SIMULATOR FOR MICROLENS PLANET SURVEYS Sergei Ipatov 1, Keith Horne 2, Khalid Alsubai 3, Dan Bramich 4, Martin Dominik 2,*, Markus Hundertmark 2, Christine Liebig 2, Colin Snodgrass 5, Rachel Street 6, Yiannis Tsapras 6 1) Alsubai Est. for Scientific Studies, Doha, Qatar; 2) Univ. of St. Andrews, St. Andrews, Scotland, United Kingdom; 3) Qatar Foundation, Doha, Qatar; 4) European Southern Observatory, Garching bei München, Germany; 5) Max Planck Institute for Solar System Research, Katlenburg-Lindau, Germany; 6) Las Cumbres Observatory Global Telescope Network, Santa Barbara, USA * Royal Society University Research Fellow †supported by Qatar National Research Fund (QNRF), member of Qatar Foundation (grant NPRP ) This source file for the poster can be also found on Contact:

Abstract We summarize the status of a computer simulator for microlens planet surveys. The simulator generates synthetic light curves of microlensing events observed with specified networks of telescopes over specified periods of time. The main purpose is to assess the impact on planet detection capabilities of different observing strategies, and different telescope resources, and to quantify the planet detection efficiency of our actual observing network, so that we can use the observations to constrain planet abundance distributions. At this stage we have developed models for sky brightness and seeing, calibrated by fitting to data from the OGLE survey and RoboNet observations in Time intervals during which events are observable are identified by accounting for positions of the Sun, the Moon and other restrictions on telescope pointing. Simulated observations are then generated for an algorithm that adjusts target priorities in real time with the aim of maximizing planet detection zone area summed over all the available events. The efficiency of microlens observations with a use of different telescopes is discussed. 2

Styding planets by microlensing Microlensing is unique in its sensitivity to wider-orbit (i.e. cool) planetary-mass bodies. The 15% blip lasting about 24 hrs that revealed 5-Earth-mass planet OGLE-2005-BLG- 390 impressively demonstrated the sensitivity of ongoing microlensing efforts to Super-Earths. Had an Earth-mass planet been in the same spot, it would have been detectable from a 3% signal lasting 12 hrs. The detection of less massive planets requires photometry at the few per cent level on Galactic bulge main-sequence stars, which, given the crowding levels, becomes possible with images of angular resolution below about 0.4″. 3

Detection zone at microlense observations Based on the approach presented in [1], at each time step for different events we calculate the detection zone area and the probability of detection of an exoplanet. The event with a maximum probability at a time step is chosen for observations. We define the ‘detection zone’ as the region on the lens plane (x,y) where the light curve anomaly δ(t,x,y,q) is large enough to be detected by the observations (q is the ratio of the planet to that of the star). [1] Horne K., Snodgrass C., Tsapras Y., MNRAS, 2009, v. 396, Detection zones on the lens plane indicate the regions where a planet with mass ratio q=m/M=10 -3 is detected with Δχ 2 >25. The light curve A(t) has maximum magnification A o =5, and the accuracy of the measurements is σ=(5/A 1/2 ) per cent. 4

Maximizing planet detection zone area The photometric S/N (signal to noise) ratio and hence the area w of an isolated planet detection zone scales as the square root of the exposure time : S/N = (Δt /τ) 1/2, w = g Δt 1/2. Here τ is the exposure time required to reach S/N=1. The 'goodness' g i of an available target depends on the target's brightness and magnification, the telescope and detector characteristics, and observing conditions (air mass, sky brightness, seeing). The simulator evaluates 'goodness' of available targets in real time, and observes the one offering the greatest increase in w with exposure time. Moves to a new target occur when the increase in w for the new target is better than the current target, accounting for the slew time required to move to the new target. As the CCD camera takes a finite time t read to read out, and the telescope takes a finite time t slew to slew from one target and settle into position on the next, the on- target exposure time accumulated during an observation time t is Δt=t- t slew –n t read (t slew ~ s, t read ~10-20 s). At a time step Δt the detection area of i-th event increases by g i [(Δt+t done ) 1/2 - t done 1/2 ], where t done is an exposure time already has been done. For a new target, the area is g i (Δt-t slew ) 1/2. For a choice of a best event, we compared g i [(t plan +t done ) 1/2 - t done 1/2 ] for a current target with g i [t slew 1/2 ] for a new target, where t plan =2t slew. All but one of the targets require slew time before the exposure can begin. See [1] for details. [1] Horne K., Snodgrass C., Tsapras Y., MNRAS, 2009, v. 396,

Target observability The observability of a target is limited by its own position on the sky, as well as that of the Sun and the Moon, and telescopes moreover have pointing restrictions. Taking the LT (from as example, we particularly require: Air mass of target > 3 or Cos (zenith of the Sun) < sin (-8.8 o ) or Altitude of a target: alt altmax=87 o or Hour angle: ha hamax. For LT there are no limits on ha: hamin=-12 h and hamax=12 h. For 1-m telescopes, hamin=-5 h and hamax=5 h. Telescopes considered: 1.3m OGLE - The Optical Gravitational Lensing Experiment - Las Campanas, Chile. 2m FTS - Faulkes Telescope South - Siding Springs, Australia. 2m FTN - Faulkes Telescope North - Haleakela, Hawaii. 2m LT - Liverpool Telescope - La Palma, Canary Islands. Three 1m CTIO - Cerro Tololo Inter-American Observatory in Chile. Three 1m SAAO - South African Astronomical Observatory. Two 1m SSO - Siding Spring Observatory near Coonabarabran, New South Wales, Australia. 1m MDO - McDonald observatory in Texas. 6

Observations analyzed for construction of sky model: For studies of sky brightness for FTS, FTN, and LT, we considered those events observed in 2011 for which.dat files are greater than 1 kbt: FTS - 39 events; FTN - 19 events, LT – 20 events. For OGLE we considered 20 events ( ). Calculations of I sky (0) and the coefficients (k 1 and k o ) presented in the tables and on the plots were based on χ 2 optimization of the straight line fit (y=k 1 ·x+k o, χ 2 =∑[(y i -k 1 ·x i -k o )/σ i ] 2, σ i 2 is variance). The value of I sky (0) (sky brightness at zenith) was chosen in such a way that the sum of squares of differences between observational and model sky brightness magnitudes were minimum in the case when the Moon is below the horizon. The used sky model was based mainly on [2] K. Krisciunas & B. Schaefer, 1991, PASP, v. 103,

Dependences of seeing on air mass and values of sky brightness at zenith obtained based on analysis of observations Values I sky (0) of sky brightness at zenith (I magnitude per square arcsec) and b o in the χ 2 optimization b=b 1 ·a+b o (where a is air mass) for the Moon below the horizon at throughput t hruput =0.324 and an extinction coefficient e xtmag =0.05 (for e xtmag equal to 0 and 0.1, values of I sky (0) differed by less than 0.3%; I sky (0) is one of parameters of the model used): 9 TelescopeFTSFTNLTOGLE I sky (0) b0b TelescopeFTSFTNLTOGLE ko ko k1 k σ Seeing (FWHM in arcsec) vs. air mass (χ 2 optimization): seeing=k o +k 1 ×(airmass-1)

Seeing (in arcsec) vs. air mass. FTS observations of 39 events. A thick straight line is based on χ 2 optimization (y= k o + k 1 (x-1), k o =1.334, k 1 =0.519). Thinner straight lines differ from this line by +/- Ϭ ( Ϭ =0.367). Non-straight lines show mean and median values (the line for the mean value is thicker). 10

Sky brightness (mag) vs. air mass for Moon below the horizon. Different points are for OGLE observations of 20 different events. The lines are for the χ 2 optimization (b=b 1 ·a+b o ) with different b o (different values for different events) and the same b 1. The most solid line is for the model for which b o is the same for all events. For Moon below the horizon, the values of b o (which characterize sky zenith brightness near different events) differ typically but not more that 1 mag. 11

Sky brightness residuals (mag) vs. air mass for the model with different b o for FTS observations of 39 events. Left plot is for all positions of the Moon and the Sun. Right plot is for the Moon below the horizon and solar elevation < - 18 o. 12

Sky brightness residuals vs. solar elevation The influence of solar elevation on sky brightness began to play a role at θ Sun >-14 o, and was considerable at θ Sun >-7 o. For example, if we consider only FTS observations with the Moon below the horizon, then sky brightness residual s br can be up to -3 mag at -8 o -1 mag at θ Sun -0.4 mag at θ Sun <-14 o. Sky brightness residuals (in mag.) vs. solar elevation for FTS observations of 39 events. Red signs are for the Moon below the horizon. 13

Time intervals when it is better observe events Time intervals for events selected for observations with OGLE (at actual times of peaks of light curves). Considered events:

Light curves (with error bars) for events selected for observations with OGLE (at actual times of peaks of light curves). Considered events:

Comparison of the efficiency of telescopes for microlense observations Our simulator suggests what events it is better to observe at specific time intervals with a specific telescope in order to increase the probability of finding new exoplanets using microlense observations. For estimates of the probability, for best events we considered w sum =∑ g i [(Δt+t done ) 1/2 - t done 1/2 ] (where Δt=2t slew for an event observed at a current time, and Δt=t slew and t done =0 for other events) and r wsumt =(w sum /w sumOGLE )/(t sum /t sumOGLE ), where t sum is the total time during considered time interval when it is possible to observe at least one event. For best events we also calculated w sumo =∑ g i [(t s +t done ) 1/2 - t done 1/2 ] and r wsumto =(w sumo /w sumoOGLE )/(t sum /t sumOGLE ) with t s =20 s. For a typical run, the value of t sum /t sumOGLE is about 0.66, 0.56, and 0.62 for FTN, LT, and MDO, respectively, but it can vary for different considered time intervals. For other considered sites, the ratio differed from 1 by less than 0.1. In our calculations, r wsumt and r wsumto for observations with a 1-m telescope (located at CTIO, SAAO, SSO, or MDO) equipped with the Sinistro ccd, and with a 2-m telescope (FTS, FTN, or LT) were mainly in the range and of that for OGLE, respectively (see the plot below). The ratio of w sum for FTS and SSO located at the same site usually was about 2. For the SBIG ccd, the values of w sum (and w sumo ) were smaller by a factor of ~1.2 than those for the Sinistro ccd. The difference in w sum is about 5% if for SSO we use the values of I sky (0) and the dependence of seeing vs. air mass as those for OGLE, compared to those for FTS. 16

The values of r wsumt =(w sum /w sumOGLE )/(t sum /t sumOGLE ) (the left plot) and r wsumto =(w sumo /w sumoOGLE )/(t sum /t sumOGLE ) (the right plot), which characterize the efficiency of microlense observations, vs. the number N t of a telescope in the case when 1-m telescopes (equipped with the Sinistro ccd) located at the same site observe different events at the same time. See details on the next slide. 17

Designations to the above plot: The values of r wsumt =(w sum /w sumOGLE )/(t sum /t sumOGLE ) (the left plot) and r wsumto =(w sumo /w sumoOGLE )/(t sum /t sumOGLE ) (the right plot), which characterize the efficiency of microlense observations, vs. the number N t of a telescope in the case when 1-m telescopes (equipped with the Sinistro ccd) located at the same site observe different events at the same time. Considered time interval equals T=90 days. The signs for calculations with actual values of t 0 (the time corresponding to the peak of a light-curve) and with random values of t 0 (t 0 = R NDM ∙(t mx +2t E )- t E +t o, where t E is the time scale equal to the ratio of the angular Einstein radius to the relative proper motion, R NDM is a random value between 0 and 1, t mx is the duration of the considered time interval, t o is the beginning of the interval) for 1562 events are black and red, respectively. Crests are for the time interval beginning from May 2, circles are for the interval beginning from August NtNt FTSFTNLTOGLECTIOMDOSAAOSSO

Comparison of the efficiency of 2-m telescopes with that of OGLE for a search of exoplanets OGLE observed a little more than 200 galactic bulge fields. For 1500 events in 2011, it means that typically there could be ~10 events in one field (the ratio 1500/200 is 7.5). For telescopes other than OGLE (exclusive for LT), typically there can be only one event in the field of view. In our calculations for 1562 events (and 90 days time interval), the value of w sum (or w sumo ) for the best events chosen for observations was usually considerably greater than for typical 10 other events (which are not the best at the current moment of time). Therefore, we can use the values of w sum (or w sumo ) calculated only for the best events for comparison of the efficiency of different telescopes for a search of new exoplanets. Of course, telescopes with a wider field of view are more effective for a search of new events. Nevertheless, the obtained results show that, for a search of exoplanets based on already discovered events, a 2-m LCOGT telescope on average is more effective (per unit of time of observations) than OGLE and the efficiency of a 1-m telescope with Sinistro ccd can be close to that of OGLE, as the ratio of values of w sum (or w sumo ) per unit of time for the 2-m telescopes to that for OGLE usually is in the range of , and that for 1-m telescopes is mainly in the range of

Comparison of the efficiency of observations with the use of 1-m telescopes located at the same site. In our test calculations, the values of w sum (and w sumo ) at diameter of a telescope equal to 2 m were usually greater by a factor of than those for 1-m telescope if the difference was only in the diameter d of a telescope. Such factors correspond to s s 0.4, there s is the effective area of a telescope. For two or three telescopes located at the same site for observations of the same event, the ratio of w sum (or w sumo ) to that for one telescope is about =1.25 to =1.32 or about =1.4 to =1.55, respectively. Analysis of our calculations shows that most of the time it is better to observe different events using the telescopes located at the same site than to observe the same event with two or three telescopes, but at a time close to a light-curve peak often it is better to observe the same event with all telescopes located at the same site. For 1562 events during 90 days (or larger) interval, a considerable (up to more than ½) contribution to wsum was during short time intervals corresponding to peaks of light curves, if this telescope is allowed to observe all events. 20