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Gaia16aye is a binary microlensing event and is crossing the caustic again

ATel #9507; L. Wyrzykowski (Warsaw University Observatory), G. Leto (INAF OACT), G. Altavilla (Bologna), V. Bakis (Akdeniz Univ.), N. Britavskiy (Odessa), U. Burgaz (Ege Univ.), T. Butterley (Durham Univ.), J. M. Carrasco (Institut de Ciencies del Cosmos-Univ. of Barcelona), V. S. Dhillon (Univ. of Sheffield), M. Dominik (Univ. St Andrews), A. Gomboc (Nova Gorica), L. K. Hardy (Univ. of Sheffield), S. P. Littlefair (Univ. of Sheffield), J. R. Maund (Univ. of Sheffield), A. Piascik (Liverpool JMU), L. Rhodes (Univ. of Sheffield), R. Z. Sanchez (INAF OACT), K. V. Sokolovsky (IAASARS NOA/SAI/ASC Lebedev), I. Steele (Liverpool JMU), R. W. Wilson (Durham Univ.), A. Hamanowicz, P. Mroz, M. Pawlak, K. Rybicki, M. Sitek (Warsaw), P. Mikolajczyk, Z. Kolaczkowski (Wroclaw), R. Street (LCOGT), P. Bendjoya (C2PU-Omicron), V. Bozza (Salerno), J. Dziedzic, K. Niczyj, R. Nowicki, M. Porebski, (KFnRD)
on 19 Sep 2016; 22:03 UT
Credential Certification: Lukasz Wyrzykowski (wyrzykow@astrouw.edu.pl)

Subjects: Microlensing Event, Transient

Referred to by ATel #: 9533, 9753, 9770, 9780

Galactic Plane (Cygnus) transient, Gaia16aye, nicknamed Ayers Rock (19:40:01.13 +30:07:53.4, J2000), was discovered by Gaia Science Alerts, http://gsaweb.ast.cam.ac.uk/alerts/alert/Gaia16aye, on 2016-08-05 as a flare on an otherwise quiet star. Since then, it has been followed-up intensively by the network of telescopes coordinated under OPTICON Time Domain Work Package, http://www.astro-opticon.org/. The second brightening was detected as reported by Bakis et al. (ATEL #9376).

Here we report on the third brightening episode in Gaia16aye, which we interpret as a signature of a binary microlensing event, the first ever discovered towards the Galactic Plane. The multi-filter observations since early August show achromatic changes in brightness, consistent with microlensing scenario. The shape of the first flaring resembles a caustic-exit with fairly broad finite source effect. Then, the brightness of the object increased very gradually (around 0.1mag in a month) and on 2016-09-17 it increased sharply by 2 mag (first spotted by the APT2 telescope), indicating a second caustic entry. The source reached 12 mag in the I-band. The lightcurve in i and V filters is shown here, http://www.astrouw.edu.pl/~wyrzykow/Gaia16aye-atel.png.

The expected behaviour of such lensing system is that the source will decline quickly in next days to reach the plateau between two caustics, in order to start rising again to cross the caustic again in one or couple of weeks. The source will again reach about I=12 mag for about a day and then will sharply decline and the event will be over forever.

While we are conducting the observations with primarily European-based telescopes (over 2500 photometric observations were collected so far), we strongly encourage the astronomical community from around the globe (from Northern Hemisphere) to join the observations in order to catch the fourth flare, i.e., the second caustic exit and to provide detailed light curve of the caustic crossing. This is necessary in order to determine the angular size of the source and the mass of the lensing system.

We emphasise that the source is going to be very bright in next week or so and will be accessible also by small-aperture telescopes. This could make also a great opportunity for amateurs and schools to join the observations and provide scientifically valuable data.

Spectral observations are also encouraged in order to determine the unknown properties of the source and lens.

The following observatories contributed significantly to the follow-up of Gaia16aye: APT2(Catania,Italy), Bialkow(Wroclaw,Poland), Loiano(Italy), LCOGT-net, Liverpool Telescope (La Palma,Spain), pt2m(La Palma,Spain), Obs.Montsec(Lleida, Spain), OmicronC2PU(France), Ostrowik(Warsaw,Poland),UBT60(Turkey).
The photometric calibrations were obtained in an automated fashion using the Cambridge Photometric Calibration Server (CPCS, http://gsaweb.ast.cam.ac.uk/followup/ ), designed and maintained by Sergey Koposov and Lukasz Wyrzykowski. We acknowledge ESA Gaia, DPAC and the Photometric Science Alerts Team.

Binned light curve in i and V bands of Gaia16aye.