avoid observing when air mass >1.5. V-band should reduce this
effect, R-band will be slightly worse (but will have better SNR),
and both unfiltered and BB-band (blue-blocking) will have a large
systematic effect. Since the depth is small the amateur observations
will be "noisy." If the systematics are uncorrelated then it's
legitimate to average (or even better, median combine) the LC
parameters derived from the amateur measurements. "Eyeball" averages
can be done using the plots, below. Eventually chi-squared solutions
may be justified.
RA = 11:42:11.1, DE = +26:42:24
Season = Mar 15
V = 10.68, B-V = 1.5
2 (very red)
My estimates (based on one night's all-sky photometry,
2008.02.12): B = 12.28 ± 0.04, V = 10.64 ±
0.03, R = 9.69 ± 0.03, I = 8.26 ± 0.03
Discovery Paper (Gillon et al, 2007):
HJDo = 4222.616
P = 2.64385 (9) day
Depth = ~6.5 ± 1.0
Length = not published (my measurement of the published
LC yields 0.94 ± 0.06 hr)
HST-based (Bean et al, 2008)
HJDo = 4455.278241 (26)
P = 2.643904 (5) days
Infrared (8 micron,
SST obsns by Gillon et al, 2007; 2-planet analysis
by Ribas et al, 2008):
HJDo = 1551.78 (5) which must be a typographical
P = 2.64384 (5) days
Depth = 7.4 ± 0.2 mmag
Length = not published (my measurement of the published
LC yields 0.977 ± 0.005
Schneider's Extrasolar Planets Encyclopaedia listing (http://exoplanet.eu/catalog-transit.php):
HJDo = 4280.78148 (15) & P = 2.643904
(5) day (using amateur & professional observations)
HJDo = 4280.78238 (19) & P = 2.6438956 (6) day (using amateur
& professional observations)
Depth = 7.5 ±
0.2 mmag (average of V, R, BB, C)
Length = 0.97 ± 0.02 hr
(average of V, R, BB, C)
Fp = 0.48 ± 0.08, F2 =
0.88 ± 0.10
Summary of Transits
Amateur observations. (Downloading data files
is unrestricted, but when use in a publication is underway please
notify observer either directly or via webmaster. More description
Transit depth and length appear to be constant, so
far. Median depth = 7.5 ± 0.3 mmag, length
= 0.97 ± 0.03 hour.
So far there's no
evidence for a dependence of transit depth on filter
band (which surprises me, given that this is a grazing
transit). (Recent data has not been plotted.)
Amateur Transit Light Curves
9130MXI1 Long run after egress
provides a very good check of stability and OOT model fit; good job!
Three LC versions of the same data. The bottom one is
the elast reliable because it was produced by an automatic reduction
program that still has "bugs."
Cloudy near end.
Windy most of the session.
First submission by Miguel Rodriguez,
using a 6-inch Newtonian tlescope. Good agreement with
consensus though a little noisy at high air mass end of observations.
8423nave (waiting for permission)
We're puzzled by the late arrival of this LC. Jim says the
clock was checked and there were no saturation issues. This
is the only B-band LC for GJ 436 on the AXA.
8325nave (waiting for permission)
8322gary (data lost due to computer hard disk
8317nave (waiting for permission)
permission) First submission; congratulations! Great job with
an 8-inch aperture.
8301nave (waiting for permission)
Apologies for including this noisy egress only LC.
permission) First submission by this observer. Note the quality
and 8-inch aperture.
8217mrch (need permission)
8214wltr (need permission) First-time submission;
congraulations! LC shape and timine are consistent
with the consensus.
8208mend Amazingly good quality LC, especially
for an 8-inch and first-time contributor.
8208greg The early data had too long exposure
times that saturated stars; when exposure was shortened
behavior improved. Note that the egress was "late" by
approximately the same amount as indicated by the Staels observations
of the same transit.
8208stae (need permission) This LC "confirms"
the recent trend to "lateness" - which is important
evidence for a perturbing exoplanet proposed by Ribas et
al, 2008. The high dependence upon air mass is due to observations
being unfiltered and GJ 436 being very red and all reference
stars being bluer.
8206gary Frequent uses of a hair dryer to remove
frost from the corrector plate; temp = 28 F, Dew Pt
= 22 F (RH = 78%). WWV check of time tags.
8201nave (waiting for permission)
8124srdc The clock was checked & found to
7c31gary Air mass curvature is high due to use
of a BB-filter and high air mass at the beginning.
7531gary Mid-transit = 4251.6992 (JD) = 4251.6997
(HJD). Depth = 8.5 mmag (R). Length = 0.78 hr.
7517vanm Good quality.
The closest transit was at 23.06 UT on 2008.03.24
(i.e., 8.9 hrs before mid-observing session).
These observations were a test
of a new optical configuration which accounts for the
short duration. Nevertheless, there seems to be mild evidence
for 1 mmag variations on an hourly timescale.
et al (2007) SST observations at 8 micron wavelength,
reproduced from Ribas et al (2008). Lowest panel shoes
effect of a hypothetical 0.1 degree inclination change
that could be produced by perturbations from a 5-Earth mass
outer orbit planet in a 2:1
Observatory of Geneva 1.2-meter Euler telescope at
La Silla Observatory, Chile (Gillon
et al, 2007). Mid-transit
at 2007 May 02, 02:41 UT. My measurements of this LC yield
depth ~6.5 ± 1.0 mag, length = 0.943 ± 0.064 hr.
Finder image with identifier star numbers (above)
and J-K colors (times 100, below) selected stars. GJ 436 has
V = 10.68 and Rc = 9.66.
This image has FOV = 16 x 11 'arc and FWHM ~2.5 "arc.
Tentative magnitudes for these stars is summarized in the
On the night of
2008.02.12 UT I conducted all-sky photometry measurements
of the GJ 436 region. I used the Landolt star field at RA/DE
06:52/-00:27, which also has an extensive list of Henden all-sky
observational results for many more stars than in the Landolt
list. The Landolt field was observed before and after GJ 436,
all at the same air mass. My recent experience with landolt stars
is that some of them have changed over the decades, and not
enough of them have masgnitudes for all four bands. Further, my
culled list of Henden magnitudes (=> 4 observations per star) shows
better internal consistency than the Landolt magnitudes. So for
this all-sky analysis I adopted only Henden magnitudes for calibrating
my telescope system and transferring this calibration to the GJ
436 star field. A fuller discussion of my all-sky photometry observing
and analysis procedures are available at a web page (still under
The following table
is based on 18 Henden stars (as many as 53 readings
per band) for this one night's observations:
Results of the all-sky photometry measurements of 2008.02.12.
Star numbers correspond to the labels in Fig. F2.
Normally I don't present all-sky photometry results without completing
a second observing session and analysis and verify
compatibility between the two results. In this case
I have no plans for doing this since I doubt that anyone
will use any of these results.
My favorite "reality
check" for detecting the presence of a systematic error
for one or more bands after performing an all-sky photometry
session is to plot color/color scatter diagrams.
Figure A2 and A3. Color/color scatter diagram showing the location
of GJ 436 (red square) and the 9 nearby stars (gray
squares) in relation to 1259 landolt stars.
If the solution for one filter band had a systematic error it would show
up in these plots as a group offset in the color/color
scatter diagrams involving that filter. For example,
if all B-magnitudes were high by 0.05 magnitude then the
goup of gray squares and the red square would be offset to
the right by 0.05 magnitude. It's possible that such an offset
is present in Fig. A2, but it's clear that greater vaues fo such
an offset are very unlikely. An alternative for explaining the
rightward shift of Fig. A2 gray squares is for there to be instead
a downward shift, or values for V that are to negative by about the
same 0.05 magnitude amount. This is unlikely after inspecting Fig.
A3, where there is no evidence of shifts. Presumably, all 3 bands
(V, R and I) are free of calibration error offsets (unless by some
unlikely circumstance there are offset errors in all 3 bands that excatly
compensate to produce color/color agreement with the Landolot stars).
If it is true that V, R and I are free of calibration offset errors
greater than ~0.03 magnitude, then what should be make of the funny
location for GJ 436 in Fig. A3? I claim that it is inescapable that
GJ 436 has a much greater V-I color than the Landolt stars. Since V
appears to be normal (e.g., Fig. A2), we must conclude that I is anomalous.
In other words, these color/color scatter diagrams show that GJ 436
has an I-magnitude that is brighter than normal by ~0.5 magnitude!
I'll leave it to others to explain how this could be the case.
Note: It won't matter what magnitude you assume for reference stars for
the purpose of obtaining quality light curves. These
estimated values are presented for the purpose of
identifying star colors that "match" GJ 436's color which
can be useful in minimizing extinction related systematic
errors (i.e, LC curvature that's correlated with air mass).
In this table the column for R-band will be the most accurate
since it is based on observations. The other magnitudes for Stars
1 through 6 are based on JK magnitudes. For GJ 436 the B and I magnitudes
are based on color/color correlations for main sequence stars.
All stars in the table are compatible with main sequence color/color
Gillon et al, 2007, Astron, & Astrophys., "Detection of Transits
of the Nearby Hot Neptune GJ 436" http://babbage.sissa.it/abs/0705.2219
Butler et al, 2004,
Astrophys. J. Lett.,"A Neptune-Mass Planet Orbiting
the Nearby M Dwarf GJ 436" http://adsabs.harvard.edu/abs/2004ApJ...617..580B
Ribas et al, 2008a,
Astrophys. J. Lett., "A ~5_earth Super Earth
Orbiting GJ 436?: The Poser of Near-Grazing Transits"
Ribas et al, 2008b, IAU 253, Boston, MA, 2008
Alonso et al, 2008, "Limits to the planet candidate
GJ 436c" http://arxiv.org/abs/0804.3030
Bean et al, 2008, arXiv:0806.0851v2, http://arxiv.org/abs/0806.0851
Coughlin et al, 2008, preliminary
Batygin et al, 2009, "A Quasi-Stationary Solution to Gleise 436b's Eccentricity,"
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L. Gary. Nothing on this web page is copyrighted. This site opened:
July 04, 2007. Last Update: 2009.08.02