1512.04434v1.pdf

To be submitted to Astrophysical Journal Letters

Preprint typeset using L

X style emulateapj v. 5/2/11

GAMMA RAYS FROM THE QUASAR PKS 1441+25: STORY OF AN ESCAPE

A. U. Abeysekara

, S. Archambault

, A. Archer

, T. Aune

, A. Barnacka

, W. Benbow

, R. Bird

J. Biteau

8,9,44

, J. H. Buckley

, V. Bugaev

, J. V Cardenzana

, M. Cerruti

6,45

, X. Chen

11,12

, J. L. Christiansen

L. Ciupik

, M. P. Connolly

, P. Coppi

, W. Cui

, H. J. Dickinson

, J. Dumm

, J. D. Eisch

, M. Errando

3,19,46

A. Falcone

, Q. Feng

, J. P. Finley

, H. Fleischhack

, A. Flinders

, P. Fortin

, L. Fortson

, A. Furniss

8,21

G. H. Gillanders

, S. Griffin

, J. Grube

, G. Gyuk

, M. H

utten

, N. H

akansson

, D. Hanna

, J. Holder

T. B. Humensky

, C. A. Johnson

8,47

, P. Kaaret

, P. Kar

, N. Kelley-Hoskins

, Y. Khassen

, D. Kieda

M. Krause

, F. Krennrich

, S. Kumar

, M. J. Lang

15,48

, G. Maier

, S. McArthur

, A. McCann

K. Meagher

, P. Moriarty

, R. Mukherjee

, D. Nieto

, A. O’Faol

ain de Bhr

oithe

, R. A. Ong

A. N. Otte

, N. Park

, J. S. Perkins

, A. Petrashyk

, M. Pohl

11,12

, A. Popkow

, E. Pueschel

, J. Quinn

K. Ragan

, G. Ratliff

, P. T. Reynolds

, G. T. Richards

, E. Roache

, J. Rousselle

, M. Santander

G. H. Sembroski

, K. Shahinyan

, A. W. Smith

, D. Staszak

, I. Telezhinsky

11,12

, N. W. Todd

, J. V. Tucci

J. Tyler

, V. V. Vassiliev

, S. Vincent

, S. P. Wakely

, O. M. Weiner

, A. Weinstein

, A. Wilhelm

11,12

D. A. Williams

, B. Zitzer

(VERITAS) & P. S. Smith

(SPOL) & T. W.-S. Holoien

32,35

, J. L. Prieto

33,34

C. S. Kochanek

32,35

, K. Z. Stanek

32,35

, B. Shappee

36,37

(ASAS-SN) & T. Hovatta

, W. Max-Moerbeck

T. J. Pearson

, R. A. Reeves

, J. L. Richards

, A. C. S. Readhead

(OVRO) & G. M. Madejski

(NuSTAR) &

S. G. Djorgovski

, A. J. Drake

, M. J. Graham

, A. Mahabal

(CRTS)

To be submitted to Astrophysical Journal Letters

ABSTRACT

Outbursts from gamma-ray quasars provide insights on the relativistic jets of active galactic nuclei

and constraints on the diffuse radiation fields that fill the Universe. The detection of significant

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 (

= 0

939) during April 2015, 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.

Subject headings:

cosmology: observations — diffuse radiation — gamma rays: galaxies — quasars: in-

dividual (PKS 1441+25 = VER J1443+250) — radiation mechanisms: non-thermal

Department of Physics and Astronomy, University of Utah,

Salt Lake City, UT 84112, USA

Physics Department, McGill University, Montreal, QC H3A

2T8, Canada

Department of Physics, Washington University, St. Louis,

MO 63130, USA

Department of Physics and Astronomy, University of

California, Los Angeles, CA 90095, USA

Harvard-Smithsonian Center for Astrophysics, 60 Garden

Street, Cambridge, MA 02138, USA

Fred Lawrence Whipple Observatory, Harvard-Smithsonian

Center for Astrophysics, Amado, AZ 85645, USA

School of Physics, University College Dublin, Belfield,

Dublin 4, Ireland

Santa Cruz Institute for Particle Physics and Department of

Physics, University of California, Santa Cruz, CA 95064, USA

now at Insititut de Physique Nucl ́eaire d’Orsay (IPNO),

CNRS-INP3, Univ. Paris-Sud, Universit ́e Paris-Saclay, 91400

Orsay, France

Department of Physics and Astronomy, Iowa State Univer-

sity, Ames, IA 50011, USA

Institute of Physics and Astronomy, University of Potsdam,

14476 Potsdam-Golm, Germany

DESY, Platanenallee 6, 15738 Zeuthen, Germany

Physics Department, California Polytechnic State Univer-

sity, San Luis Obispo, CA 94307, USA

Astronomy Department, Adler Planetarium and Astron-

omy Museum, Chicago, IL 60605, USA

School of Physics, National University of Ireland Galway,

University Road, Galway, Ireland

Department of Astronomy, Yale University, New Haven,

CT 06520-8101, USA

Department of Physics and Astronomy, Purdue University,

West Lafayette, IN 47907, USA

School of Physics and Astronomy, University of Minnesota,

Minneapolis, MN 55455, USA

Department of Physics and Astronomy, Barnard College,

Columbia University, NY 10027, USA

Department of Astronomy and Astrophysics, 525 Davey

Lab, Pennsylvania State University, University Park, PA 16802,

USA

now at California State University - East Bay, Hayward,

CA 94542, USA

Department of Physics and Astronomy and the Bartol

Research Institute, University of Delaware, Newark, DE 19716,

USA

Physics Department, Columbia University, New York, NY

10027, USA

Department of Physics and Astronomy, University of Iowa,

Van Allen Hall, Iowa City, IA 52242, USA

School of Physics and Center for Relativistic Astrophysics,

Georgia Institute of Technology, 837 State Street NW, Atlanta,

GA 30332-0430

Enrico Fermi Institute, University of Chicago, Chicago, IL

60637, USA

N.A.S.A./Goddard Space-Flight Center, Code 661, Green-

belt, MD 20771, USA

Department of Applied Science, Cork Institute of Technol-

ogy, Bishopstown, Cork, Ireland

University of Maryland, College Park / NASA GSFC,

College Park, MD 20742, USA

Argonne National Laboratory, 9700 S. Cass Avenue,

Argonne, IL 60439, USA

arXiv:1512.04434v1 [astro-ph.HE] 14 Dec 2015

The VERITAS Collaboration

et al.

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),

with only four belonging to the class

of flat-spectrum radio quasars (FSRQs): 3C 279 (

536, MAGIC Collaboration et al. 2008), PKS 1510-

089 (

= 0

361, H.E.S.S. Collaboration et al. 2013a),

PKS 1222+216 (

= 0

432, Aleksi ́c et al. 2011), and

S3 0218+35 (

= 0

944, Sitarek et al. 2015).

FSRQs are believed to host radiatively-efficient disks

that enrich the environment of the supermassive black

hole with ultraviolet-to-optical photons. This photon

field, the reprocessed 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 pro-

duction on the extragalactic background light (EBL),

which encompasses the ultraviolet-to-infrared emission

of all stars and galaxies since the epoch of reionization

(

10).

Direct measurements of the EBL are prone to

contamination from the bright local environment, while

strict lower limits are derived from galaxy surveys, mea-

suring 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 (

= 0

939, Shaw et al.

2012) by VERITAS. This source was observed in April

Steward Observatory, University of Arizona, 933 N. Cherry

Avenue, Tucson, AZ 85721, USA

Department of Astronomy, The Ohio State University, 140

West 18th Avenue, Columbus, OH 43210, USA

Nucleo de Astronomia de la Facultad de Ingenieria, Uni-

versidad Diego Portales, Av. Ejercito 441, Santiago, Chile

Millennium Institute of Astrophysics, Santiago, Chile

Center for Cosmology and AstroParticle Physics, The Ohio

State University, 191 W. Woodruff Ave., Columbus, OH 43210,

USA

Carnegie Observatories,

813 Santa Barbara Street,

Pasadena, CA 91101, USA

Hubble, Carnegie-Princeton Fellow

Aalto

University,

Mets ̈ahovi

Radio

Observatory,

Mets ̈ahovintie 114, 02540, Kylm ̈al ̈a, Finland

National Radio Astronomy Observatory, P.O. Box 0,

Socorro, NM 87801, USA

Cahill Center for Astronomy and Astrophysics, California

Institute of Technology, Pasadena CA, 91125, USA

CePIA, Departamento de Astronom ́ıa, Universidad de

Concepci ́on, Casilla 160-C, Concepci ́on, Chile

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

California Institute of Technology, 1200 E. California Blvd,

Pasdena CA, 91125, USA

jbiteau@ucsc.edu, biteau@ipno.in2p3.fr

matteo.cerruti@cfa.harvard.edu

errando@astro.columbia.edu

caajohns@ucsc.edu

mark.lang@nuigalway.ie

http://tevcat.uchicago.edu/

We adopt the concordance ΛCDM model (

= 0

7, Ω

M

3, Ω

= 0

7).

and May 2015 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; Thompson 2015).

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 tele-

scopes located in southern Arizona (Holder 2011). VERI-

TAS 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 ra-

dio location and at a significance of 7.7 standard devi-

ations (

) during the 15

0 h exposure (2710 ON-source

events, 13780 OFF-source events, OFF normalization

of 1/6).

Using a standard analysis with cuts opti-

mized for low-energy showers (Archambault et al. 2014,

and references therein), we measure an average flux of

Φ(

80 GeV) = (5

−

with a

photon index Γ

VHE

= 5

5 up to 200 GeV,

cor-

responding to an intrinsic index of 3

5 after cor-

rection for the EBL (Gilmore et al. 2012, “fixed”). The

day-by-day lightcurve is compatible with constant emis-

sion in that period (

/ndf

= 7

6), and fractional

variability

var

110 % at the 95 % confidence level

(Vaughan et al. 2003).

Subsequent observations in May

(MJD 57155-57166, 3

8 h exposure) showed no significant

excess (660 ON-source events, 3770 OFF-source events,

OFF normalization of 1/6), resulting in an upper limit

of Φ(

80 GeV)

−

at the 99 % con-

fidence 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, respec-

tively. The systematic uncertainty on the flux 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 August 2008 (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) within a region of inter-

est of 10

◦

radius and fixing them for sources 10

−

◦

away. We reconstruct the spectrum of PKS 1441+25 be-

tween 100 MeV and 100 GeV in four-week (MJD 54705-

57169, top panel in Fig. 1) and two-week (MJD 57001-

57169, middle panel) bins assuming a power-law model

with a free normalization and photon index (purple

points), as well as in one-day bins (pink points) fix-

ing the photon index to its best-fit average value in

MJD 57001-57169, Γ

= 1

02, slightly harder

than in the 3FGL, 2

07. The source was in a

high state during MJD 57001-57169, with an integrated

100 MeV

−

100 GeV flux that is one to two orders of mag-

nitude above the 3FGL value, (1

−

During the period contemporaneous with the VERITAS

The last spectral points at 140 and 180 GeV are significant at

the 2

4 and 3

level, respectively

All flux estimates are used for variability constraints but we

also show 99 %-confidence-level upper limits for points below 3

in Fig. 1

Gamma rays from PKS 1441+25

detection (MJD 57133-57140), the source shows a flux of

(34

−

and a hard index of 1

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

(see Fig. 2). The curvature is resilient to

changes in the analysis and the temporal window, and

fits in smaller energy ranges confirm the hint.

X-ray observations with

NuSTAR

and

Swift

were trig-

gered following the VHE detection.

NuSTAR

, a hard-

X-ray instrument sensitive to 3

−

79 keV photons (Har-

rison et al. 2013), observed the source on MJD 57137

for an exposure of 38

2 ks. The data were reduced us-

ing the

NuSTARDAS

software v1.3.1.

Swift

-XRT (Gehrels

et al. 2004) observed PKS 1441+25 between 0

3 and

10 keV in June 2010 (MJD 55359), in January 2015

(MJD 57028 & 57050), in April 2015 (MJD 57127-57138),

and in May 2015 (MJD 57155 & 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

10 and an integrated 3

−

30 keV

flux of (1

09)

−

erg cm

−

. No intranight

variability is detected.

Swift

-XRT did not significantly

detect the source in June 2010, but the 2015 observa-

tions reveal a power-law spectrum with no detectable

spectral variability and an average 0

−

10 keV photon

index of 2

24, using an absorbed model with a

hydrogen column density of 3

−

(Kalberla

et al. 2005). Significant flux variations are detected in

the period contemporaneous with VERITAS observa-

tions (

/ndf

= 25

var

= 22

9 %), with a flux-

halving time of 13

4 days based on an exponential

fit to the data in MJD 57127-57155 (

/ndf

= 8

6).

Simultaneously with the XRT observations,

Swift

UVOT (Roming et al. 2005) took photometric snapshots

of PKS 1441+25 in six optical-to-ultraviolet filters. Flux

densities were extracted using

uvotmaghist

and circu-

lar ON-source and background regions of 5 arcsec and

15 arcsec radius, respectively. PKS 1441+25 has been

observed in the V-band since January 2012 within the

All-Sky Automated Survey for Supernovae (ASAS-SN;

Shappee et al. 2014), using the quadruple 14-cm “Bru-

tus” telescope in Hawaii. The fluxes from both experi-

ments in Fig. 2 are dereddenned using

(

−

) = 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

unfiltered optical observations of PKS 1441+25 since

2005 within the Catalina Real-Time Transient Survey

(CRTS, Drake et al. 2009). Observed magnitudes were

converted into the V-band using the empirical method

described in Drake et al. (2013). The SPOL spectropo-

larimeter (Schmidt et al. 1992) has monitored the linear

optical polarization of PKS 1441+25 in 5000

−

7000

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

1 % to

57120

57130

57140

57150

57160

deg

angle

Polarization

100

105

110

115

120

SPOL

57120

57130

57140

57150

57160

fraction

Polarization

SPOL

57120

57130

57140

57150

57160

-1

-2

erg cm

-15

x10

(O-UV)

0.5

1.5

2.5

V - ASAS-SN

V - Catalina

-UVOT / ASAS-SN / Catalina

Swift

57120

57130

57140

57150

57160

-1

-2

erg cm

-12

(2-10 keV)

0.5

1.5

-XRT

Swift

57120

57130

57140

57150

57160

0.1-100 GeV

Index

1.7

1.8

1.9

2.1

2.2

-LAT

Fermi

57120

57130

57140

57150

57160

-1

-2

-8

(0.1-100 GeV)

-LAT

Fermi

57120

57130

57140

57150

57160

-1

-2

-11

(> 80 GeV)

-5

VERITAS

57020

57040

57060

57080

57100

57120

57140

57160

0.1-100 GeV

Index

1.7

1.8

1.9

2.1

2.2

2.3

-LAT

Fermi

57020

57040

57060

57080

57100

57120

57140

57160

-1

-2

-8

(0.1-100 GeV)

100

150

200

-LAT

Fermi

55000

55500

56000

56500

57000

-1

-2

-8

(0.1-100 GeV)

-LAT

Fermi

55000

55500

56000

56500

57000

-1

-2

erg cm

-15

x10

(V)

0.5

1.5

Catalina

55000

55500

56000

56500

57000

(15 GHz)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

OVRO

MM/YY

12/08

12/09

12/10

12/11

12/12

12/13

12/14

DD/MM

08/04

15/04

22/04

29/04

06/05

13/05

20/05

MM/YY

12/14

01/15

02/15

03/15

04/15

05/15

MJD

55000

55500

56000

56500

57000

MJD

57020

57040

57060

57080

57100

57120

57140

57160

MJD

57120

57130

57140

57150

57160

Fig. 1.—

Top:

observations of PKS 1441+25 from 2008 to 2015.

Middle:

observations from December 2014 to May 2015.

Bottom:

observations in April and May. The gray dashed lines mark the

period considered for the analyses in Sec. 3 and 4.

The VERITAS Collaboration

et al.

[ MeV ]

Energy

-10

-8

-6

-4

-2

]

-1

-2

[ erg cm

Energy flux

-14

-13

-12

-11

-10

April 2015

VERITAS

-LAT

Fermi

NuSTAR

Swift

ASAS-SN

Other observations

- May 2015

Swift

- January 2015

Swift

- June 2010

Swift

Archival (ASDC)

Broad-band modeling

Synchrotron

Self Compton

External Compton

(torus)

[ Hz ]

Frequency

]

-1

[ erg s

Luminosity

[ MeV ]

Energy

]

-1

-2

[ erg cm

Energy flux

-12

-11

-10

VERITAS

-LAT

Fermi

Corrected for EBL

Observed

[ MeV ]

Energy

-3

-2

]

-1

-2

[ erg cm

Energy flux

-13

-12

NuSTAR

-XRT

Swift

Fig. 2.—

Multiwavelength emission of PKS 1441+25. Side panels show the X-ray (top) and gamma-ray emission (bottom) in April 2015

(MJD 57133-57140). The various exposures and the model are discussed in Sec. 2 and 3, respectively.

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 fea-

ture located 1

2 milliarcsec downstream, at position angle

−

◦

. Both features have relatively high fractional po-

larization (

∼

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

field. The fractional polarization level of the core fea-

ture is among the highest seen in the MOJAVE program

(Lister et al. 2011).

The 2008-2015 observations of PKS 1441+25 shown in

Fig. 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 coefficient

= 0

02, differing

from zero by 5

based on the

-distribution of shuffled

lightcurve points. Similarly, the analysis of the optical

and HE lightcurves yields

= 0

02, differing from

zero by 4

. The discrete correlation functions display

broad, zero-centered peaks with widths of

∼

100 days, in-

dicating no significant time lags beyond this time scale.

During the period marked by gray dashed lines in Fig. 1,

observations on daily timescales from optical wavelengths

to X-rays reveal fractional flux variations smaller than

25 %, compatible with the upper limits set by

Fermi

-LAT

and VERITAS (30 % and 110 % at the 95 % confidence

level, respectively). Such flux variations are small with

respect to the four orders of magnitude spanned in

νF

enabling the construction of a quasi-contemporaneous

spectral energy distribution in Sec. 3.

EMISSION SCENARIO

The spectral energy distribution, with the X-ray-to-

VHE data averaged over the active phase in April 2015

(MJD 57133-57140), is shown in Fig 2. The optical-to-

X-ray spectrum is well described by a power law with

photon index Γ = 2

01 from 2 eV to 30 keV, in-

cluding a 10 % intrinsic scatter in the fit procedure that

accounts for the small-amplitude optical-to-UV variabil-

ity. This spectrum suggests a single synchrotron compo-

nent peaking below 2 eV

∼

Hz, created by an

electron population of index

= 2Γ

−

∼

02.

As expected in FSRQs (Fossati et al. 1998), the emission

of PKS 1441+25 is dominated by the gamma-ray com-

ponent, well-described by a single component peaking at

−

GeV.

The detection of gamma rays up to 200 GeV, about

400 GeV in the galaxy’s frame, suggests that the emit-

ting region is located beyond the BLR, or else pair pro-

duction would suppress any VHE flux even for a flat

BLR geometry (Tavecchio & Ghisellini 2012). The el-

evated 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 flare.

This behavior contrasts with other observations of bright

FSRQs, displaying different flux variations at different

wavelengths (e.g. Abdo et al. 2010), more in line with

multi-component scenarios. The flare of PKS 1441+25

appears to be one of the few events whose detailed tem-

poral and spectral multiwavelength features are consis-

tent with the emission of a single component beyond the

BLR.

The BLR size can be derived using the estimated black-

hole mass,

= 10

(Shaw et al. 2012),

assuming

BLR

√

disk

erg s

−

(Kaspi

et al. 2007) and an accretion disk luminosity that is a

fraction

= 10 % of the Eddington luminosity. Alterna-

tively,

disk

can be estimated from the BLR luminosity as

disk

BLR

, with

BLR

= 10

erg s

−

(Xiong

& Zhang 2014). Both estimates yield

BLR

03 pc,

Gamma rays from PKS 1441+25

setting a lower limit on the distance between the black

hole and the emitting region of

000 Schwarzschild

radii.

We show in Fig. 2 that a stationary model can re-

produce 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 opti-

cal 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 emis-

sion is outside of the BLR. We parametrize the electron-

positron population by a fixed broken power law with in-

dices

= 2 and

= 3

58 between

min

= 1 and

max

, with a break at

break

= 1

(jet frame). The

particle energy density is on the order of the magnetic

energy density,

= 1

5, with a total luminosity of

erg s

−

, and an infrared photon energy density of

−

erg cm

−

= 10

K (galaxy frame), char-

acteristic 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,

var

= (1 +

)

/δ

R/c

, be comparable to

the flux-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

= 4

∼

1 pc and Doppler

factor

∼

18. The magnetic field,

∼

80 mG, is tan-

gled within the emitting frame, but compressed trans-

versely to the motion within the observer’s frame, which

would explain the high optical-polarization degree,

Following Sasada et al. (2014), the ratio of

over the

maximum theoretical polarization Π

= (

+1)

(

constrains the angle

at which the emitting region is

viewed. We find that the Doppler factor and the geom-

etry of the system are well reproduced by a jet Lorentz

factor Γ

jet

∼

12 and

∼

◦

, which is within the jet

opening angle,

jet

= 1

jet

∼

◦

The location of the emission can be roughly estimated

assuming that the whole cross-section of the jet con-

tributes to the radiation (Tavecchio et al. 2010), as

∼

tan

jet

∼

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 variabil-

ity is seen from optical to X-ray wavelengths, despite the

statistics being sufficient 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), B ̈ottcher (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 field being

two to ten times lower than that inferred for other VHE

FSRQs. The magnetic field 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.

EXTRAGALACTIC BACKGROUND LIGHT

The redshift of PKS 1441+25,

= 0

939, and its detec-

tion 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 in-

trinsic VHE hardness can then constrain the EBL inten-

sity (Aharonian et al. 2006). In a scenario where the HE

and VHE energy photons originate from the same compo-

nent (but see Stern & Poutanen 2014, for emission within

the BLR), the unattenuated HE observations set an up-

per limit on the intrinsic hardness. Based on Sec. 3, we

neglect any additional emission component and fit, as in

Biteau & Williams (2015), an absorbed power law with

free EBL normalization,

, to the VERITAS spectrum.

We minimize

∑

..n

(

−

Γ log(

i

)

−

ατ

(

i

)

i

+ Θ(Γ

LAT

−

Γ)

(Γ

LAT

−

Γ)

(1)

where

, and Γ are free parameters,

is fixed to

120 GeV,

{

}

..n

are the fluxes measured by VERI-

TAS at energies

{

}

..n

, and Θ is the Heaviside func-

tion. The

Fermi

-LAT log-parabolic spectrum in Fig. 2

shows a photon index Γ

LAT

= 2

43 at 30 GeV,

where the absorption by the EBL is smaller than 5 %

(Franceschini et al. 2008; Dom ́ınguez et al. 2011; Gilmore

et al. 2012). We account for the systematic uncertainty

on the VERITAS photon index, 0

20, by imposing a max-

imum hardness with uncertainty

= 0

⊕

20 = 0

47,

where

⊕

indicates a quadratic sum. We finally marginal-

ize the equivalent likelihood, exp(

−

2), over the VER-

ITAS energy scale. The logarithm of the latter is as-

sumed to be Gaussian, with zero mean and width 0

corresponding to a 20 % systematic uncertainty. Equa-

tion 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

= 0 and

constrained to

α <

5 at the 95 % confidence level for

the model of Gilmore et al. (2012). This result is almost

independent of the choice of model (Franceschini et al.

2008; Dom ́ınguez et al. 2011).

Considering both the peak and full width at half max-

imum (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 Fig. 3 is compatible and competitive below

m with other state-of-the-art gamma-ray measure-

ments from Ackermann et al. (2012), H.E.S.S. Collabo-

ration et al. (2013b), and Biteau & Williams (2015). Al-

though

is compatible with zero, there is no significant

tension with local constraints (see Biteau & Williams

2015, brown and orange arrows in Fig. 3), since the dif-

ferences are only 1

and 1

in the peak and FWHM

regions, respectively.

The VERITAS Collaboration

et al.

m ]

[

Wavelength

-1

]

-1

-2

[ nW m

EBL intensity

Biteau & Williams (2015) - 68% c.l.

H.E.S.S. (2013) - 68% c.l.

-LAT (2012) - 68% c.l.

Fermi

VERITAS - 95% c.l. upper limit

PKS 1441-25 - cross section peak

PKS 1441-25 - cross section FWHM

Lower limits

galaxy counts

Upper limits

direct observations

Fig. 3.—

Near-ultraviolet to near-infrared spectrum of the EBL.

The upper limit from this work is shown in blue, in regions corre-

sponding to the peak and FWHM of the cross section (1

< τ <

2).

DISCUSSION

The low energy threshold of VERITAS enabled the de-

tection above 80 GeV of one of the most distant VHE

gamma-ray sources (

= 0

939), in a redshift range pre-

viously accessible only to space-borne gamma-ray obser-

vatories. We obtain stringent constraints on the EBL

intensity below 1

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 stud-

ies above 1

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 flux at all wave-

lengths.

The correlation between the radio, optical,

and HE lightcurves, unusual for this class of sources

(Max-Moerbeck et al. 2014b), together with slow multi-

wavelength variability, suggest that the multi-band flare

was produced by a single region located

∼

−

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 observa-

tions, 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 Office of Science, the U.S. Na-

tional Science Foundation and the Smithsonian Institu-

tion, and by NSERC in Canada, with additional support

from NASA Swift GI grant NNX15AR38G. We acknowl-

edge the excellent work of the technical support staff at

the Fred Lawrence Whipple Observatory and at the col-

laborating institutions in the construction and operation

of the instrument. The VERITAS Collaboration is grate-

ful to Trevor Weekes for his seminal contributions and

leadership in the field of VHE gamma-ray astrophysics,

which made this study possible.

ASAS-SN thanks LCOGT, NSF, Mt. Cuba Astronom-

ical 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 sup-

ported in part by NASA grants NNX08AW31G and

NNX11A043G, and NSF grants AST-0808050 and AST-

1109911.

The National Radio Astronomy Observatory is a fa-

cility 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 (Lis-

ter et al. 2009).

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