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srodney Added a section discussing the ICL
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\subsection{Intracluster Light}\label{sec:ICL}
To estimate the mass of intracluster stars along the line of sight to
the \spock events, we follow the procedure of Kelly et al. (in prep)
and Morishita et al. (in prep). This entails fitting and removing the
surface brightness of individual galaxies in the field, then fitting a
smooth profile to the residual surface brightness of intracluster
light (ICL). The surface brightness is then converted to a projected
stellar mass surface density by assuming a Chabrier
\citeyear{Chabrier:2003} initial mass function and an exponentially
declining star formation history. For further details, see Kelly et
al. (in prep). This procedure leads to an estimate for the
intracluster stellar mass of
$\log (\Sigma_{\star} / (M_{\odot}\,{\rm kpc}^{-2})) = 6.9\pm0.4$.
This is very similar to the value of $6.8^{+0.4}_{-0.3}$ inferred for
the probable caustic crossing star {\it Icarus} (Kelly et al., in
prep).
\subsection{Expected Timescale for Microlensing Events}\label{sec:Microlensing}
A commonly observed example of microlensing-induced transient effects
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velocity, the timescale $\tau$ is on the order of hours to days, which is well
matched to the observed rise and decline timescales of the \spock
events.
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side of the caustic \citep{Schneider:1986, MiraldaEscude:1991}. With
a more complex lens comprising many compact objects, the light curve
would exhibit a superposition of many such sharp peaks
\citep{Lewis:1993}.
The peculiar transient {\it Icarus}, observed
in
another of behind the Hubble
Frontier
Fields, Fields cluster MACS J1149.6+2223, has been proposed as the
first observed example of such a stellar caustic crossing event
(P. Kelly et al., in prep).
Kelly et al. find that such events may be
expected to appear more frequently in strongly lensed galaxies that
have small angular separation from the center of a massive cluster. In
such a situation, our line of sight to the lensed background galaxy
passes through a dense web of overlapping micro-lenses caused by the
intracluster stars distributed around the center of the cluster. This
has the effect of ``blurring'' the magnification profile across the
cluster critical curve, making it more likely that a single (and rare)
massive star in the background galaxy gets magnified by the required
factor of $\sim10^5$ to become visible as a transient caustic crossing
event. On this basis the \spock host galaxy images are suitably
positioned for caustic crossing transients, as they are seen through a
relatively high density of intracluster stars---comparable to that
observed for the {\it Icarus} transient (Methods \ref{sec:ICL}).
The characteristic timescale of
such a
canonical caustic crossing event
would be on the order of hours or days (Methods
\ref{sec:Microlensing}), which is comparable to the timescales
observed for the \spock events. Furthermore, since gravitational
lensing is achromatic, the color of a caustic crossing transient will
be constant. Using simplistic linear interpolations of the observed
light curves (Methods \ref{sec:LightCurves}) we find that the inferred
color curves for both \spock events are marginally consistent with
this expectation of an unchanging color (Methods
\ref{sec:ColorCurves}).
Although the phenomenology of each \spock event is compatible with a
microlensing explanation, when applying this scenario to the MACS0416
field it is still difficult to explain {\it two} events with similar
decay timescales at such distinct locations on the sky. This is
primarily because a caustic-crossing transient event must appear at
the location of the lensing critical curve.
It is possible for such
microlensing transients to appear offset from the central spine of the
critical curve, and this may in fact be observed in the case of {\it Icarus}
due to a ``blurring'' of the curve caused by the intracluster stars
It is possible to ubstantial clumping of mass along the line of sight through the lensing cluster (from the
The consensus from our
six lens models is that there is only a single critical curve passing
roughly mid-way between the two observed \spock locations.
The lens models could, however, be modified so that instead of just
two host images (11.1 and 11.2 in Figure~\ref{fig:Discovery}), the
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