
\def\hst{{\it HST}}
\def\spitzer{{\emph Spitzer}}
\def\bull{$\bullet$~}
%\newcounter{ctr}
\def\pp{\parshape 2 0truein 6.1truein .3truein 5.5truein}
\def\reference{\noindent\pp}
\def\hal{H$\alpha$}
\def\kms{km s$^{-1}$}
\def\apj{ApJ}
\def\aj{AJ}
\def\mnras{MNRAS}
\def\apjs{ApJS}
\def\araa{ARA\&A}
\def\pasj{PASJ}
\def\lam{$\lambda$}
%
%Luis's definitions
\def\aa{{A\&A}}
\def\aas{{ A\&AS}}
\def\aj{{AJ}}
\def\al{$\alpha$}
\def\bet{$\beta$}
\def\amin{$^\prime$}
\def\annrev{{ARA\&A}}
\def\apj{{ApJ}}
\def\apjl{{ApJL}}
\def\apjs{{ApJS}}
\def\asec{$^{\prime\prime}$}
\def\baas{{BAAS}}
\def\cc{cm$^{-3}$}
\def\chandra{{\it Chandra}}
\def\deg{$^{\circ}$}
\def\cc{cm$^{-3}$}
\def\e#1{$\times$10$^{#1}$}
\def\etal{{et al. }}
\def\flamb{ergs s$^{-1}$ cm$^{-2}$ \AA$^{-1}$}
\def\flux{ergs s$^{-1}$ cm$^{-2}$}
\def\fnu{ergs s$^{-1}$ cm$^{-2}$ Hz$^{-1}$}
\def\hst{{\it HST}}
\def\kms{km\thinspaces$^{-1}$}
\def\lamb{$\lambda$}
\def\loiii{$L_{\rm [O {\tiny III}]}$}
\def\lum{ergs s$^{-1}$}
\def\mbh{$M_{\rm BH}$}
\def\micron{{$\mu$m}}
\def\mnras{{MNRAS}}
\def\msigma{$M_{\mathrm{BH}}-\sigma_{\ast}$}
\def\mlbulge{$M_{\mathrm{BH}}-L_{\rm bul}$}
\def\mbulge{$M_{\mathrm{BH}}-M_{\rm bul}$}
\def\mlr{$\Gamma_r$}
\def\mlk{$\Gamma_K$}
\def\nat{{Nature}}
\def\pasp{{PASP}}
\def\perang{\AA$^{-1}$}
\def\percm2{cm$^{-2}$}
\def\sigmastar{$\sigma_{\ast}$}
\def\solum{$L_\odot$}
\def\msun{$M_\odot$}
\def\xmm{{\it XMM-Newton}}
\def\lax{{$\mathrel{\hbox{\rlap{\hbox{\lower4pt\hbox{$\sim$}}}\hbox{$<$}}}$}}
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\def\heii{\ion{He}{2}}
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\def\oiii{[\ion{O}{3}]}
\def\ni{[\ion{N}{1}]}
\def\oi{[\ion{O}{1}]}
\def\nii{[\ion{N}{2}]}
\def\hei{\ion{He}{1}}
\def\sii{[\ion{S}{2}]}
\def\siii{[\ion{S}{3}]}

\def\farcm{\hbox{$.\mkern-4mu^\prime$}}
\def\farcs{\hbox{$.\mkern-4mu^{\prime\prime}$}}


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
%    phase1-GO.tex  (use only for General Observer and Snapshot proposals; use phase1-AR.tex for Archival Research and
%                      Theory proposals use phase1-DD.tex for GO/DD proposals and use phase1-MC.tex for GO/MC rapid response
%                       proposals).
%
%    HUBBLE SPACE TELESCOPE
%    PHASE I OBSERVING PROPOSAL TEMPLATE 
%     FOR CYCLE 24 (2016)
%
%    Version 1.1, August 12, 2015.
%
%    Guidelines and assistance
%    =========================
%     Cycle 23 Announcement Web Page:
%
%         http://www.stsci.edu/hst/proposing/docs/cycle23announce 
%
%    Please contact the STScI Help Desk if you need assistance with any
%    aspect of proposing for and using HST. Either send e-mail to
%    help@stsci.edu, or call 1-800-544-8125; from outside the United
%    States, call [1] 410-338-1082.
%
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% The template begins here. Please do not modify the font size from 12 point.

\documentclass[12pt]{article}
\usepackage{graphicx}
\usepackage{phase1}

\begin{document}

%   1. SCIENTIFIC JUSTIFICATION
%       (see Section 9.1 of the Call for Proposals)
%
%
\justification          % Do not delete this command.
% Enter your scientific justification here. 

%{\it Using HST/WFC3-IR imaging from the UV through the IR for 
%5XX active galactic nuclei, we will leverage simultaneous 
%spectral energy distributions and high spatial resolution to determine 
%whether their peculiar spectral energy distributions are consistent 
%with binary black holes.}

\noindent
{\bf The Final Parsec Problem}:
Despite decades of work, we still do not know whether pairs of
supermassive black holes (BHs) merge efficiently or stall at $\sim 1$
pc due to a lack of stars to harden the binary on these scales (e.g.,
Begelman et al. 1980). The uncertainty in the physical processes that
drive the binaries inward, and thus in the time spent by the binary at
any given radius, directly translates into uncertainty in the expected
level of gravitational radiation detectable by pulsar timing 
arrays (e.g., Shannon et al. 2015; Taylor et
al. 2016). In principle, one would like to search directly for spatially
separated pairs of accreting supermassive BHs, to directly measure the
distribution of orbital separations. However, even with radio
interferometry, it is only possible to resolve pc scales for
relatively nearby systems, and this technique has yielded only one
bona fide SMBHB with a projected separation of 7 pc (Rodriguez et
al. 2006) despite very systematic searches (Burke-Spolaor 2011). 
  {\it There are still no proven direct electromagnetic signatures
of binary BHs at cosmological 
distances. \hst\ may allow us to detect the signatures of a binary-cleared 
gap in the accretion disk for the first time.}

A variety of attempts have been made to detect the electro-magnetic
signatures of SMBHBs. One class of searches looks for the radial
velocity offsets associated with orbital motion (Boroson \& Lauer
2009, Eracleous et al.\ 2012, Decarli et al.\ 2013, Ju et al.\ 2013,
Shen et al. 2013). These searches require long time baselines and are
only sensitive to binaries at $\sim 0.1$pc separations (Wang, L.\ in
prep.). They have yet to yield ironclad binary systems.  Another
technique searches for a quasi-periodic signal that may be expected as
the secondary BH torques the larger-scale accretion disk in its orbit
(e.g., Haiman et al. 2009).  Searches using the Catalina Real-time
Transient Survey have uncovered $\sim 100$ candidates that may show
quasi-periodic variability (Graham et al.\ 2015; Liu et al. 2015). If
the variability is indeed due to orbital motion, the inferred orbital
periods are years, corresponding to separations of 100-1000s of AU.

\medskip

\noindent
{\bf A Spectral Search for an Accretion Disk Gap}: 
As promising as quasi-periodic variability searches have been, 
they are typically limited to $z < 1$, where the sources are bright 
enough to be monitored with small aperture telescopes.
We seek a complementary approach that can work at higher redshift 
($z \sim 2$), where the merger rate was higher and the bulk of the quasar
activity occured. A promising complementary approach may come from 
studying the spectral energy distributions (SEDs) of quasars in 
search of anamolies that may point to differences in the accretion 
disk structure.

Specifically, during the sub-pc to AU-scale binary phase, we expect
that the inner UV-emitting accretion disk will be evacuated, clearing
a gap in the accretion disk (e.g., Figure 1; Roedig et al. 2014 and references
therein).  This gap leads to a dip in the SED at UV-optical
wavelengths. At shorter wavelengths, the UV SED will rise again due 
to ``mini-disks'' around the SMBHB. This phenomenon should be 
observable as a $U-$shaped SED. In Figure 1, we
show the fractional spectral change expected from such a cleared gap,
as a function of binary separation. 

\vskip -0.1in
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\vbox{
\hbox{
\hskip 0.8in
%\psfig{file=u3789rot.eps,height=3.truein,angle=-90}
\includegraphics[width=0.7\textwidth]{gapmini.pdf}
}
}
\vskip 0.1in
{\small
\noindent{\it Figure 1} --- {\bf Top}: Cartoon demonstrating 
the configuration leading to an SED with a gap (Hayasaki et al.\ 2007).
{\bf Bottom}: Theoretically expected $U-$shaped spectra resulting 
from the gap in the accretion disk. The radius $a_g = 100 R_s$. 
Note that as the radius of the secondary moves 
outwards, the temperature of the gap gets cooler and the central 
wavelength gets longer. Thus, we search for gaps ranging between 1500 
and 10,000 \AA.}

\vskip 0.1in
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

{\it We request follow-up imaging
of the six candidate SMBHBs that we selected from the SDSS, to
determine whether the $U-$shaped dip apparent in their non-simultaneous
spectral energy distribution is best described by a BH binary clearing 
gap in the accretion disk.}

We choose our sample of subparsec SMBHB candidates from the SDSS DR7
list of over $100,000$ quasars (Shen et al. 2011). We select
candidates with $0.5 < z < 2.5$ so that we can detect the UV rise
beyond the gap (80,000 objects) and with detections in all \emph{WISE}
bands (removing $<100$ targets).  From this list of $\sim 80,000$, we
select those quasars whose average SDSS flux (averaged over all 5
bands) is less than the average \emph{WISE} flux (averaged over all 4 bands),
as expected for a gap signature in the visible.  This broad search
returns a sample of 28 candidates. We then select those objects with a
rise both in the blue (\emph{Galex} or SDSS $u$-band) and red (SDSS
$i-$ and $z-$bands, \emph{WISE}). We remove strong blazars, where temporal
variability causes the anomalies in the SED, and quasars with very
bright stars nearby that contaminated the \emph{Galex} and 
\emph{WISE} photometry, leaving {\it six} subparsec SBHB candidates 
(see detailed example in Figure 2, and all SEDs in Figure 3).

Based on the broad-band SEDs alone, we have identified two primary
sources of contamination for our binary BH hypothesis. (1) An
intrinsically reddened quasar with a blue host galaxy could 
masquerade as a gap, and we identify and remove one such target with
pre-existing \hst\ imaging (Urrutia et al. 2007). (2) Alternatively, 
we could be seeing a standard quasar with a boosted red
SED from synchrotron. There are three targets in our sample
that are detected at 20 cm in the FIRST survey (Becker et al. 1994) that may be
contaminated by jet emission.  With the quasi-simultaneous observations
from the UV through the radio proposed here, we can rule out both
sources of contamination. {\it Only the \hst\ has the combination of high
angular resolution and UV sensitivity that we need to confirm the
nature of these objects.} \smallskip

\noindent
{\bf The Need for HST}:
We propose to obtain F225W, F336W, F606W, F814W, and F160W imaging 
of each of our 6 SMBHB candidates,
using only one orbit per target. In addition, we request 
Ku-band imaging from J-VLA to determine the likelihood that the red 
continuum arises from synchrotron (Table 1). We have the following main goals:

\noindent
$\bullet$~Obtain simultaneous photometry from the UV through the NIR,
that spans the dip we think is caused by the gap in the accretion
disk. We wish to be certain that the UV rise is not caused by
non-simultaneous data. \hst\ is the {\it only} instrument capable of
obtaining the required UV imaging to span the observed dip that is the
signature of binarity.

\noindent
$\bullet$~Rule out that the observed UV emission may come from star
formation by both searching for spatially resolved UV/blue emission
and fitting the blue SED.

\noindent
$\bullet$~Examine the morphology of the host galaxy in the rest-frame
optical using the deep F160W image. If we are observing close
binaries, we might certainly expect to see signs of a recent
interaction. In one case (SED4; Table 1, Figure 2), existing \hst\
imaging reveals clear signs that this galaxy recently underwent a
merger (e.g., disturbed morphology and tidal tails).

\noindent
$\bullet$~Use the broad bandwidth of the JVLA to determine whether the 
red excess that we observe with 2MASS+\emph{WISE} may be due to synchrotron 
emission for the three sources with radio detections.  

\medskip

We have uncovered the most promising examples of AU-scale binary BHs
with a disk gap. This method is one of the only ways to identify the
electro-magnetic signatures of SMBHBs. Only with the simultaneous
UV-NIR coverage and high spatial resolution of \hst\ can we verify the
nature of these sources, bringing us one step closer to characterizing
the sub-pc binary BH population. \\

\medskip

\noindent
{\bf Bibliography}
$\bullet$ Barth, A.~J., et al.\ 2015, \apjs, 217, 26
$\bullet$ Becker, R.~H., et al.\ 1994, Astronomical Data 
Analysis Software and Systems III, 61, 165 
$\bullet$ Begelman, M.~C., et al.\ 1980, Nat, 287, 307
$\bullet$ Boroson, T.~A., \& Lauer, T.~R.\ 2009, \nat, 458, 53
$\bullet$ Burke-Spolaor, S.\ 2011, \mnras, 410, 2113 
$\bullet$ Decarli, R., et al.\ 2013, \mnras, 433, 1492
$\bullet$ Eracleous, M., et al.\ 2012, \apjs, 201, 23
$\bullet$ Graham, M.~J., et al.\ 2015, \mnras, 453, 1562 
$\bullet$ Haiman, Z., et al.\ 2009, \apj, 700, 1952
$\bullet$ Hayasaki, et al.\ 2007, \pasj, 59, 427
$\bullet$ Ju, W., et al.\ 2013, \apj, 777, 44
$\bullet$ Liu, T., et al.\ 2015, \apjl, 803, L16 
$\bullet$ Rodriguez, C., et al.\ 2006, \apj, 646, 49
$\bullet$ Roedig, C., et al.\ 2014, \apj, 785, 115
$\bullet$ Shannon, R.~M., et al.\ 2015, Science, 349, 1522 
$\bullet$ Shen, Y., et al.\ 2013, \apj, 775, 49
$\bullet$ Taylor, S.~R., et al.\ 2016, ApJL, 819, L6
$\bullet$ Urrutia, T., et al.\ 2008, \apj, 674, 80


\vskip -0.15in
\vbox{
\hbox{
\hskip +0.25in
%\psfig{file=u3789rot.eps,height=3.truein,angle=-90}
\includegraphics[width=0.98\textwidth]{object.pdf}
}
}
\vskip 0.1in

\vskip -0.1in
\vbox{
\hbox{
\hskip +0.4in
%\psfig{file=u3789rot.eps,height=3.truein,angle=-90}
\includegraphics[width=0.8\textwidth]{prop3.pdf}
}
}
\vskip 0.1in
{\small
\noindent{\it Figure 2} --- 
The broad-band SED for one of our best candidates, SED4 (black
circles) showing the locations of the bands requested here in green
diamonds (Vartanyan et al. in prep). Overplotted is the Richards et
al. (2006) composite quasar spectrum (blue) and the best-fit composite
SED+reddening (red); reddening alone cannot explain this
dip.  The ACS/F435W image of this target shows a point-source
dominated nucleus, as well as tidal features indicative of recent
merging. We emphasize that simultaneous UV-NIR imaging is required to
verify that this observed $U-$shaped spectrum is not an artifact of
variability.

\vskip -0.3in
\vbox{
\hbox{
\hskip +0.5in
%\psfig{file=u3789rot.eps,height=3.truein,angle=-90}
\includegraphics[width=0.75\textwidth]{test2col.pdf}
}
}
\vskip 0.1in
{\small
\noindent{\it Figure 3} --- The broad-band SEDs for our full sample, 
relative to the best-fit composite quasar template (shown in blue in the 
top panel) with reddening applied (red lines). 
We demonstrate explicitly here that 
simple dust reddening of a quasar SED cannot explain the $U-$shaped 
SEDs that we observe. There is another source of UV emission causing 
the SED to rise again into the UV. A possible source of the rise in the 
UV is a significant contribution from star formation in the host galaxy, 
which can only be ruled out with the \hst\ UV imaging requested here.

\vskip 0.1in

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%   2. DESCRIPTION OF THE OBSERVATIONS
%       (see Section 9.2 of the Call for Proposals)
%
%
\describeobservations   % Do not delete this command.
% Enter your observing description here.

Our primary goals, which set our integration times are to (a) detect
each quasar in all five bands (F225W, F336W, F606W, F814W, F160W) with
at least $5~\sigma$ significance, which sets our minimum exposure time
(b) detect any spatially resolved galaxy component in the UV if it
exists and (c) search for signs of recent interactions in the
rest-frame optical.  Thus, we spend the majority of our integration
time on the UV and NIR filters.

In the UV, we imagine that our faintest target (NUV$=22.5$ AB mag) is
spatially extended with a radius of $1.5$ arcsec (13 kpc at
$z=1.5$). In this scenario, we will achieve a $5\, \sigma$ detection
in a $0.2$ arcsec radius beam with a total exposure time of 600sec in
F225W. To determine the depth in the NIR, we imagine that only $1 \%$
of the light is in the extended galaxy, yielding a surface brightness
of 22 mag arcsec$^{-2}$ for the faintest galaxies. To achieve a
$10 \, \sigma$ detection in a 0.2 arcsec beam requires 800 sec at F160W. 
We will of course reach much deeper surface brightness limits in radial 
averages.

In the UVIS bands, we will use a three-point dither pattern to
facilitate cosmic-ray and hot-pixel removal. In the IR, we will use
the WFC3-IR-DITHER-BOX in order to properly sample the PSF.  We will
use subarrays (512$\times$512 and IRSUB512 respectively) to avoid buffer
dumps and shorten read-out times.

The exposure times for each filter are as follows:

$\bullet$ F225W: Determine whether we still detect the dip in the SED 
that indicates a gap. Determine whether the source is resolved in the 
UV, to determine whether the host galaxy contributes significantly to the rise 
in the UV; $3 \times 190$~sec.

$\bullet$ F336W: Fill in the SED with simultaneous coverage, ensure 
$5 \sigma$ detection; $3 \times 50$~sec.

$\bullet$ F606W: Fill in the SED with simultaneous coverage, ensure 
$5 \sigma$ detection; $3 \times 40$~sec.

$\bullet$ F814W: Fill in the SED with simultaneous coverage, ensure 
$5 \sigma$ detection; $3 \times 40$~sec.

$\bullet$ F160W: In addition to filling in the 
SED, we also seek potential evidence for recent merger activity. 
We can reach surface brightness limits of $22$ mag arcsec$^{-2}$. Note that at 
$z = 2$ (our highest redshift source) we still probe beyond the 4000\AA\ 
break in the rest-frame of our galaxies; $4 \times 200$~sec.

Thanks to the use of sub-arrays, we can fit all five bands into a
single orbit and we need not worry about buffer dumps. The orbit for
each galaxy will proceed as follows: F160W-F814W-F606W-F336W-F225W.
We spend 25 min on overheads (6 for guide-star acquisition, 10 min for
readout, 3 min for filter changes, 5 min for dithering, and 1 min to
change from IR to UVIS). To maximize our dynamic range in F160W, we
will use STEP25, with NSAMP=12, and take a 4-point dither sequence.

We request a total of {\bf 6 orbits} to image the entire sample.


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%   3. SPECIAL REQUIREMENTS
%       (see Section 9.3 of the Call for Proposals)
%
%
\specialreq             % Do not delete this command.
% Justify your special requirements here, if any.

Our program absolutely requires the UV imaging requested herein.
Without the simultaneous SED measurement, we cannot link the 
$U-$shaped spectrum observed in the SDSS+\emph{Galex} imaging with a 
gap in the accretion disk.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%   4. COORDINATED OBSERVATIONS
%       (see Section 9.4 of the Call for Proposals)
%
%
\coordinatedobs          % Do not delete this command.
% Enter your coordinated observing plans here, if any.

A total on-source exposure time with the Jansky VLA of 41 minutes, or
a total observation time (including overheads) of {\bf one hour}, with the
Ku-band receiver will allow us to make a strong assessment of whether
radio synchrotron is the cause of the apparent red excess in these
objects. VLA observations in A- or B-configuration will be suitable to
resolve any core component from extended jets, and thus will provide a
suitable assessment of the core's synchrotron contribution to the
SED. The choice of Ku measurement further aims to separate core
contribution, which likely has a flat or rising spectrum toward higher
frequency, from an extended contribution, which typically has a
declining spectrum.

We determined this VLA exposure time using a 15GHz radio flux
extrapolated from the FIRST to the \emph{WISE} data points (Table 1). We then
assume a ratio of 30:1 for core-to-lobe flux contribution at 1 GHz,
and aim to collect a 10$\sigma$ measurement in two 2\,GHz bands, thus
requiring an RMS sensitivity $\sigma_{15}$ that is 300 times lower
than the extrapolated 15GHz point (2, 2, and 4 minutes on-source for
SED2, SED4, and SED5 respectively). For non-detections, we simply aim
to reach a full-4\,GHz-bandwidth sensitivity limit of 10\,$\mu$Jy (11
minutes for each of the three FIRST non-detections). These times were
determined using the VLA exposure calculator with 25 antennae, 2\,GHz
bandwidth at a mean of 17\,GHz, and accounting for expected 15\% RFI
losses and adding 20 minutes of overhead time. Our 2\,GHz bands will
be placed at mean frequencies of 13 and 17\,GHz. This will allow a
spectral measurement, or if no detection is made, will imply that
radio synchrotron is not a strong contributor to the SEDs.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%   5. JUSTIFY DUPLICATIONS
%       (see Section 9.5 of the Call for Proposals)
%
%
\duplications           % Do not delete this command.
% Enter your duplication justifications here, if any.

Object SED4 in our table has been observed previously by \hst\ with 
ACS/F435W and ACS/F814W. These observations (Figure 2) show us that the 
source is dominated by a point source, and reveal tidal features 
indicative of recent merging activity (Vartanyan et al.\ in prep). 
However, they do not provide a simultaneous UV-NIR SED, which is required 
to demonstrate unambiguously that the observed $U-$shaped SED is real. 
Thus, we request additional imaging in all five bands here.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\end{document}          % End of proposal. Do not delete this line.
                        % Everything after this command is ignored.

