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+++ b/bibliography/biblio.bib
...
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@article{Umetsu:2016,
Author = {{Umetsu}, K. and {Zitrin}, A. and {Gruen}, D. and {Merten}, J. and {Donahue}, M. and {Postman}, M.},
Journal = {\apj},
Month = apr,
Pages = {116},
Title = {{CLASH: Joint Analysis of Strong-lensing, Weak-lensing Shear, and Magnification Data for 20 Galaxy Clusters}},
Volume = 821,
Year = 2016}
@article{Umetsu:2014,
Author = {{Umetsu}, K. and {Medezinski}, E. and {Nonino}, M. and {Merten}, J. and {Postman}, M. and {Meneghetti}, M. and {Donahue}, M. and {Czakon}, N. and {Molino}, A. and {Seitz}, S. and {Gruen}, D. and {Lemze}, D. and {Balestra}, I. and {Ben{\'{\i}}tez}, N. and {Biviano}, A. and {Broadhurst}, T. and {Ford}, H. and {Grillo}, C. and {Koekemoer}, A. and {Melchior}, P. and {Mercurio}, A. and {Moustakas}, J. and {Rosati}, P. and {Zitrin}, A.},
Journal = {\apj},
Month = nov,
Pages = {163},
Title = {{CLASH: Weak-lensing Shear-and-magnification Analysis of 20 Galaxy Clusters}},
Volume = 795,
Year = 2014}
@article{Fregeau:2004,
Author = {{Fregeau}, J.~M. and {Cheung}, P. and {Portegies Zwart}, S.~F. and {Rasio}, F.~A.},
Journal = {\mnras},
diff --git a/spock_natureastronomy_finalsubmission.tex b/spock_natureastronomy_finalsubmission.tex
index ef5d9c1..06bcf59 100644
--- a/spock_natureastronomy_finalsubmission.tex
+++ b/spock_natureastronomy_finalsubmission.tex
...
\newcommand{\EHU}{Fisika Teorikoa, Zientzia eta Teknologia Fakultatea, Euskal Herriko Unibertsitatea UPV/EHU}
\newcommand{\Basque}{IKERBASQUE, Basque Foundation for Science, Alameda Urquijo, 36-5 48008 Bilbao, Spain}
\newcommand{\Berkeley}{Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA}
\newcommand{\Miller}{Miller Senior Fellow, Miller Institute for Basic Research in Science, University of California, Berkeley, CA 94720, USA}
\newcommand{\STScI}{Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218, USA}
\newcommand{\Ferrara}{Dipartimento di Fisica e Scienze della Terra, Universit\`{a} degli Studi di Ferrara, via Saragat 1, I-44122, Ferrara, Italy}
\newcommand{\INAF}{INAF, Osservatorio Astronomico di Bologna, via Ranzani 1, I-40127 Bologna, Italy}
...
G.~B.~Caminha\altaffilmark{\affilref{Ferrara}},
G.~Chiriv{\`i}\altaffilmark{\affilref{MPIA}},
J.~M.~Diego\altaffilmark{\affilref{IFCA}},
A.~V.~Filippenko\altaffilmark{\affilref{Berkeley}}, A.~V.~Filippenko\altaffilmark{\affilref{Berkeley},\affilref{Miller}},
R.~J.~Foley\altaffilmark{\affilref{UCSC}},
O.~Graur\altaffilmark{\affilref{NYU},\affilref{AMNH},\affilref{CfA}},
C.~Grillo\altaffilmark{\affilref{Milan},\affilref{DARK}},
...
S.~H.~Suyu\altaffilmark{\affilref{MPIA},\affilref{ASIAA},\affilref{Garching}},
T.~Treu\altaffilmark{\affilref{UCLA},\affilref{Packard}},
B.~J.~Weiner\altaffilmark{\affilref{Arizona}},
L.~L.~R.~Williams\altaffilmark{\affilref{Minnesota}} L.~L.~R.~Williams\altaffilmark{\affilref{Minnesota}}, \&
A.~Zitrin\altaffilmark{\affilref{BenGurion}}
}
...
in the lensed background galaxies that would otherwise be undetectable.
The \fullmacs0416 cluster (hereafter \macs0416) is one of the most
efficient lenses in the sky, and in 2014 it was observed with high-cadence
imaging from the
{\it Hubble Space
Telescope Telescope} (\HST). Here we describe two
unusual transient events that appeared behind \macs0416 in a strongly lensed galaxy at redshift
$z=1.0054\pm0.0002$. These transients---designated
\spockone and \spocktwo and collectively nicknamed ``Spock''---were
faster and fainter than any supernova (SN), but significantly more luminous
than a classical nova. They reached peak luminosities of $\sim10^{41}$
erg s$^{-1}$ ($M_{AB}<-14$ mag) in
$\lesssim$5 $\lesssim5$ rest-frame days, then faded
below detectability in roughly the same time span. Models of the
cluster lens suggest that these events may be {\it spatially}
coincident at the source plane, but are most likely not {\it
...
As shown in Figure~\ref{fig:SpockDetectionImages}, the \spock events
appeared in \HST imaging collected as part of
the Hubble Frontier Fields (HFF) survey\cite{Lotz:2017}, a
multi-cycle multicycle
program for deep imaging of
6 six massive galaxy clusters and associated
``blank sky'' fields observed in parallel. \HST is not an efficient
wide-field survey telescope, and the HFF survey was not designed with
the discovery of peculiar extragalactic transients as a core
...
time-delayed events that were missed by the \HST imaging of this
field.
%***Steve: Below, ``Supplementary Note~\ref{sec:LensModelVariations}''
% results in a question mark in the PDF file -- fix.
The models also predict absolute magnification values between about
$\mu=10$ and $\mu=200$ for both events. This wide range is
due caused
primarily
to by the close proximity of the lensing critical curve (the
region of theoretically infinite magnification) for sources at $z=1$.
The lensing configuration consistently adopted for this cluster
assumes that the arc comprises two mirror images of the host galaxy
...
intersects both of the \spock locations (see Supplementary
Note~\ref{sec:LensModelVariations}). Such lensing configurations can
qualitatively reproduce the observed morphology of the \spock host
galaxy, but they are
disfavored disfavoured by a purely quantitative assessment of
the positional strong-lensing constraints.
\subsection{Ruling Out Common Astrophysical Transients.}
...
energy released by even the most extreme stellar
flare\cite{Karoff:2016} falls far short of the observed energy release
from the \spock transients. We can also rule out active galactic
nuclei (AGN), which are
disfavored disfavoured by the quiescence of the \spock
sources between the two observed episodes and the absence of any of
the broad emission lines that are often observed in AGN.
Additionally, no x-ray emitting point source was detected in 7 epochs
from 2009 to 2014, including \Chandra X-ray Space Telescope imaging
that was coeval with the peak of infrared emission from \spocktwo.
%***Steve: Below, ``\citep{Fregeau:2004}''
% results in a question mark in the PDF file -- fix.
% In fact, ALL of the citations in this next paragraph lead to
% question marks in the PDF file...
Dynamically induced stellar collisions or close interactions in a
dense stellar cluster\citep{Fregeau:2004} could in principle produce a
series of optical transients. Similarly, the collision of a jovian
...
that observed for the \spock events, although it is unclear whether
the UV/optical emission could match the observed \spock light curves.
These scenarios warrant further scrutiny, so that predictions of the
light curve light-curve shape and anticipated rates can be more rigorously
compared to the \spock observations.
Many types of stellar explosions can generate isolated transient
...
(\Lpk) versus decline time\cite{Kulkarni:2007}.
Figure~\ref{fig:PeakLuminosityDeclineTime} shows our two-dimensional
constraints on \Lpk and the decline timescale \t2 (the time over which
the transient declines by
2 mag) 2~mag) for the \spock events,
accounting for the range of lensing magnifications ($10<\mu<200$)
derived from the cluster lens models. The \spockone and \spocktwo
events are largely consistent with each other, and if both events are
...
transients\cite{Drout:2014}, Ca-rich SNe\cite{Kasliwal:2012}, and
luminous red novae\cite{Kulkarni:2007}.
%***Steve: Below, ``\cite{Li:1998,''
% results in a question mark in the PDF file -- fix.
The SN-like transients that come closest to matching the observed
light curves of the two \spock events are the ``kilonova'' class and
the ``.Ia'' class. Kilonovae are a category of optical/near-infrared
...
explosion, as is the case for all normal SN explosions. For .Ia
events, even if an AM CVn system could produce repeated He shell
flashes of similar luminosity, the period of recurrence would be
$\sim10^5$ yr, making these effectively
non-recurrent nonrecurrent sources. Models
invoking a stellar merger or the collision of a planet with its parent
star have a similar difficulty. In these cases the star may survive
the encounter, but the rarity of these collision events makes it
highly unlikely to detect two such transients from the same galaxy in
a single year.
%***Steve: Below, ``(see Supplementary Figure~\ref{fig:HostProperties}
% and Supplementary Table~\ref{tab:HostProperties})''
% results in question marks in the PDF file -- fix.
Although the two events were most likely not {\it temporally}
coincident, all of our lens models indicate that it is entirely
plausible for the two \spock events to be {\it spatially} coincident:
a single location at the source plane can be mapped to both \spock
locations to within the positional accuracy of the model
reconstructions
($\sim$0.6\arcsec ($\sim0.6$\arcsec in the lens plane). This is
supported by the fact that the host-galaxy
colors colours and spectral indices
at each \spock location are indistinguishable within the uncertainties
(see Supplementary Figure~\ref{fig:HostProperties} and Supplementary
Table~\ref{tab:HostProperties}). Thus, to accommodate all of the
...
\subsection{Luminous Blue Variable.}
%***Steve: Below, ``(see Supplementary
% Figure~\ref{fig:LBVLightCurveComparison})''
% results in a question mark in the PDF file -- fix.
The transient sources
categorized categorised as LBVs are the result of eruptions
or explosive episodes from massive stars ($>10$ \Msun). The class is
exemplified by examples such as P Cygni, $\eta$ Carinae (\etaCar), and
S Doradus\cite{Smith:2011b, Kochanek:2012}. Although most giant LBV
...
accounting for the \spock events as two separate episodes.
Two well-studied LBVs that provide a plausible match to the observed
\spock events are ``SN
2009ip''\cite{Maza:2009} 2009ip''\cite{Maza:2009, Pastorello:2013} and
NGC3432-LBV1\cite{Pastorello:2010}. Both exhibited multiple brief
transient episodes over a span of months to years. Unfortunately,
for these outbursts we have only upper limits on the decline
...
In addition to those relatively short and very bright giant eruptions,
most LBVs also commonly exhibit a slower underlying variability. P
Cygni and \etaCar, for example, slowly rose and fell in brightness by
$\sim$1 to 2 $\sim1$--2 mag over a timespan of several years before and after
their historic giant eruptions. Such variation has not been detected
at the \spock locations. Nevertheless, given the broad
range of light-curve
behaviors behaviours seen in LBV events, we cannot reject
this class as a possible explanation for the \spock system.
The total radiated energy of the \spock events is
in the range
$10^{44}
within the range of plausible values for a major LBV outburst. From
this measurement we can derive constraints on the luminosity of the
...
cycle is directly observed, the object is classified as a recurrent
nova (RN).
%***Steve: Below, ``Supplementary
% Figure~\ref{fig:RecurrentNovaLightCurveComparison}
% results in a question mark in the PDF file -- fix.
The light curves of many RN systems in the Milky Way are similar in
shape to the \spock episodes, exhibiting a sharp rise ($<10$ days in
the rest-frame) and a similarly rapid decline (see Supplementary
...
recurrence timescale for \spock in the rest frame is $120\pm30$ days,
which would be a singularly rapid recurrence period for a RN system.
The RNe in our own Galaxy have recurrence timescales of 10--98
years\cite{Schaefer:2010}. yr\cite{Schaefer:2010}. The fastest measured recurrence timescale
belongs to M31N 2008-12a, which has exhibited a new outburst every
year from 2008 through 2016\cite{Tang:2014, Darnley:2016}
Although this M31 record-holder demonstrates that very rapid
recurrence is possible, classifying \spock as a RN would still require
a very extreme mass-transfer rate to accommodate the
$<1$ year $<1$~yr
recurrence.
%***Steve: Below, ``Supplementary Figure
% \ref{fig:RecurrentNovaRecurrenceComparison}
% results in a question mark in the PDF file -- fix.
Another major concern with the RN hypothesis is that the two \spock
events are substantially brighter than all known novae---perhaps by as
much as 2 orders of magnitude. This is exacerbated by the
...
are caused by a single RN system, then that progenitor system would be
among the most extreme white dwarf binary systems yet known.
\subsection{Microlensing.}\label{sec:MicroLensing}
In the presence of strong gravitational lensing it is possible to
...
first observed example of such a stellar caustic crossing
event\cite{Kelly:2017}. Such events may be expected to appear more
frequently in strongly lensed galaxies that have small angular
separation from the
center centre of a massive cluster. In such a situation,
our line of sight to the lensed background galaxy passes through a
dense web of overlapping microlenses caused by the intracluster stars
distributed around the
center centre 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
...
high density of intracluster stars (see Methods)---comparable to that
observed for the MACS J1149 LS1 transient.
%***Steve: Below, ``(Supplementary
% Figure~\ref{fig:ColorCurves})
% results in a question mark in the PDF file -- fix.
The characteristic timescale of a canonical caustic-crossing event
would be on the order of hours or days (see Supplementary
Information), which is comparable to the timescales observed for the
\spock events. Gravitational lensing is achromatic as long as the size
of the source is consistent across the spectral energy distribution
(SED). This means that the
color colour of a caustic-crossing transient will
be roughly constant. Using simplistic linear interpolations of the
observed light curves (see Methods), we find that the inferred
color colour
curves for both \spock events are marginally consistent with this
expectation of an unchanging
color colour (Supplementary
Figure~\ref{fig:ColorCurves}).
In the baseline lensing configuration adopted above---where a single
...
counting the number of strongly lensed galaxies in the HFF clusters
that have sufficiently high magnification that a source with
$M_{V}=-14$ mag would be detected in \HST imaging. There are only six
galaxies that satisfy that
criteria, criterion, all with $0.5
for an average of 80 days. Treating \spockone and \spocktwo as
separate events leads to a very rough rate estimate of 1.5 \spock-like
...
to make it exceedingly unlikely that two unrelated fast optical
transients would appear in the same galaxy in a single year.
Furthermore, we have observed no other transient events with similar
luminosities and
light curve light-curve shapes in high-cadence surveys of five
other Frontier Fields clusters. Indeed, all other transients detected
in the primary HFF survey have been fully consistent with normal SNe.
Thus, we have no evidence to suggest that transients of this kind are
...
surface explosions from a single RN, or (3) they were each caused by
the rapidly changing magnification as two unrelated massive stars
crossed over lensing caustics. We cannot make a definitive choice
between these hypotheses, principally
due owing to the scarcity of
observational data and the uncertainty in the location of the
lensing critical curves.
...
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% END MAIN REFERENCES
% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% BEGIN POST-MATTER: ACKNOWLEDGMENTS, AUTHOR INFO, FIG LEGENDS
\clearpage
\medskip
Correspondence and requests for materials
should be addressed to S.A.R.~(email: [email protected]).
\medskip
{\bf Acknowledgments}
The authors thank Mario Livio and Laura Chomiuk for helpful discussion
of this paper, as well as Stephen Murray and Neil Gehrels for
assistance with the \Chandra and \Swift data, respectively.
Some/all Some of the data presented in this paper were obtained from the
Mikulski Archive for Space Telescopes (MAST).
STScI is operated by the
Association of Universities for Research in Astronomy, Inc., under
NASA contract NAS5-26555. Support for MAST
for
non-HST non-{\it HST} data is provided by the
NASA National Aeronautics and Space Administration (NASA)
Office of Space Science via grant
NNX09AF08G NNX09AF08G, and
by other grants and contracts.
This research has made use of the NASA/IPAC Extragalactic Database
(NED) which is operated by the Jet Propulsion Laboratory, California
Institute of Technology, under contract with
the National Aeronautics
and Space Administration. NASA.
Financial support for this work was provided to S.A.R., O.G., and
L.G.S. by NASA through grant HST-GO-13386 from the Space Telescope
Science Institute (STScI), which is operated by Associated
Universities for Research in Astronomy, Inc. (AURA), under NASA
contract NAS 5-26555.
J.M.D J.M.D. acknowledges support of
the projects
AYA2015-64508-P (MINECO/FEDER, UE),
AYA2012-39475-C02-01 AYA2012-39475-C02-01, and the
consolider project CSD2010-00064 funded by the Ministerio de Economia
y Competitividad. A.V.F. and P.L.K. are grateful for financial
assistance from the Christopher R. Redlich Fund, the TABASGO
Foundation, and NASA/STScI grants
14528, 14872, GO-14208, GO-14528, GO-14872,
and
14922. GO-14922; A.V.F. is also grateful to the Miller Institute for
Basic Research in Science (U.C. Berkeley). The work
of A.V.F. was conducted in part at the Aspen Center for Physics, which
is supported by NSF grant PHY-1607611; he thanks the Center for its
hospitality during the neutron stars workshop in June and July 2017.
R.J.F. and the UCSC group
is are supported in part by NSF grant
AST-1518052 and from fellowships
to R.J.F. from the Alfred P.\ Sloan
Foundation and the David and Lucile Packard
Foundation to R.J.F. Foundation.
C.G. acknowledges support by VILLUM FONDEN Young Investigator
Programme through grant
no. 10123.
J.H. was supported by a VILLUM
FONDEN Investigator grant (project number 16599). M.J. was supported
by the Science and Technology Facilities Council (grant
number
ST/L00075X/1) and used the DiRAC Data Centric system at Durham
University, operated by the Institute for Computational Cosmology on
behalf of the STFC DiRAC HPC Facility (\url{www.dirac.ac.uk}).
M.J. was funded by BIS National E-infrastructure capital grant
ST/K00042X/1, STFC capital grant ST/H008519/1,
and STFC DiRAC
Operations grant
ST/K003267/1 ST/K003267/1, and Durham University. DiRAC is part of
the National E-Infrastructure. R.K. was supported by Grant-in-Aid for
JSPS Research Fellow (16J01302). M.O. acknowledges support in part
by World Premier International Research Center Initiative (WPI
Initiative), MEXT, Japan, and JSPS KAKENHI Grant Number 26800093 and
15H05892. J.R. acknowledges support from the ERC starting grant
336736-CALENDS. G.C. and S.H.S. thank the Max Planck Society for
support through the Max Planck Research Group of S.H.S.
T.T. and the The
GLASS team
and T.T. were funded by NASA through
HST NASA/STScI grant
HST-GO-13459 from STScI. GO-13459. L.L.R.W. would like to thank
the Minnesota Supercomputing
Institute at the University of Minnesota for providing resources and
support.
\medskip
{\bf Author Contributions}
S.~A.~R.~designed S.A.R.~designed observations, processed the \HST data,
organized organised the
analysis analysis, and wrote the manuscript.
M.~B., T.~B., G.~B.~C., G.~C.,
J.~M.~D., A.~H., M.~J., R.~K., M.~O., J.~R., K.~S., S.~H.~S.,
L.~L.~R.~W., M.B., T.B., G.B.C., G.C.,
J.M.D., A.H., M.J., R.K., M.O., J.R., K.S., S.H.S.,
L.L.R.W., and
A.~Z.~contributed A.Z.~contributed to the lensing analysis with
construction and/or interpretation of a cluster lens model.
I.~B.,
G.~B., C.~G., S.~H., B.~M., A.~M., P.~R., K.~B.~S., J.~S., I.B.,
G.B., C.G., S.H., B.M., A.M., P.R., K.B.S., J.S., and
B.~J.~W.~collected, B.J.W.~collected, processed, and/or analyzed data on the host galaxy
and other galaxies in the cluster field.
R.~J.~F., S.~W.~J.,
P.~L.~K., C.~M., O.~G., J.~H., A.~G.~R., R.J.F., S.W.J.,
P.L.K., C.M., O.G., J.H., A.G.R., and
L.-G.~S.~contributed L.-G.S.~contributed to
the evaluation of models of astrophysical transients.
A.~V.~F. A.V.F. and
T.~T.~assisted T.T.~assisted with the observational program design and editing of
the manuscript.
\medskip
...
\item \MPIA
\item \IFCA
\item \Berkeley
\item \Miller
\item \UCSC
\item \NYU
\item \AMNH
...
\medskip
{\bf Competing Interests} The authors declare that they have no competing financial interests.
\medskip
{\bf Correspondence} Correspondence and requests for materials
should be addressed to S.A.R.~(email: [email protected]).
\medskip
{\bf Supplementary Information}
Supplementary figures,
tables tables, and notes are available online.
% END OF POST-MATTER STUFF
% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
...
gravitational lensing of the cluster. Two columns on the right side
show the discovery of the two transient events in optical and infrared
light, respectively. In these final two columns the top row is a
template image, the
center centre row shows the epoch when each transient
appeared, and the bottom row is the difference image.}
\end{figure*}
\begin{figure*}
\caption{\label{fig:SpockDelayPredictions}
Predictions for the reappearance episodes of both \spockone
and \spocktwo
due to caused by gravitational lensing time delays, as listed in
Table~\ref{tab:LensModelPredictions}. The top panel shows photometry
collected at the NW position (host-galaxy image 11.2) where the first
event (\spockone) appeared in
January, January 2014. Optical
measurements from ACS are in blue and green, and infrared observations
from WFC3-IR are in red and orange. Each blue bar in the lower panel shows
one lens model prediction for the dates when that same physical event
...
\end{figure*}
\begin{figure*}[tbp]
\caption{
\label{fig:SpockDelayPredictions}
Predictions for the reappearance episodes of both \spockone
and \spocktwo due to gravitational lensing time delays, as listed in
Table~\ref{tab:LensModelPredictions}. The top panel shows photometry
collected at the NW position (host-galaxy image 11.2) where the first
event (\spockone) appeared in January, 2014. Optical
measurements from ACS are in blue and green, and infrared observations
from WFC3-IR are in red and orange. Each blue bar in the lower panel shows
one lens model prediction for the dates when that same physical event
(\spockone) would have also appeared in the SE location (galaxy image
11.1), due to gravitational lensing time delay. The lower panel plots
photometry from the SE position (11.1). On the right side we see the
second observed event (\spocktwo). The red bars above
show model predictions for when the NW host image 11.2 would have
exhibited the gravitationally delayed image of the \spocktwo\ event.
The width of each bar encompasses the 68\% confidence region for a
single model, and darker regions indicate an overlap from multiple
models.
}
\end{figure*}
\begin{figure*}[tbp]
\caption{
\label{fig:SpockCriticalCurves}
...
\caption{
\label{fig:LightCurves}
Light curves for the two transient events, \spockone\ on the left
and \spocktwo\ on the right. Measured fluxes in
micro-Janskys microJanskys are
plotted against rest-frame time at $z=1.0054$, relative to the time of
the peak observed flux for each event. The corresponding Modified
Julian Date (MJD) in the observer frame is marked on the top axis for
...
\label{fig:PeakLuminosityDeclineTime}
Peak luminosity vs. decline time for \spock and assorted categories of
explosive transients. Observed constraints of the \spock events are
plotted as overlapping
colored coloured bands, along the left side of the
figure. \spockone is shown as cyan and blue bands, corresponding to
independent constraints drawn from the F435W and F814W light curves,
respectively. For \spocktwo the scarlet and maroon bands show
...
class from neutron star mergers. Grey bands in both panels show the
MMRD relation for classical novae. In the right panel, circles mark
the observed peak luminosities and decline times for classical novae,
while black
`+' ``+'' symbols mark recurrent novae from our own Galaxy. The
large cross labeled at the bottom shows the
rapid recurrence rapid-recurrence nova M31N
2008-12a. Each orange diamond marks a separate short transient event
from the two rapid LBV outburst systems, SN
2009ip\cite{Pastorello:2013} and NGC3432-LBV1 (also known as SN
...
\colhead{$\lvert\mu_{\rm NW}\rvert$} & \colhead{$\lvert\mu_{\rm SE}\rvert$} &
\colhead{$\lvert\mu_{11.3}\rvert$} &
\colhead{$\Delta t_{\rm NW:SE}$} & \colhead{$\Delta t_{\rm NW:11.3}$} \\
\colhead{} & \colhead{} & \colhead{} & \colhead{} & \colhead{(days)} &
\colhead{(years)}} \colhead{(yr)}}
\startdata
CATS & 196$^{+140}_{-53}$ & 46$^{+2}_{-1}$ & 3.3$^{+0.0}_{-0.0}$ & -1.7$^{+2.0}_{-1.9}$ & -3.7$^{+0.1}_{-0.2}$\\[0.5em]
GLAFIC & 29$^{+43}_{-10}$ & 84$^{+103}_{-38}$ & 3.0$^{+0.2}_{-0.2}$ & 4.1$^{+5.5}_{-3.4}$ & -5.0$^{+0.5}_{-0.6}$\\[0.5em]
...
\subsection{Discovery.}\label{sec:Discovery}
The transient \spock\ was discovered in \HST imaging collected as part
of the Hubble Frontier Fields (HFF) survey
(HST-PID: 13496, PI: (HST-PID GO-13496, PI Lotz),
a
multi-cycle multicycle program observing
6 six massive galaxy clusters and
associated ``blank sky'' parallel fields\cite{Lotz:2017}. Several
\HST observing programs have provided additional observations
supplementing the core HFF program. One of these is the FrontierSN
program
(HST-PID: 13386, PI: (HST-PID GO-13386, PI Rodney), which aims to identify and study
explosive transients found in the HFF and related
programs\citet{Rodney:2015a}. The FrontierSN team discovered
\spock\ in two separate HFF observing campaigns on the galaxy cluster
\MACS0416. The first was an imaging campaign in
January, January 2014 during
which the MACS0416 cluster field was observed in the F435W, F606W, and
F814W optical bands using the Advanced Camera for Surveys Wide Field
Camera (ACS-WFC). The second concluded in
August, August 2014, and imaged
the cluster with the infrared detector of \HST's Wide Field Camera 3
(WFC3-IR) using the F105W, F125W, F140W, and F160W bands.
...
subtracted from the astrometrically registered HFF images. In the case
of MACS0416, the templates were constructed from images collected as
part of the Cluster Lensing And Supernova survey with Hubble (CLASH,
HST-PID:12459, PI:Postman)\cite{Postman:2012}. HST-PID GO-12459, PI Postman)\cite{Postman:2012}. The resulting
difference images are
visually inspected for new point sources, and any new transients of
interest (primarily SNe) are monitored with additional
\HST imaging or ground-based spectroscopic observations as needed.
...
\subsection{Photometry.}\label{sec:Photometry}
%***Steve: Below, ``Tables~\ref{tab:spockonephot} and
% \ref{tab:spocktwophot}
% result in question marks in the PDF file -- fix.
The follow-up observations for \spock\ included \HST imaging
observations in infrared and optical bands using the WFC3-IR and
ACS-WFC detectors, respectively. Tables~\ref{tab:spockonephot} and
\ref{tab:spocktwophot} present photometry of the \spock\ events from
all available \HST observations. The flux was measured on difference
images, first using aperture photometry with a 0\farcs3 radius, and
also by fitting with an empirical
point spread point-spread function (PSF). The
PSF model was defined using \HST observations of the
G2V G2~V standard star
P330E, observed in a separate calibration program. A separate PSF
model was defined for each filter, but owing to the long-term
stability of the \HST PSF we used the same model in all epochs. All
...
Spectroscopy of the \spock\ host galaxy was collected using three
instruments on the Very Large Telescope (VLT). Observations with the
VLT's X-shooter cross-dispersed echelle spectrograph\citep{Vernet:2011} were taken on October 19,
21 21, and 23, 2014
(Program 093.A-0667(A),
PI: PI J. Hjorth) with the slit
centered centred on the
position of \spocktwo. The total integration time was
4.0 hours 4.0~hr for
the
NIR near-infrared (NIR) arm of X-shooter,
3.6 hours 3.6~hr for the
VIS visual (VIS)
arm, and
3.9 hours 3.9~hr for
the UVB arm. The spectrum did not provide any detection of the
transient source itself (as we will see below, it had already faded
back to its quiescent state by that time). However, it did provide an
...
two measures of the photometric redshift of the host: $z=1.00\pm0.02$
from the BPZ algorithm\citep{Benitez:2000}, and $z=0.92\pm0.05$ from
the EAZY program\citep{Brammer:2008}. Both were derived from \HST
photometry of the host images 11.1 and 11.2, spanning
4350--16000 4350--16,000 \AA.
Additional VLT observations were collected using the Visible
Multi-object Spectrograph (VIMOS)\citep{LeFevre:2003}, as part of the
CLASH-VLT large program (Program 186.A-0.798;
P.I.: PI
P. Rosati)\citep{Rosati:2014}, which collected
$\sim$4000 $\sim4000$ reliable
redshifts over 600 arcmin$^2$ in the \macs0416
field\citep{Grillo:2015,Balestra:2016}. These massively multi-object
observations could potentially have provided confirmation of the
redshift of the \spock host galaxy with separate spectral line
identifications in each of the three host-galaxy images. For the
\macs0416 field the CLASH-VLT program collected
1 hour 1~hr of useful
exposure time in good seeing conditions with the Low Resolution Blue
grism. Unfortunately, the wavelength range of this grism
(3600-6700 (3600--6700
\AA) does not include any strong emission lines for a source at
$z=1.0054$, and the signal-to-noise ratio (S/N) was not sufficient to provide
any clear line identifications for the three images of the \spock host
...
The VLT Multi Unit Spectroscopic Explorer (MUSE)\citep{Henault:2003,Bacon:2012} observed the NE portion of
the MACS0416 field---where the \spock host images are located---in
December, December 2014 for
2 hours 2~hr of integration time (ESO program
094.A-0115,
PI: PI J.\,Richard). These observations also confirmed the
redshift of the host galaxy with clear detection of the
\forbidden{O}{ii} doublet. Importantly, since MUSE is an integral
field spectrograph, these observations also provided a confirmation of
...
matching \forbidden{O}{ii} line at the same wavelength\cite{Caminha:2017}.
A final source of spectroscopic information relevant to \spock is the
Grism Lens Amplified Survey from Space (GLASS;
PID:
HST-GO-13459; PI:T. HST-PID
GO-13459; PI T. Treu)\citep{Schmidt:2014,Treu:2015a}. The GLASS
program collected slitless spectroscopy on the \macs0416 field using
the WFC3-IR G102 and G141 grisms on \HST, deriving redshifts for
galaxies down to a magnitude limit $H<23$. As with the VLT VIMOS
...
\subsection{Gravitational Lens Models.}\label{sec:LensingModels}
The seven lens models used to provide estimates of the plausible range
of magnifications and time delays are as
follows: follows.
\begin{itemize}
\item{\it CATS:} The model of \citeref{Jauzac:2014}, version 4.1,
generated with the {\tt LENSTOOL} software
(\url{http://projects.lam.fr/repos/lenstool/wiki})\citep{Jullo:2007}
using strong lensing constraints. This model
parameterizes parametrises cluster
and galaxy components using pseudo-isothermal elliptical mass
distribution (PIEMD) density profiles\citep{Kassiola:1993,
Limousin:2007}.
...
the {\tt GLAFIC} software
(\url{http://www.slac.stanford.edu/~oguri/glafic/})\citep{Oguri:2010b}
with strong-lensing constraints. This model assumes simply
parametrized parametrised mass distributions, and model parameters are
constrained using positions of more than 100 multiple images.
\item{\it GLEE:} A new model built using the {\tt GLEE}
software\citep{Suyu:2010b, Suyu:2012} with the same strong-lensing
constraints used in \citeref{Caminha:2017}, representing mass
distributions with simply
parameterized parametrised mass profiles.
\item{\it GRALE:} A free-form, adaptive grid model developed using
the GRALE software tool\citep{Liesenborgs:2006, Liesenborgs:2007,
Mohammed:2014, Sebesta:2016}, which implements a genetic algorithm
...
{\tt SWUnited} modeling method\citep{Bradac:2005, Bradac:2009}, in
which an adaptive pixelated grid iteratively adapts the mass
distribution to match both strong- and weak-lensing constraints.
Time delay Time-delay predictions are not available for this model.
\item{\it WSLAP+:} Created with the {\tt WSLAP+} software
(\url{http://www.ifca.unican.es/users/jdiego/LensExplorer})\citep{Sendra:2014}:
Weak and Strong Lensing Analysis Package plus member galaxies (Note:
...
\MACS0416 in \citeref{Zitrin:2013a}.
\end{itemize}
Early versions of the {\it SWUnited}, {\it CATS}, {\it
ZLTM} ZLTM}, and {\it
GRALE} models were originally distributed as part of the Hubble
Frontier Fields lens modeling project
(\url{https://archive.stsci.edu/prepds/frontier/lensmodels/}), in
...
models also made use of spectroscopic redshifts in the cluster
field\cite{Mann:2012, Christensen:2012, Grillo:2015, Caminha:2017}.
Input weak-lensing constraints were derived from data collected at the
Subaru
Telescope\cite{Umetsu:2014, telescope\cite{Umetsu:2014, Umetsu:2016} and archival imaging.
\subsection{X-ray Nondetections.}\label{sec:Xray}
...
detected near the locations of the \spock events (N. Gehrels, private
communication). The field was also observed by \Chandra with the
ACIS-I instrument for three separate programs. On June 7, 2009 it was
observed for GO program 10800770
(PI: (PI H.\,Ebeling). It was revisited
for GTO program 15800052
(PI: (PI S.\,Murray) on November 20, 2013 and for
GO program 15800858
(PI: (PI C.\, Jones) on June 9, August 31, November
26, and December 17, 2014. These \Chandra images show no evidence for
an x-ray emitting point source near the \spock locations on those
dates (S. Murray, private communication).
The \Chandra observations that were closest in time to the observed
\spock events were those taken in August and
November, November 2014. The
August 31 observations were coincident with the observed peak of
rest-frame optical emission for the \spocktwo event (on MJD
56900). The November 26 observations correspond to 44 rest-frame days
after the peak of the \spocktwo event. If the \spock events are
UV/optical nova
eruption, eruptions, then these observations most likely did not
coincide with the nova system's supersoft x-ray phase. For a RN system
the x-ray phase typically initiates after a short delay, and persists
for a span of only a few weeks. For example, the most rapid
recurrence recurrent
nova known, M31N 2008-12a, has exhibited a supersoft x-ray phase from
6 to 18 days after the peak of the optical emission
\citep{Henze:2015a}.
\subsection{Light Curve \subsection{Light-Curve Fitting.}\label{sec:LightCurves}
Due %***Steve: Below, ``Supplementary Figure~\ref{fig:LinearLightCurveFits}''
% results in a question mark in the PDF file -- fix.
Owing to the rapid decline timescale, no observations were collected for
either event that unambiguously show the declining portion of the
light curve. Therefore, we must make some assumptions for the shape of
the light curve in order to quantify the peak luminosity and the
corresponding timescales for the rise and the decline. We first
approach this with a simplistic model that is piecewise linear in
magnitude
vs vs. time. Supplementary Figure~\ref{fig:LinearLightCurveFits} shows
examples of the resulting fits for the two events. For each fit we
use only the data collected within 3 days of the brightest observed
magnitude, which allows us to fit a linear rise separately for the
F606W and F814W light curves
for of \spockone and the F125W and F160W
light curves
for of \spocktwo. To quantify the covariance between the
true peak brightness, the rise
time time, and the decline timescale, we use
the following
procedure: procedure.
\begin{enumerate}
\item
make Make an assumption for the date of peak, $t_{\rm
pk}$; pk}$.
\item
measure Measure the peak magnitude at $t_{\rm pk}$ from the linear fit
to the rising light-curve
data; data.
\item
assume Assume the source reaches a minimum brightness (maximum
magnitude) of 30 AB mag at the epoch of first observation after the
peak; peak.
\item
draw Draw a line for the declining light curve between the assumed
peak and the assumed minimum
brightness; brightness.
\item
use Use that declining light-curve line to measure the timescale for
the event to drop by 2 mag,
$t_2$; $t_2$.
\item
make Make a new assumption for $t_{\rm pk}$ and repeat.
\end{enumerate}
%***Steve: Below, ``Supplementary Figure~\ref{fig:LinearLightCurveFits}''
% results in a question mark in the PDF file -- fix.
As shown in Supplementary Figure~\ref{fig:LinearLightCurveFits}, the resulting
piecewise linear fits are simplistic, but nevertheless approximately
capture the observed
behavior behaviour for both events. Furthermore, since
this toy model is not physically motivated, it allows us to remain
agnostic for the time being as to the astrophysical source(s) driving
these transients. From these fits we can see that \spockone most
likely reached a peak magnitude between 25 and 26.5 AB mag in both
F814W and F435W, and had a decline timescale $t_2$ of less than 2 days
in the rest frame. The observations of \spocktwo provide less
stringent constraints, but we see that it had a peak
magnitude between
23 and 26.5 AB mag in F160W and exhibited a decline time of
less than
seven $<7$
days. These fits also illustrate the generic fact that a higher
peak brightness corresponds to a longer rise time and a faster decline
timescale, independent of the specific model used. Changes to the
arbitrary constraints we placed on these linear fits do not
...
At any assumed value for the time of peak brightness this linear
interpolation gives an estimate of the peak magnitude. We then convert
that to a luminosity
(e.g., $\nu ($\nu L_\nu$ in erg s$^{-1}$) by first
correcting for the luminosity distance assuming a standard \LCDM
cosmology, and then accounting for an assumed lensing magnification,
$\mu$. The range of plausible lensing magnifications ($10<\mu<100$)
is derived from the union of our seven independent lens models. This
results in a grid of possible peak luminosities for each event as a
function of magnification and time of peak. As we are using linear
light curve light-curve fits, the assumed time of peak is equivalent to an
assumption for the decline time, which we quantify as $t_2$, the time
over which the transient declines by 2
magnitudes. mag.
\subsection{LBV Build-up Timescale and Quiescent Luminosity.}\label{sec:LBVbuildup}
...
depends on the precise shape of the light
curve\cite{Smith:2011b}. Note that earlier work\cite{Smith:2011b} has
used $t_{1.5}$ instead of $t_2$, which amounts to a different
light-curve shape light-curve-shape term, $\zeta$. Adopting \Lpk$\approx10^{41}$ erg
s$^{-1}$ and \t2$\approx$1 day (as shown in
Fig.~\ref{fig:PeakLuminosityDeclineTime}), we find that the total
radiated energy is $E_{\rm rad}\approx10^{46}$ erg. A realistic range
for this estimate would span $10^{44}due
owing to
uncertainties in the magnification, bolometric luminosity correction,
decline time, and light-curve shape. These uncertainties
notwithstanding, our estimate falls well within the range of plausible
...
\subsection{RN Light-Curve Comparison.}\label{sec:RNLightCurves}
%***Steve: Below,
% ``Supplementary Figure~\ref{fig:RecurrentNovaLightCurveComparison}''
% results in a question mark in the PDF file -- fix. Ditto for the
% next figure referenced in that paragraph.
There are ten known RNe in the Milky Way galaxy, and seven of
these exhibit outbursts that decline rapidly, fading by
two magnitudes 2~mag
in
less than ten $<10$ days\citep{Schaefer:2010}.
Supplementary Figure~\ref{fig:RecurrentNovaLightCurveComparison}
compares the \spock light curves to a composite light curve (the
gray grey
shaded region), which encompasses the
V band light curve $V$-band light-curve
templates\citep{Schaefer:2010} for all seven of these
galactic Galactic RN
events. The Andromeda galaxy (M31) also hosts at least one RN with a
rapidly declining light curve. The 2014 eruption of this well-studied
nova, M31N 2008-12a, is shown as a solid black line in Supplementary
Figure~\ref{fig:RecurrentNovaLightCurveComparison}, fading by
2 mag 2~mag in
$<3$ days. This comparison demonstrates that the rapid decline of
both of the \spock transient events is fully consistent with the
eruptions of known RNe in the local universe.
\subsection{RN Luminosity and Recurrence Period.}\label{sec:RNLuminosityRecurrence}
%***Steve: Below, ``Figure~\ref{fig:RecurrentNovaRecurrenceComparison}''
% results in a question mark in the PDF file -- fix. Ditto for the
% next figure referenced in that paragraph.
To examine the recurrence period and peak brightness of the \spock
events relative to RNe, we rely on a pair of papers that evaluated an
extensive grid of nova models through multiple cycles of outburst and
...
recurrence period as fast as one year is expected only for a RN system
in which the primary white dwarf is both very close to the
Chandrasekhar mass limit (1.4 \Msun) and also has an extraordinarily
rapid
mass transfer mass-transfer rate ($\sim10^{-6}$ \Msun yr$^{-1}$). The models
of \citeref{Yaron:2005} suggest that such systems should have a very
low peak amplitude (barely consistent with the lower limit for \spock)
and a low peak luminosity
($\sim$100 ($\sim100$ times less luminous than the
\spock events).
The closest analog for the \spock events from the population of known
RN systems is the nova M31N\,2008-12a. \citeref{Kato:2015} provided a
theoretical model that can account for the key observational
characteristics of this remarkable nova: the very rapid recurrence
timescale ($<$1 yr), fast optical light curve
($\t2\sim2$ ($\t2\approx2$ days), and
short supersoft x-ray phase
(6-18 (6--18 days after optical
outburst)\citep{Henze:2015a}. To match these observations,
\citeref{Kato:2015} invoke a 1.38 \Msun white dwarf primary,
drawing mass from a companion at a rate of $1.6\times10^{-7}$ \Msun
...
light (ICL). The surface brightness is then converted to a projected
stellar mass surface density by assuming a
Chabrier\cite{Chabrier:2003} initial mass function and an
exponentially declining
star formation star-formation history. This procedure leads
to an estimate for the intracluster stellar mass of $\log
(\Sigma_{\star} / (\Msun\,{\rm kpc}^{-2})) = 6.9\pm0.4$. This is
very similar to the value of $6.8^{+0.4}_{-0.3}$ inferred for the
probable
caustic crossing caustic-crossing star M1149 LS1\cite{Kelly:2017}.
\subsection{Colour Curves.}\label{sec:ColorCurves}
\subsection{Color Curves.}\label{sec:ColorCurves} %***Steve: Below, ``Supplementary Figure~\ref{fig:ColorCurves}''
% results in a question mark in the PDF file -- fix.
At $z=1$ the observed optical and infrared bands translate to
rest-frame ultraviolet (UV) and optical wavelengths, respectively. To
derive rest-frame UV and optical
colors colours from the observed photometry,
we start with the measured magnitude in a relatively blue band (F435W
and F606W for \spockone and F105W, F125W, F140W for \spocktwo). We
then subtract the coeval magnitude for a matched red band (F814W for
\spockone, F125W or F160W for \spocktwo), derived from the linear fits
to those bands. To adjust these to rest-frame filters, we apply
K
corrections\citep{Hogg:2002}, K-corrections\citep{Hogg:2002}, which we compute by
defining a crude SED via linear interpolation between the observed
broad bands for each transient event at
each every epoch. For consistency
with past published results, we include in each
K correction K-correction a
transformation from AB to Vega-based magnitudes. The resulting UV and
optical
colors colours are plotted in Supplementary Figure~\ref{fig:ColorCurves}. Both
\spockone and \spocktwo show little or no
color colour variation over the
period where
color colour information is available. This lack of
color colour
evolution is compatible with all three of the primary hypotheses
advanced, as it is possible to have no discernible
color colour evolution
from either an LBV or RN over this short time span, and microlensing
events inherently exhibit an unchanging
color. colour.
If these two events are from a single source then one could construct
a composite SED from rest-frame UV to optical wavelengths by combining
...
\subsection{Rates.}\label{sec:RatesMethods}
%***Steve: Below, ``Figure~\ref{fig:StronglyLensedGalaxies}''
% results in a question mark in the PDF file -- fix.
To derive a rough estimate of the rate of \spock-like transients, we
first define the set of strongly lensed galaxies in which a similarly
faint and fast transient could have been detected in the HFF
...
out in the CLASH and CANDELS programs\cite{Graur:2014,Rodney:2014}.
For a transient with peak brightness $M_{V}>-14$ mag to be detected,
the host galaxy must be amplified by strong lensing with a
magnification $\mu>20$ at
$z\sim1$, $z\approx1$, growing to $\mu>100$ at
$z\sim2$. $z\approx2$.
Using photometric redshifts and magnifications derived from the GLAFIC
lens models of the six HFF clusters, we find $N_{\rm gal}=6$ galaxies
that satisfy this criterion, with $0.5
...
entirety of the primary HFF campaigns on each cluster, but excludes
all of the ancillary data collection periods from supplemental \HST
imaging programs. The average control time for an HFF cluster is
$t_{c} =
0.22$ years 0.22$~yr (80 days). Treating each \spock event as a
separate detection, we can derive a rate estimate using $R = 2 /
(N_{\rm gal}\,t_c)$. This yields $R=1.5$ events galaxy$^{-1}$
year$^{-1}$. yr$^{-1}$.
Future examination of the rate of such transients should consider the
total stellar mass and the star-formation rates of the galaxies
...
% DATA AVAILABILITY STATEMENT
The data that support the plots within this paper and other findings
of this study are available from the corresponding author upon
reasonable request. All \HST data
utilized utilised for this work are
available through the Mikulski Archive for Space Telescopes (MAST).
...
\providecommand{\eprint}[2][]{\url{#2}}
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