GAMMA-RAYS FROM THE QUASAR PKS
1441
+
25: STORY OF AN ESCAPE
A. U. Abeysekara
1
, S. Archambault
2
, A. Archer
3
, T. Aune
4
, A. Barnacka
5
, W. Benbow
6
, R. Bird
7
, J. Biteau
8
,
9
,
J. H. Buckley
3
, V. Bugaev
3
, J. V. Cardenzana
10
, M. Cerruti
6
,
11
, X. Chen
12
,
13
, J. L. Christiansen
14
, L. Ciupik
15
,
M. P. Connolly
16
, P. Coppi
17
, W. Cui
18
, H. J. Dickinson
19
, J. Dumm
19
, J. D. Eisch
10
, M. Errando
3
,
20
, A. Falcone
21
,
Q. Feng
18
, J. P. Finley
18
, H. Fleischhack
19
, A. Flinders
1
, P. Fortin
6
, L. Fortson
19
, A. Furniss
8
,
22
, G. H. Gillanders
16
,
S. Grif
fi
n
2
, J. Grube
15
, G. Gyuk
15
, M. Hütten
13
, N. Håkansson
12
, D. Hanna
2
, J. Holder
23
, T. B. Humensky
24
,
C. A. Johnson
8
, P. Kaaret
25
,P.Kar
1
, N. Kelley-Hoskins
13
, Y. Khassen
7
, D. Kieda
1
, M. Krause
13
, F. Krennrich
10
,
S. Kumar
23
, M. J. Lang
16
, G. Maier
13
, S. McArthur
18
, A. McCann
2
, K. Meagher
26
, P. Moriarty
16
, R. Mukherjee
20
,
D. Nieto
24
,A.O
’
Faoláin de Bhr
Ó
ithe
13
, R. A. Ong
4
, A. N. Otte
26
, N. Park
27
, J. S. Perkins
28
, A. Petrashyk
24
, M. Pohl
12
,
13
,
A. Popkow
4
, E. Pueschel
7
, J. Quinn
7
, K. Ragan
2
, G. Ratliff
15
, P. T. Reynolds
29
, G. T. Richards
26
, E. Roache
6
,
J. Rousselle
4
, M. Santander
20
, G. H. Sembroski
18
, K. Shahinyan
19
, A. W. Smith
30
, D. Staszak
2
, I. Telezhinsky
12
,
13
,
N. W. Todd
3
, J. V. Tucci
18
, J. Tyler
2
, V. V. Vassiliev
4
, S. Vincent
13
, S. P. Wakely
27
, O. M. Weiner
24
, A. Weinstein
10
,
A. Wilhelm
12
,
13
, D. A. Williams
8
, B. Zitzer
31
(
VERITAS
)
,
P. S. Smith
32
(
SPOL
)
,
T. W.-S. Holoien
33
,
34
, J. L. Prieto
35
,
36
, C. S. Kochanek
33
,
34
, K. Z. Stanek
33
,
34
, B. Shappee
37
,
44
(
ASAS-SN
)
,
T. Hovatta
38
, W. Max-Moerbeck
39
, T. J. Pearson
40
, R. A. Reeves
41
, J. L. Richards
18
, A. C. S. Readhead
40
(
OVRO
)
,
G. M. Madejski
42
(
NuSTAR
)
,
and
S. G. Djorgovski
43
, A. J. Drake
43
, M. J. Graham
43
, and A. Mahabal
43
(
CRTS
)
1
Department of Physics and Astronomy, University of Utah, Salt Lake City, UT 84112, USA
2
Physics Department, McGill University, Montreal, QC H3A 2T8, Canada
3
Department of Physics, Washington University, St.
Louis, MO 63130, USA
4
Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA
5
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
6
Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center for Astrophysics, Amado, AZ 85645, USA;
matteo.cerruti@cfa.harvard.edu
7
School of Physics, University College Dublin, Bel
fi
eld, Dublin 4, Ireland
8
Santa Cruz Institute for Particle Physics and Department of Physics, University of California, Santa Cruz, CA 95064, USA;
caajohns@ucsc.edu
9
Now at Institut de Physique Nucléaire d
’
Orsay
(
IPNO
)
, CNRS-IN2P3, Univ.
Paris-Sud, Université Paris-Saclay, F-91400 Orsay, France;
jbiteau@ucsc.edu
,
biteau@ipno.in2p3.fr
10
Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA
11
Now at Sorbonne Universités, UPMC Univ. Paris Diderot, Sorbonne Paris Cité, CNRS, Laboratoire de Physique Nucléaire et de Hautes Energies
(
LPNHE
)
,
4 place Jussieu, F-75252, Paris Cedex 5, France;
mcerruti@lpnhe.in2p3.fr
12
Institute of Physics and Astronomy, University of Potsdam, D-14476 Potsdam-Golm, Germany
13
DESY, Platanenallee 6, D-15738 Zeuthen, Germany
14
Physics Department, California Polytechnic State University, San Luis Obispo, CA 94307, USA
15
Astronomy Department, Adler Planetarium and Astronomy Museum, Chicago, IL 60605, USA
16
School of Physics, National University of Ireland Galway, University Road, Galway, Ireland;
mark.lang@nuigalway.ie
17
Department of Astronomy, Yale University, New Haven, CT 06520-8101, USA
18
Department of Physics and Astronomy, Purdue University, West Lafayette, IN 47907, USA
19
School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA
20
Department of Physics and Astronomy, Barnard College, Columbia University, NY 10027, USA;
errando@astro.columbia.edu
21
Department of Astronomy and Astrophysics, 525 Davey Lab, Pennsylvania State University, University Park, PA 16802, USA
22
Now at California State University
—
East Bay, Hayward, CA 94542, USA
23
Department of Physics and Astronomy and the Bartol Research Institute, University of Delaware, Newark, DE 19716, USA
24
Physics Department, Columbia University, New York, NY 10027, USA
25
Department of Physics and Astronomy, University of Iowa, Van Allen Hall, Iowa City, IA 52242, USA
26
School of Physics and Center for Relativistic Astrophysics, Georgia Institute of Technology, 837 State Street NW, Atlanta, GA 30332-0430, USA
27
Enrico Fermi Institute, University of Chicago, Chicago, IL 60637, USA
28
N.A.S.A.
/
Goddard Space-Flight Center, Code 661, Greenbelt, MD 20771, USA
29
Department of Applied Science, Cork Institute of Technology, Bishopstown, Cork, Ireland
30
University of Maryland, College Park
/
NASA GSFC, College Park, MD 20742, USA
31
Argonne National Laboratory, 9700 S.
Cass Avenue, Argonne, IL 60439, USA
32
Steward Observatory, University of Arizona, 933 N.
Cherry Avenue, Tucson, AZ 85721, USA
33
Department of Astronomy, The Ohio State University, 140 West 18th Avenue, Columbus, OH 43210, USA
34
Center for Cosmology and AstroParticle Physics, The Ohio State University, 191 W. Woodruff Ave., Columbus, OH 43210, USA
35
Nucleo de Astronomia de la Facultad de Ingenieria, Universidad Diego Portales, Av.
Ejercito 441, Santiago, Chile
36
Millennium Institute of Astrophysics, Santiago, Chile
37
Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA 91101, USA
38
Aalto University, Metsähovi Radio Observatory, Metsähovintie 114, FI-02540, Kylmälä, Finland
The Astrophysical Journal Letters,
815:L22
(
7pp
)
, 2015 December 20
doi:10.1088
/
2041-8205
/
815
/
2
/
L22
© 2015. The American Astronomical Society. All rights reserved.
1
39
National Radio Astronomy Observatory, P.O.
Box 0, Socorro, NM 87801, USA
40
Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA
41
CePIA, Departamento de Astronomía, Universidad de Concepción, Casilla 160-C, Concepción, Chile
42
W.
W.
Hansen Experimental Physics Laboratory, Kavli Institute for Particle Astrophysics and Cosmology, Department of Physics and
SLAC National Accelerator Laboratory, Stanford University, Stanford, CA 94305, USA
43
California Institute of Technology, 1200 E.
California Blvd, Pasdena, CA 91125, USA
Received 2015 August 10; accepted 2015 November 17; published 2015 December 15
ABSTRACT
Outbursts from gamma-ray quasars provide insights on the relativistic jets of active galactic nuclei and constraints
on the diffuse radiation
fi
elds that
fi
ll the universe. The detection of signi
fi
cant emission above 100 GeV from a
distant quasar would show that some of the radiated gamma-rays escape pair-production interactions with low-
energy photons, be it the extragalactic background light
(
EBL
)
, or the radiation near the supermassive black hole
lying at the jet
’
s base. VERITAS detected gamma-ray emission up to
∼
200 GeV from PKS 1441
+
25
(
z
=
0.939
)
during 2015 April, a period of high activity across all wavelengths. This observation of PKS
1441
+
25 suggests
that the emission region is located thousands of Schwarzschild radii away from the black hole. The gamma-ray
detection also sets a stringent upper limit on the near-ultraviolet to near-infrared EBL intensity, suggesting that
galaxy surveys have resolved most, if not all, of the sources of the EBL at these wavelengths.
Key words:
cosmology: observations
–
diffuse radiation
–
gamma rays: galaxies
–
quasars: individual
(
PKS 1441
+
25
=
VER J1443
+
250
)
–
radiation mechanisms: non-thermal
1. A NEW VERY HIGH ENERGY QUASAR
The extragalactic gamma-ray sky is dominated by the
emission of blazars, active galactic nuclei whose jets are pointed
within a few degrees of Earth. About sixty blazars have been
detected at very high energy
(
VHE;
E
>
100 GeV
)
,
45
with only
four belonging to the class of
fl
at-spectrum radio quasars
(
FSRQs
)
:3C
279
(
z
=
0.536, MAGIC Collaboration
et al.
2008
)
,PKS
1510
–
089
(
z
=
0.361, H.E.S.S. Collaboration
et al.
2013a
)
,PKS
1222
+
216
(
z
=
0.432, Aleksi
ć
et al.
2011
)
,
and S3
0218
+
35
(
z
=
0.944, Sitarek et al.
2015
)
.
FSRQs are believed to host radiatively ef
fi
cient disks that
enrich the environment of the supermassive black hole with
ultraviolet-to-optical photons. This photon
fi
eld, the repro-
cessed emission from the clouds of the broad line region
(
BLR
)
, and the infrared radiation from the
“
dusty torus
”
can all
interact with gamma-rays through pair production, preventing
the escape of VHE radiation from the base of the jet
(
Donea &
Protheroe
2003
)
.
VHE gamma-rays that do escape will face pair production on
the extragalactic background light
(
EBL
)
, which encompasses
the ultraviolet-to-infrared emission of all stars and galaxies
since the epoch of reionization
(
z
10
)
.
46
Direct measure-
ments of the EBL are prone to contamination from the bright
local environment, while strict lower limits are derived from
galaxy surveys, measuring the light emitted by known
populations of sources
(
Madau & Pozzetti
2000
)
.
VHE detections of high-redshift FSRQs constrain both the
EBL and the gamma-ray emission regions in blazars. This letter
reports the detection of VHE gamma-rays from the FSRQ
PKS
1441
+
25
(
z
=
0.939, Shaw et al.
2012
)
by VERITAS.
This source was observed in 2015 April and May following the
VHE discovery by MAGIC
(
Mirzoyan
2015
)
, triggered by
multiwavelength activity
(
Pacciani
2015
)
and a spectral
hardening at high energies
(
HE, 100 MeV
<
E
<
100 GeV;
D. Thompson 2015, private communication on behalf of the
Fermi
-LAT team
)
.
2. OBSERVATIONS OF PKS
1441
+
25
PKS
1441
+
25 was detected from 2015 April 21
(
MJD 57133
)
to April 28
(
MJD 57140
)
with VERITAS, an
array of four imaging atmospheric Cherenkov telescopes
located in southern Arizona
(
Holder
2011
)
. VERITAS imaged
gamma-ray induced showers from the source above 80 GeV,
enabling the detection of PKS
1441
+
25
(
VER J1443
+
250
)
at
a position consistent with its radio location and at a signi
fi
cance
of 7.7
standard
deviations
(
σ
)
during the 15.0 hr exposure
(
2710 ON-source events, 13780 OFF-source events, OFF
normalization of 1
/
6
)
. Using a standard analysis with cuts
optimized for low-energy showers
(
Archambault et al.
2014
,
and references therein
)
, we measure an average
fl
ux of
Φ
(
>
80 GeV
)
=
(
5.0
±
0.7
)
×
10
−
11
cm
−
2
s
−
1
with a photon
index
Γ
VHE
=
5.3
±
0.5 up to 200 GeV,
47
corresponding to an
intrinsic index of 3.4
±
0.5 after correction for the EBL
(
Gilmore et al.
2012
,
“
fi
xed
”
)
. The day-by-day lightcurve is
compatible with constant emission in that period
(
χ
2
/
ndf
=
7.4
/
6
)
, and fractional variability
F
var
<
110% at the
95% con
fi
dence level
(
Vaughan et al.
2003
)
.
48
Subsequent
observations in May
(
MJD
57155
–
57166, 3.8 hr exposure
)
showed no signi
fi
cant excess
(
660 ON-source events, 3770
OFF-source events, OFF normalization of 1
/
6
)
, resulting in an
upper limit of
Φ
(
>
80 GeV
)
<
4.3
×
10
−
11
cm
−
2
s
−
1
at the
99% con
fi
dence level. These results have been cross checked
with an independent calibration and analysis. Monte-Carlo
simulations indicate systematic uncertainties on the VHE
energy scale and photon index of 20% and 0.2, respectively.
The systematic uncertainty on the
fl
ux of this source is
estimated to be 60%, including the energy-scale uncertainty
discussed in Archambault et al.
(
2014
)
.
The LAT pair-conversion telescope onboard the
Fermi
satellite has surveyed the whole sky in the HE band since 2008
August
(
Atwood et al.
2009
)
. We analyzed the LAT data using
the public science tools
v10r0p5
(
Pass-8
)
leaving free the
parameters of sources from the 3FGL
(
Acero et al.
2015
)
44
Hubble, Carnegie-Princeton Fellow.
45
http:
//
tevcat.uchicago.edu
/
46
We adopt the concordance
Λ
CDM model
(
h
0
=
0.7,
Ω
M
=
0.3,
Ω
Λ
=
0.7
)
.
47
The last spectral points at 140 and 180 GeV are signi
fi
cant at the 2.4 and
3.0
σ
level, respectively.
48
All
fl
ux estimates are used for variability constraints but we also show 99%-
con
fi
dence-level upper limits for points below 3
σ
in Figure
1
.
2
The Astrophysical Journal Letters,
815:L22
(
7pp
)
, 2015 December 20
Abeysekara et al.
within a region of interest of 10
°
radius and
fi
xing them for
sources 10
°
–
20
°
away. We reconstruct the spectrum of
PKS
1441
+
25 between 100 MeV and 100 GeV in four-week
(
MJD 54705
–
57169, top panel in Figure
1
)
and two-week
(
MJD 57001
–
57169, middle panel
)
bins assuming a power-law
model with a free normalization and photon index
(
purple
points
)
,aswellasinone-daybins
(
pink points
)
fi
xing
the photon index to its best-
fi
t average value in MJD
57001
–
57169,
Γ
HE
=
1.97
±
0.02, slightly harder than in the
3FGL, 2.13
±
0.07. The source was in a high state during
MJD
57001
–
57169, with an integrated 100 MeV
–
100 GeV
fl
ux
that is one to two orders of magnitude above the 3FGL value,
(
1.3
±
0.1
)
×
10
−
8
cm
−
2
s
−
1
. During the period contempora-
neous with the VERITAS detection
(
MJD 57133
–
57140
)
,the
source shows a
fl
ux of
(
34
±
4
)
×
10
−
8
cm
−
2
s
−
1
and a hard
index of 1.75
±
0.08. Although a power-law model is used for
robustness in the lightcurve determination, the spectrum shows a
hint of curvature, with a log parabola preferred over a power law
by 3.2
σ
(
see Figure
2
)
. The curvature is resilient to changes in
the analysis and the temporal window, and
fi
ts in smaller energy
ranges con
fi
rm the hint.
X-ray observations with
NuSTAR
and
Swift
were triggered
following the VHE detection.
NuSTAR
, a hard-X-ray instru-
ment sensitive to 3
–
79 keV photons
(
Harrison et al.
2013
)
,
observed the source on MJD
57137 for an exposure of
38.2 ks. The data were reduced using the
NuSTARDAS
software v1.3.1.
Swift
-XRT
(
Gehrels et al.
2004
)
observed
PKS
1441
+
25 between 0.3 and 10 keV in 2010 June
(
MJD 55359
)
, in 2015 January
(
MJD 57028 and 57050
)
,
in 2015 April
(
MJD 57127
–
57138
)
, and in 2015 May
(
MJD 57155 and 57160
)
. Data taken in photon-counting mode
were calibrated and cleaned with
xrtpipeline
using
CALDB 20140120 v.014. ON-source and background events
were selected within regions of 20-pixel
(
∼
46 arcsec
)
and 40-
pixel radius, respectively. The XRT and
NuSTAR
spectral
analyses were performed with
XPSEC
v12.8.2, requiring at
least 20 counts per bin. The
NuSTAR
spectrum is matched by a
power law with a photon index of 2.30
±
0.10 and an integrated
3
–
30 keV
fl
ux of
(
1.25
±
0.09
)
×
10
−
12
erg cm
−
2
s
−
1
.Nointra-
night variability is detected.
Swift
-XRT did not signi
fi
cantly
detect the source in 2010 June, but the 2015 observations reveal
a power-law spectrum with no detectable spectral variability and
an average 0.3
–
10 keV photon index of 2.35
±
0.24, using an
absorbed model with a hydrogen column density of
3.19
×
10
20
cm
−
2
(
Kalberla et al.
2005
)
. Signi
fi
cant
fl
ux
variations are detected in the period contemporaneous with
VERITAS observations
(
χ
2
/
ndf
=
25.9
/
3,
F
var
=
22.6
±
0.9%
)
,
with a
fl
ux-halving time of 13.9
±
1.4 days based on an
exponential
fi
t to the data in MJD
57127
–
57155
(
χ
2
/
ndf
=
8.6
/
6
)
.
Simultaneously with the XRT observations,
Swift
-UVOT
(
Roming et al.
2005
)
took photometric snapshots of PKS
1441
+
25 in six optical-to-ultraviolet
fi
lters. Flux densities were
extracted using
uvotmaghist
and circular ON-source and
background regions of 5 and 15 arcsec radius, respectively.
PKS
1441
+
25 has been observed in the
V
-band since 2012
January within the All-Sky Automated Survey for Supernovae
(
ASAS-SN; Shappee et al.
2014
)
, using the quadruple 14-cm
“
Brutus
”
telescope in Hawaii. The
fl
uxes from both experi-
ments in Figure
2
are dereddenned using
E
(
B
−
V
)
=
0.043,
consistent with the column density used for the XRT analysis
(
Jenkins & Savage
1974
)
. The 0.68-m Catalina Schmidt
Telescope
(
AZ
)
has also performed long-term un
fi
ltered optical
observations of PKS 1441
+
25 since 2005 within the Catalina
Real-time Transient Survey
(
CRTS, Drake et al.
2009
)
.
Figure 1.
Top: observations of PKS
1441
+
25 from 2008 to 2015. Middle:
observations from 2014 December to 2015 May. Bottom: observations in April
and May. The gray dashed lines mark the period considered for the analyses in
Sections
3
and
4
.
3
The Astrophysical Journal Letters,
815:L22
(
7pp
)
, 2015 December 20
Abeysekara et al.
Observed magnitudes were converted into the
V
-band using the
empirical method described in Drake et al.
(
2013
)
. The SPOL
spectropolarimeter
(
Schmidt et al.
1992
)
has monitored the
linear optical polarization of PKS
1441
+
25 in 5000
–
7000
Å
,
with observations at the 1.54-m Kuiper Telescope, at the 6.5-m
MMT,
and
at
the
Steward
Observatory
2.3-m Bok Telescope
(
AZ
)
. The source shows a high degree
of polarization, with values ranging from 37.7
±
0.1% to
36.2
±
0.1% between MJD
57133 and MJD
57140.
The OVRO 40-m telescope
(
Richards et al.
2011
)
has
monitored PKS
1441
+
25 at 15 GHz since late 2009. A 15 GHz
VLBA image obtained by the MOJAVE program
(
Lister
et al.
2009
)
on 2014 March 30
(
MJD 56381
)
shows a compact
core and a bright, linearly polarized jet feature located 1.2 mas
downstream, at position angle
−
68
°
. Both features have
relatively high fractional polarization
(
∼
10%
)
, and electric
vectors aligned with the jet direction, at an angle of 102
°
similar to that measured by SPOL, indicating a well-ordered
transverse magnetic
fi
eld. The fractional polarization level of
the core feature is among the highest seen in the MOJAVE
program
(
Lister et al.
2011
)
.
The 2008
–
2015 observations of PKS
1441
+
25 shown in
Figure
1
reveal a brightening of the source in the radio, optical,
and HE bands starting around MJD
56900. A simple Pearson
test
(
see caveats in Max-Moerbeck et al.
2014a
)
applied to the
radio and HE long-term lightcurves shows a correlation
coef
fi
cient
r
=
0.75
±
0.02, differing from zero by 5.4
σ
based
on the
r
-distribution of shuf
fl
ed lightcurve points. Similarly,
the analysis of the optical and HE lightcurves yields
r
=
0.89
±
0.02, differing from zero by 4.8
σ
. The discrete
correlation functions display broad, zero-centered peaks with
widths of
∼
100 days, indicating no signi
fi
cant time lags
beyond this time scale. During the period marked by gray
dashed lines in Figure
1
, observations on daily timescales from
optical wavelengths to X-rays reveal fractional
fl
ux variations
smaller than 25%, compatible with the upper limits set by
Fermi
-LAT and VERITAS
(
30% and 110% at the 95%
con
fi
dence level, respectively
)
. Such
fl
ux variations are small
with respect to the four orders of magnitude spanned in
ν
F
ν
,
enabling the construction of a quasi-contemporaneous spectral
energy distribution in Section
3
.
3. EMISSION SCENARIO
The spectral energy distribution, with the X-ray-to-VHE
data averaged over the active phase in 2015 April
(
MJD 57133
–
57140
)
, is shown in Figure
2
. The optical-to-X-
ray spectrum is well described by a power law with photon
index
Γ
=
2.29
±
0.01 from 2 to 30 keV, including a 10%
intrinsic scatter in the
fi
t procedure that accounts for the
small-amplitude optical-to-UV variability. This spectrum
suggests a single synchrotron component peaking below
2eV
∼
5
×
10
14
Hz, created by an electron population of
index
p
=
2
Γ
−
1
∼
3.58
±
0.02. As expected in FSRQs
(
Fossati et al.
1998
)
, the emission of PKS
1441
+
25 is
dominated by the gamma-ray component, well-described by a
single component peaking at
3
.3 GeV.
1.1
1.8
-
+
The detection of gamma-rays up to 200 GeV, about 400 GeV
in the galaxy
’
s frame, suggests that the emitting region is
located beyond the BLR, or else pair production would
suppress any VHE
fl
ux even for a
fl
at BLR geometry
(
Tavecchio & Ghisellini
2012
)
. The elevated radio state,
correlated with the optical and HE brightening, also suggests
synchrotron emission outside of the BLR where synchrotron
self-absorption is smaller. The hypothesis of large-scale
emission is strengthened by the week-long duration of the
optical-to-gamma-ray
fl
are. This behavior contrasts with other
observations of bright FSRQs, displaying different
fl
ux
variations at different wavelengths
(
e.g., Abdo et al.
2010
)
,
more in line with multi-component scenarios. The
fl
are of
PKS
1441
+
25 appears to be one of the few events whose
detailed temporal and spectral multiwavelength features are
consistent with the emission of a single component beyond
the BLR.
The BLR size can be derived using the estimated black-hole
mass,
MM
10
BH
7.83 0.13
=
(
Shaw et al.
2012
)
, assuming
Figure 2.
Multiwavelength emission of PKS
1441
+
25. Side panels show the X-ray
(
top
)
and gamma-ray emission
(
bottom
)
in 2015 April
(
MJD 57133
–
57140
)
. The
various exposures and the model are discussed in Sections
2
and
3
, respectively.
4
The Astrophysical Journal Letters,
815:L22
(
7pp
)
, 2015 December 20
Abeysekara et al.
r
L
10 cm
10 erg s
BLR
17
disk
45
1
́
-
(
Kaspi et al.
2007
)
and
an accretion disk luminosity that is a fraction
η
=
10% of the
Eddington luminosity. Alternatively,
L
disk
can be estimated
from the BLR luminosity as
L
disk
;
10
×
L
BLR
, with
L
BLR
=
10
44.3
erg s
−
1
(
Xiong & Zhang
2014
)
. Both estimates
yield
r
BLR
;
0.03 pc, setting a lower limit on the distance
between the black hole and the emitting region of
r
5000
Schwarzschild radii.
We show in Figure
2
that a stationary model can reproduce
the data, using the numerical code of Cerruti et al.
(
2013
)
and
the EBL model of Gilmore et al.
(
2012
)
. Non-stationary Klein
–
Nishina effects on electron cooling could be important for this
source
(
Moderski et al.
2005
)
. Nonetheless, the straight power
law observed from optical to X-rays suggests that the
upscattering electrons see photon energies that are low enough
to stay out of the Klein
–
Nishina regime. This again indicates
that the emission is outside of the BLR. We parametrize the
electron
–
positron population by a
fi
xed broken power law with
indices
p
1
=
2 and
p
2
=
3.58 between
γ
min
=
1 and
γ
max
=
7
×
10
5
, with a break at
γ
break
=
1.2
×
10
4
(
jet
frame
)
. The particle energy density is on the order of the
magnetic energy density,
u
e
/
u
B
=
1.5, with a total luminosity
of 6
×
10
45
erg s
−
1
, and an infrared photon energy density of
1.4
×
10
−
5
erg cm
−
3
at
T
=
10
3
K
(
galaxy frame
)
, character-
istic of thermal radiation from the torus. We broke the
degeneracies among the parameters of the model by requiring
that the minimum variability timescale in the observer frame,
t
var
=
(
1
+
z
)
/
δ
×
R
/
c
, be comparable to the
fl
ux-halving
timescale of about two weeks observed in X rays, where
we have the best statistics to probe variability. The data
are then modeled with an emitting region of radius
R
=
4
×
10
17
cm
∼
0.1 pc and Doppler factor
δ
∼
18. The
magnetic
fi
eld,
B
∼
80 mG, is tangled within the emitting
frame, but compressed transversely to the motion within the
observer
’
s frame, which would explain the high optical-
polarization degree, PD. Following Sasada et al.
(
2014
)
, the
ratio of PD over the maximum theoretical polarization
Π
S
=
(
p
+
1
)
/
(
p
+
7
/
3
)
constrains the angle
θ
at which the
emitting region is viewed. We
fi
nd that the Doppler factor and
the geometry of the system are well reproduced by a jet Lorentz
factor
Γ
jet
∼
12 and
θ
∼
2
°
.6, which is within the jet opening
angle,
θ
jet
=
1
/
Γ
jet
∼
4
°
.8.
The location of the emission can be roughly estimated
assuming that the whole cross-section of the jet contributes to
the radiation
(
Tavecchio et al.
2010
)
,as
r
R
tan
jet
q
~~
1.5 pc
,
or 200,000 Schwarzschild radii. The region is not
expected to be much more compact than in this model, as no
fast large-amplitude variability is seen from optical to X-ray
wavelengths, despite the statistics being suf
fi
cient to detect
doubling timescales as short as days. The region could still be
further away from the black hole if its size was only fraction of
the jet cross-section.
The model parameters are similar to those obtained by
MAGIC Collaboration et al.
(
2008
)
, Böttcher et al.
(
2008
)
,
Tavecchio et al.
(
2011
)
, and Barnacka et al.
(
2014
)
for other
FSRQs, but a remarkably high break energy in the electron
spectrum is needed to explain the optical-to-X-ray synchrotron
emission. The break in the electron distribution is consistent
with radiative cooling, but is pushed to higher energies due to
the magnetic
fi
eld being two to ten times lower than that
inferred for other VHE FSRQs. The magnetic
fi
eld is also in
equipartition with the particle population, minimizing the
energy budget required to produce the synchrotron emission.
Finally, the jet Lorentz factor is two to four times smaller than
that required for other VHE FSRQs, highlighting again the
reasonable energetics of this scenario.
4. EXTRAGALACTIC BACKGROUND LIGHT
The redshift of PKS
1441
+
25,
z
=
0.939, and its detection
up to 200 GeV provide an exceptional opportunity to study
the EBL.
Gamma-rays interact with EBL photons through pair
production, yielding an observed spectrum that is softer than
the intrinsic spectrum. Imposing a maximum intrinsic VHE
hardness can then constrain the EBL intensity
(
Aharonian
et al.
2006
)
. In a scenario where the HE and VHE energy
photons originate from the same component
(
but see Stern &
Poutanen
2014
, for emission within the BLR
)
, the unattenuated
HE observations set an upper limit on the intrinsic hardness.
Based on Section
3
, we neglect any additional emission
component and
fi
t, as in Biteau & Williams
(
2015
)
,an
absorbed power law with free EBL normalization,
α
, to the
VERITAS spectrum. We minimize
e
,1
in
i
EE
E
2
1..
0
log
2
2
LAT
LAT
2
2
ii
i
0
()
()
()
()
()()
å
c
ff
s
s
=
- ́
+QG -G ́
G-G
at
f
=
-G
-
G
where
α
,
f
0
, and
Γ
are free parameters,
E
0
is
fi
xed to 120 GeV,
i
in
1..
{
}
f
=
are the
fl
uxes measured by VERITAS at energies
E
,
ii
n
1..
{
}
=
and
Θ
is the Heaviside function. The
Fermi
-LAT
log-parabolic spectrum in Figure
2
shows a photon index
Γ
LAT
=
2.76
±
0.43 at 30 GeV, where the absorption by the
EBL is smaller than 5%
(
Franceschini et al.
2008
; Domínguez
et al.
2011
; Gilmore et al.
2012
)
. We account for the systematic
uncertainty on the VERITAS photon index, 0.20, by imposing
a maximum hardness with uncertainty
σ
Γ
=
0.43
⊕
0.20
=
0.47, where
⊕
indicates a quadratic sum. We
fi
nally margin-
alize the equivalent likelihood, exp
(
−
χ
2
/
2
)
, over the VER-
ITAS energy scale. The logarithm of the latter is assumed to be
Gaussian, with zero mean and width 0.2, corresponding to a
20% systematic uncertainty. Equation
(
1
)
allows for an intrinsic
VHE spectrum that is softer, but not harder, than the HE
spectrum. This yields an EBL normalization that is consistent
with
α
=
0 and constrained to
α
<
1.5 at the 95% con
fi
dence
level for the model of Gilmore et al.
(
2012
)
. This result is
almost independent of the choice of model
(
Franceschini
et al.
2008
; Domínguez et al.
2011
)
.
Considering both the peak and full width at half maximum
(
FWHM
)
of the cross section integrated along the line of sight
(
as in Biteau & Williams
2015
, with various evolutions tested
)
,
the VERITAS observations constrain the near-ultraviolet to
near-infrared EBL. Our constraint shown in Figure
3
is
compatible and competitive below 1
μ
m with other state-of-
the-art gamma-ray measurements from Ackermann et al.
(
2012
)
, H.E.S.S. Collaboration et al.
(
2013b
)
, and Biteau &
Williams
(
2015
)
. Although
α
is compatible with zero, there is
no signi
fi
cant tension with local constraints
(
see Biteau &
Williams
2015
, brown and orange arrows in Figure
3
)
, since
5
The Astrophysical Journal Letters,
815:L22
(
7pp
)
, 2015 December 20
Abeysekara et al.
the differences are only 1.5
σ
and 1.7
σ
in the peak and FWHM
regions, respectively.
5. DISCUSSION
The low energy threshold of VERITAS enabled the
detection above 80 GeV of one of the most distant VHE
gamma-ray sources
(
z
=
0.939
)
, in a redshift range previously
accessible only to space-borne gamma-ray observatories. We
obtain stringent constraints on the EBL intensity below 1
μ
m
and conclude that galaxy surveys have resolved most, if not all,
of the sources of the EBL in this region. This provides an
excellent baseline for studies above 1
μ
m where the redshifted
ultraviolet emission of primordial stars could be detected
(
Dwek et al.
2005
; Biteau & Williams
2015
)
.
The VHE detection of the highly polarized source PKS
1441
+
25 is contemporaneous with a period of hard HE emission
and of enhanced
fl
ux at all wavelengths. The correlation
between the radio, optical, and HE lightcurves, unusual for this
class of sources
(
Max-Moerbeck et al.
2014b
)
, together with
slow multiwavelength variability, suggest that the multi-band
fl
are was produced by a single region located
∼
10
4
–
10
5
Schwarzschild radii away from the black hole, which is
consistent with the VHE-gamma-ray escape condition.
PKS
1441
+
25 is by far the dimmest HE emitter of all VHE-
detected FSRQs listed in the 3FGL catalog. While HE activity
remains a prime trigger of VHE observations, searches for new
VHE-emitting quasars could also factor in radio-to-optical
brightening and synchrotron-dominated X-ray emission, as
reported for PKS
1441
+
25. These criteria will be of particular
interest if applied to distant FSRQs, possibly opening a new
observational window on the jets of blazars and on the
transformation of the universe
’
s light content with cosmic time.
This research is supported by grants from the U.S.
Department of Energy Of
fi
ce of Science, the U.S. National
Science Foundation and the Smithsonian Institution, and by
NSERC in Canada, with additional support from NASA Swift
GI grant NNX15AR38G. We acknowledge the excellent work
of the technical support staff at the Fred Lawrence Whipple
Observatory and at the collaborating institutions in the
construction and operation of the instrument. The VERITAS
Collaboration is grateful to Trevor Weekes for his seminal
contributions and leadership in the
fi
eld of VHE gamma-ray
astrophysics, which made this study possible.
ASAS-SN thanks LCOGT, NSF, Mt. Cuba Astronomical
Foundation, OSU
/
CCAPP and MAS
/
Chile for their support.
The observations at Steward Observatory are funded through
NASA Fermi GI grant NNX12AO93G.
CRTS is supported by the NSF grants AST-1313422 and
AST-1413600.
The OVRO 40-m monitoring program is supported in part by
NASA grants NNX08AW31G and NNX11A043G, and NSF
grants AST-0808050 and AST-1109911.
The National Radio Astronomy Observatory is a facility of
NSF operated under cooperative agreement by Associated
Universities, Inc.
This research has made use of data from the MOJAVE
database that is maintained by the MOJAVE team
(
Lister
et al.
2009
)
.
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