Subjects: Astronomy >> Astrophysical processes submitted time 2023-02-19
Patrick L. Kelly
Wenlei Chen
Amruth Alfred
Thomas J. Broadhurst
Jose M. Diego
Najmeh Emami
Alexei V. Filippenko
Allison Keen
Sung Kei Li
Jeremy Lim
Ashish K. Meena
Masamune Oguri
Claudia Scarlata
Tommaso Treu
Hayley Williams
Liliya L. R. Williams
Rui Zhou
Adi Zitrin
Ryan J. Foley
Saurabh W. Jha
Abstract： Once only accessible in nearby galaxies, we can now study individual stars across much of the observable universe aided by galaxy-cluster gravitational lenses. When a star, compact object, or multiple such objects in the foreground galaxy-cluster lens become aligned, they can magnify a background individual star, and the timescale of a magnification peak can limit its size to tens of AU. The number and frequency of microlensing events therefore opens a window into the population of stars and compact objects, as well as high-redshift stars. To assemble the first statistical sample of stars in order to constrain the initial mass function (IMF) of massive stars at redshift z=0.7-1.5, the abundance of primordial black holes in galaxy-cluster dark matter, and the IMF of the stars making up the intracluster light, we are carrying out a 192-orbit program with the Hubble Space Telescope called "Flashlights," which is now two-thirds complete owing to scheduling challenges. We use the ultrawide F200LP and F350LP long-pass WFC3 UVIS filters and conduct two 16-orbit visits separated by one year. Having an identical roll angle during both visits, while difficult to schedule, yields extremely clean subtraction. Here we report the discovery of more than a dozen bright microlensing events, including multiple examples in the famous "Dragon Arc" discovered in the 1980s, as well as the "Spocks" and "Warhol" arcs that have hosted already known supergiants. The ultradeep observer-frame ultraviolet-through-optical imaging is sensitive to hot stars, which will complement deep James Webb Space Telescope infrared imaging. We are also acquiring Large Binocular Telescope LUCI and Keck-I MOSFIRE near-infrared spectra of the highly magnified arcs to constrain their recent star-formation histories.
Peer Review Status:
Awaiting Review
Subjects: Astronomy >> Astrophysical processes submitted time 2023-02-19
Ashish Kumar Meena
Wenlei Chen
Adi Zitrin
Patrick L. Kelly
Miriam Golubchik
Rui Zhou
Amruth Alfred
Tom Broadhurst
Jose M. Diego
Masamune Oguri
Liliya L. R. Williams
Alexei V. Filippenko
Sung Kei Li
Abstract： We report the discovery of a transient seen in a strongly lensed arc at redshift $z_{\rm s}=1.2567$ in \emph{Hubble Space Telescope} imaging of the Abell 370 galaxy cluster. The transient is detected at $29.51\pm0.14$ AB mag in a WFC3/UVIS F200LP difference image made using observations from two different epochs, obtained in the framework of the \emph{Flashlights} program, and is also visible in the F350LP band ($m_{\rm F350LP}\sim30.53$ AB). The transient is observed on the negative-parity side of the critical curve at a distance of $\sim 0.6''$ from it, greater than previous examples of lensed stars. The large distance from the critical curve yields a significantly smaller macro-magnification, but our simulations show that bright, O/B-type supergiants can reach sufficiently high magnifications to be seen at the observed position and magnitude. In addition, the observed transient image is a trailing image with an observer-frame time delay of $\sim+0.8$ days from its expected counterpart, so that any transient lasting for longer than that should have also been seen on the minima side and is thus excluded. This, together with the blue color we measure for the transient ($m_{\rm F200LP} - m_{\rm F350LP} \sim [-0.7,-1]$ AB mag), rules out most other transient candidates such as (kilo)novae, for example, and makes a lensed star the prime candidate. Assuming the transient is indeed a lensed star as suggested, many more such events should be detected in the near future in cluster surveys with the \emph{Hubble Space Telescope} and \emph{James Webb Space Telescope}.
Peer Review Status:Awaiting Review
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