The Square Kilometer Array

Dayton Jones, K6DJ Principal Scientist (Retired), Jet Propulsion Laboratory, California Institute of Technology

Senior Research Scientist, Space Science Institute, Boulder

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Radio Astronomy

  Provides unique information about the Universe from

the Big Bang to the Solar System

–  non-thermal processes –  highest angular resolution –  unaffected by dust

HST

A Universe of hydrogen gas

A very different view -- but signal is very weak !

Why is Radio Astronomy important? A Universe of stars

“Dark Ages” - before the stars

?

Square-Kilometer Array

COBE, WMAP, & Planck satellites

Cosmic Microwave Background

(~ 400 K years)

radio

First stars & galaxies -  Epoch of

Reionization (~ 400 M years)

HST, JWST

light

Epoch of Reionization (and before)

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Present (~ 13.7 B years)

Big Bang

SKA will be able to image, not just detect, the EoR

  Distribution of first sources of ionizing radiation

  Tomography of intergalactic medium

  Incredibly rich data set

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3 December 2004

Square Kilometer Array

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Strong-Field Gravity

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Observation of Gravitational Waves from a Binary Black Hole Merger

B. P. Abbott et al.*

(LIGO Scientific Collaboration and Virgo Collaboration)(Received 21 January 2016; published 11 February 2016)

On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-WaveObservatory simultaneously observed a transient gravitational-wave signal. The signal sweeps upwards infrequency from 35 to 250 Hz with a peak gravitational-wave strain of 1.0 ! 10!21. It matches the waveformpredicted by general relativity for the inspiral and merger of a pair of black holes and the ringdown of theresulting single black hole. The signal was observed with a matched-filter signal-to-noise ratio of 24 and afalse alarm rate estimated to be less than 1 event per 203 000 years, equivalent to a significance greaterthan 5.1!. The source lies at a luminosity distance of 410!160

!180 Mpc corresponding to a redshift z " 0.09!0.03!0.04 .

In the source frame, the initial black hole masses are 36!5!4M⊙ and 29!4

!4M⊙, and the final black hole mass is62!4

!4M⊙, with 3.0!0.5!0.5M⊙c2 radiated in gravitational waves. All uncertainties define 90% credible intervals.

These observations demonstrate the existence of binary stellar-mass black hole systems. This is the first directdetection of gravitational waves and the first observation of a binary black hole merger.

DOI: 10.1103/PhysRevLett.116.061102

I. INTRODUCTION

In 1916, the year after the final formulation of the fieldequations of general relativity, Albert Einstein predictedthe existence of gravitational waves. He found thatthe linearized weak-field equations had wave solutions:transverse waves of spatial strain that travel at the speed oflight, generated by time variations of the mass quadrupolemoment of the source [1,2]. Einstein understood thatgravitational-wave amplitudes would be remarkablysmall; moreover, until the Chapel Hill conference in1957 there was significant debate about the physicalreality of gravitational waves [3].Also in 1916, Schwarzschild published a solution for the

field equations [4] that was later understood to describe ablack hole [5,6], and in 1963 Kerr generalized the solutionto rotating black holes [7]. Starting in the 1970s theoreticalwork led to the understanding of black hole quasinormalmodes [8–10], and in the 1990s higher-order post-Newtonian calculations [11] preceded extensive analyticalstudies of relativistic two-body dynamics [12,13]. Theseadvances, together with numerical relativity breakthroughsin the past decade [14–16], have enabled modeling ofbinary black hole mergers and accurate predictions oftheir gravitational waveforms. While numerous black holecandidates have now been identified through electromag-netic observations [17–19], black hole mergers have notpreviously been observed.

The discovery of the binary pulsar systemPSR B1913!16by Hulse and Taylor [20] and subsequent observations ofits energy loss by Taylor and Weisberg [21] demonstratedthe existence of gravitational waves. This discovery,along with emerging astrophysical understanding [22],led to the recognition that direct observations of theamplitude and phase of gravitational waves would enablestudies of additional relativistic systems and provide newtests of general relativity, especially in the dynamicstrong-field regime.Experiments to detect gravitational waves began with

Weber and his resonant mass detectors in the 1960s [23],followed by an international network of cryogenic reso-nant detectors [24]. Interferometric detectors were firstsuggested in the early 1960s [25] and the 1970s [26]. Astudy of the noise and performance of such detectors [27],and further concepts to improve them [28], led toproposals for long-baseline broadband laser interferome-ters with the potential for significantly increased sensi-tivity [29–32]. By the early 2000s, a set of initial detectorswas completed, including TAMA 300 in Japan, GEO 600in Germany, the Laser Interferometer Gravitational-WaveObservatory (LIGO) in the United States, and Virgo inItaly. Combinations of these detectors made joint obser-vations from 2002 through 2011, setting upper limits on avariety of gravitational-wave sources while evolving intoa global network. In 2015, Advanced LIGO became thefirst of a significantly more sensitive network of advanceddetectors to begin observations [33–36].A century after the fundamental predictions of Einstein

and Schwarzschild, we report the first direct detection ofgravitational waves and the first direct observation of abinary black hole system merging to form a single blackhole. Our observations provide unique access to the

*Full author list given at the end of the article.

Published by the American Physical Society under the terms ofthe Creative Commons Attribution 3.0 License. Further distri-bution of this work must maintain attribution to the author(s) andthe published article’s title, journal citation, and DOI.

PRL 116, 061102 (2016)Selected for a Viewpoint in Physics

PHY S I CA L R EV I EW LE T T ER Sweek ending

12 FEBRUARY 2016

0031-9007=16=116(6)=061102(16) 061102-1 Published by the American Physical Society

Laser Interferometer Gravitational-wave Observatory (LIGO)

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Strong-Field Gravity

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propagation time, the events have a combined signal-to-noise ratio (SNR) of 24 [45].Only the LIGO detectors were observing at the time of

GW150914. The Virgo detector was being upgraded,and GEO 600, though not sufficiently sensitive to detectthis event, was operating but not in observationalmode. With only two detectors the source position isprimarily determined by the relative arrival time andlocalized to an area of approximately 600 deg2 (90%credible region) [39,46].The basic features of GW150914 point to it being

produced by the coalescence of two black holes—i.e.,their orbital inspiral and merger, and subsequent final blackhole ringdown. Over 0.2 s, the signal increases in frequencyand amplitude in about 8 cycles from 35 to 150 Hz, wherethe amplitude reaches a maximum. The most plausibleexplanation for this evolution is the inspiral of two orbitingmasses, m1 and m2, due to gravitational-wave emission. Atthe lower frequencies, such evolution is characterized bythe chirp mass [11]

M ! "m1m2#3=5

"m1 $m2#1=5! c3

G

!5

96!!8=3f!11=3 _f

"3=5

;

where f and _f are the observed frequency and its timederivative and G and c are the gravitational constant andspeed of light. Estimating f and _f from the data in Fig. 1,we obtain a chirp mass of M≃ 30M⊙, implying that thetotal mass M ! m1 $m2 is ≳70M⊙ in the detector frame.This bounds the sum of the Schwarzschild radii of thebinary components to 2GM=c2 ≳ 210 km. To reach anorbital frequency of 75 Hz (half the gravitational-wavefrequency) the objects must have been very close and verycompact; equal Newtonian point masses orbiting at thisfrequency would be only ≃350 km apart. A pair ofneutron stars, while compact, would not have the requiredmass, while a black hole neutron star binary with thededuced chirp mass would have a very large total mass,and would thus merge at much lower frequency. Thisleaves black holes as the only known objects compactenough to reach an orbital frequency of 75 Hz withoutcontact. Furthermore, the decay of the waveform after itpeaks is consistent with the damped oscillations of a blackhole relaxing to a final stationary Kerr configuration.Below, we present a general-relativistic analysis ofGW150914; Fig. 2 shows the calculated waveform usingthe resulting source parameters.

III. DETECTORS

Gravitational-wave astronomy exploits multiple, widelyseparated detectors to distinguish gravitational waves fromlocal instrumental and environmental noise, to providesource sky localization, and to measure wave polarizations.The LIGO sites each operate a single Advanced LIGO

detector [33], a modified Michelson interferometer (seeFig. 3) that measures gravitational-wave strain as a differ-ence in length of its orthogonal arms. Each arm is formedby two mirrors, acting as test masses, separated byLx ! Ly ! L ! 4 km. A passing gravitational wave effec-tively alters the arm lengths such that the measureddifference is ΔL"t# ! "Lx ! "Ly ! h"t#L, where h is thegravitational-wave strain amplitude projected onto thedetector. This differential length variation alters the phasedifference between the two light fields returning to thebeam splitter, transmitting an optical signal proportional tothe gravitational-wave strain to the output photodetector.To achieve sufficient sensitivity to measure gravitational

waves, the detectors include several enhancements to thebasic Michelson interferometer. First, each arm contains aresonant optical cavity, formed by its two test mass mirrors,that multiplies the effect of a gravitational wave on the lightphase by a factor of 300 [48]. Second, a partially trans-missive power-recycling mirror at the input provides addi-tional resonant buildup of the laser light in the interferometeras a whole [49,50]: 20Wof laser input is increased to 700Wincident on the beam splitter, which is further increased to100 kW circulating in each arm cavity. Third, a partiallytransmissive signal-recycling mirror at the output optimizes

FIG. 2. Top: Estimated gravitational-wave strain amplitudefrom GW150914 projected onto H1. This shows the fullbandwidth of the waveforms, without the filtering used for Fig. 1.The inset images show numerical relativity models of the blackhole horizons as the black holes coalesce. Bottom: The Keplerianeffective black hole separation in units of Schwarzschild radii(RS ! 2GM=c2) and the effective relative velocity given by thepost-Newtonian parameter v=c ! "GM!f=c3#1=3, where f is thegravitational-wave frequency calculated with numerical relativityand M is the total mass (value from Table I).

PRL 116, 061102 (2016) P HY S I CA L R EV I EW LE T T ER S week ending12 FEBRUARY 2016

061102-3

Pulsar Timing Array

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Galactic Census of Pulsars

  Complete survey of pulsars in our galaxy

  Find large number of millisecond pulsars for timing

  Find rare pulsar-black hole binary systems

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SKA: 1.4 GHz/400 MHz/1024 T/G = 0.25 Jy 600 s

3 December 2004

Square Kilometer Array

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The importance of sensitivity Arecibo

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VLA 1980

SKA Sensitivity Reaches New Classes of Objects

HST SKA 2020

SKA�s large field-of-view for surveys and transient events in 109 galaxies !

HST SKA 6cm

ALMA

15 M

pc a

t z

= 2 SKA 20 cm

SKA

SKA

~ 1 deg. 16

  The SKA is the next generation radio telescope at meter/centimeter wavelengths

–  ~ $3B project, construction start ~ 2017

–  17 countries, 55 institutions involved

–  US deeply involved in planning SKA, but not currently a member

–  > $500M already committed to tech. tech. development and development and prototypes

SKA Technology Overview

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Three Antenna Types Needed Parabolic dishes at high frequencies: 1.0 GHz to ~10-20 GHz

Dense aperture array at medium frequencies: 0.3-1.0 GHz

Sparse dipole array at low frequencies: 0.07-0.3 GHz

SKA Animation

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Terrestrial Interference Drives Site Selection

FORTÉ satellite: 131 MHz

Forte satellite: 131MHz

O " O" South Africa Western Australia

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Possible SKA configuration in Africa

SKA Core Site in the Karoo desert region of South Africa

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SKA Prototype in South Africa

12-m composite antenna for MeerKAT prototype array

Antenna mounts arriving at site

Antenna construction building

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SKA combines many small antennas over a large area

200km

Dense Core + Remote Stations

Wide range of baseline lengths

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SKA Prototype in Australia

12-m antennas for Australian SKA Pathfinder (ASKAP) array

SKA Core Site in Australia

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SKA Engineering Challenge: Lower cost through mass-produced antennas and receivers

•  Example: Allen Telescope Array

•  6.1 m offset parabolas

•  0.5-11 GHz (simultaneously)

•  42 antennas installed so far

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One-piece 15 x 18 m composite reflector developed by China

Lower cost through simple mechanical & electrical systems

  LOFAR, MWA, and LWA (shown) serve as SKA prototypes for low frequency

  Uncooled LNAs, simple antenna kits

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Example: SKA Low Freq Antennas and Receivers

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Fig. 6. Current antenna design.

Fig. 7. Feeding of current antenna including the LNA boards.

SKA LF Antenna Impedance

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0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45

ï100

ï50

0

50

100

150

200

250

300

350

Freq /GHz

Z /O

hm

Imaginary part (measured)Imaginary part (simulated)Real part (measured)Real part (simulated)

imaginary part

real part

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MMIC Technology to Array Receiver Cost •Multifunction MMIC packaging of Ka band dual-downconverter for the DSN array reduces size and replication cost by an order of magnitude

Multi-Function MMIC Module

Assembly of Connectorized, Single-Function Parts

LO IF RF

LO

IF

IF

RF

IF

RF RF

11 cm

50 cm

High Frequency Receivers

Lower cost through mass-produced wideband feeds

  Enables high survey speed and extreme flexibility

  Uncooled LNA are OK

  Digital signal processing is challenging

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SKA Poster Multiple Simultaneous Beams Decade Bandwidth Feeds

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Locus for 1dB Gain Loss at 1420 MHz for F/D=0.437

2m Diameter Gregorian Subreflector2m Diameter Gregorian Subreflector

37-Element Focal Plane Array of 0.7 to 1.4 GHz Kildal-Type Feeds

FOV 173 deg2 at 700 MHz

Single Dual-Polarized Feed for 0.1 to 1.5 GHz

Feed is 1.38m Square

Low-Frequency Prime Focus Feed Options for US SKA 12/16m Reflector Feed is 0.46 λMAX Square. Reflector Beamwidths are 2.2O at 700 MHz, 1.1O at 1420 MHz

Chalmers Log-Periodic Feed – Single Polarized 1-6 GHz Test Model

Dual-Polarized 1.2 to 11 GHz Under Construction

Decade Bandwidth Feeds

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OLSSON et al.: THE ELEVEN ANTENNA 371

Fig. 3. Computed co- and cross-polar far-field patterns in the 45 plane of theantenna in Fig. 2. BOR -components only.

III. NUMERICAL DESIGN

We started by considering a simple feed consisting of twoparallel halfwave dipoles separated by a half wavelength and lo-cated above a ground plane. This is as explained in the introduc-tion an old feed which gives a pattern with equal E- and H-planesand with phase center in the ground plane [3]. Our idea was thento replace each single dipole by log-periodic series-fed dipoles,with a logarithmically varying dipole length, spacing and heightabove the ground plane, see Fig. 2. The initial studies were madewith wire dipoles, and we soon found out that the dipoles neededto be folded and mounted together in such a way that the feedgap of each folded dipole is connected to a gap in the normallyconnected folded part of the previous dipole. The studies werelater concentrated more on strip models of the dipoles to enableprinting on a normal dielectric circuit board.

To calculate impedance characteristics and radiation patternsof the feed we have mainly used the method of moments (MoM)as implemented in the commercial code WIPL-D [29], but wealso used the in-house MoM code described in [30] for someinitial studies and validation.

Here we will present numerical results obtained for the feedin Fig. 2 operating from 0.15 to 1.5 GHz, thereby covering adecade bandwidth. Note that we believe that, in principle, thereexists no theoretical limit to the bandwidth of the log-periodicEleven antenna, but our objective in the SKA application is todesign decade bandwidth feeds. In practice the bandwidth is nat-urally limited by the size of the feed at low frequencies and bythe diameter of the feeding cables at higher frequencies. Fig. 2shows the most important longest dimensions of the feed. Thesesizes are also given in terms of the wavelength at 100 MHz. Allthe dimensions of the dipoles such as length L, separation D,height above ground plane h, and strip width, are scaled withfrequency to provide the log-periodic shape. The scaling factork is 1.1161. The feed is excited with a delta voltage source be-tween the two strips of the split transmission line originating inthe center, and the dielectric support is neglected. Furthermorethe initial simulations were done for one linear polarization toreduce computational effort. The finite ground plane is includedin the analysis. Fig. 3 shows typical far-field patterns in the 45plane between 0.15 and 1.5 GHz similar to the patterns shown

Fig. 4. Computed directivity of the antenna in Fig. 2.

Fig. 5. Computed return loss of the antenna in Fig. 2.

Fig. 6. Computed phase center location of the antenna in Fig. 2.

in [22] for an eleven antenna covering 1 to 12 GHz. Fig. 4 showsthe directivity of the feed as a function of frequency. In Fig. 5we see the return loss of the feed at the feed point. This consti-tutes the main drawback of the feed. The return loss is not betterthan 5.0 dB at the worst frequencies, but we are confident it canbe improved. One of the major advantages of the feed is shownin Fig. 6 which presents the phase center location as a function

SKA computing challenges

  Parallel processing –  1 EFlop ~ 1 billion cores –  How do we program these?

  Computing power requirements –  1Eflop = 350MW at IBM Blue Gene efficiency –  Need one to two orders of magnitude improvement

  Data flow –  Data rate ~ 1 EB/day

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Computing Potentially Possible

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SKA Member Countries SKA Timeline

  2000 Initial SKA Concept Developed

  2005 International Agreement on Sites

  2014 Prototype Design, PDR

  2015-16 Detailed Design

  2017-23 SKA1 Construction

  2020 SKA1 Early Science

  2018-2021 SKA2 Design

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