Gravitational Waves: Volume 2: Astrophysics and Cosmology

823 Pages·2016·13.37 MB·English

by  Michele Maggiore

GRAVITATIONAL WAVES

Gravitational Waves
Volume 2
Astrophysics and Cosmology
Michele Maggiore
DépartementdePhysiqueThéorique
UniversitédeGenève
3
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Preface to Volume 2
In the preface to Vol. 1 we wrote: “The physics of gravitational waves
is in a very special period. [...] As a result of these experimental ef-
forts, there are good reasons to hope that the next decade will witness
the direct detection of gravitational waves and the opening of the field of
gravitational-wave astronomy and, possibly, cosmology.” The writing of
Vol.2took10moreyears,andduringthattimethesehopeshaveindeed
been fulfilled. The first direct detection of gravitational waves (GWs)
took place in September 2015, when the two detectors of the LIGO
Observatory, at the very beginning of the first science run of advanced
LIGO, observed the signal from a coalescing black-hole binary. The re-
sult was announced by the LIGO and Virgo collaborations in February
2016. For this discovery Reiner Weiss, Barry Barish and Kip Thorne
were awarded the 2017 Nobel Prize for Physics “for decisive contribu-
tions to the LIGO detector and the observation of gravitational waves”.
Severalmorebinaryblack-holecoalescenceshavenowbeenobserved,in-
cluding a triple coincidence between the two LIGO interferometers and
the Virgo interferometer. Furthermore, in August 2017, the GWs from
the coalescence of a neutron star binary were observed by LIGO and
Virgo, in coincidence with a γ-ray burst detected by Fermi-GBM and
by INTEGRAL. The electromagnetic counterpart was then identified
and observed by several telescopes in all bands of the electromagnetic
spectrum. Thus, the GW window has been opened, and we are starting
to look through it. With the planned improvements in the sensitivi-
ties of ground-based interferometers, we expect that GWs from astro-
physical sources will soon be observed routinely while, in the near and
mid-term future, we expect that several other detectors, such as pul-
sar timing arrays, the planned LISA space interferometer, and possibly
third-generation ground-based detectors such as the Einstein Telescope,
will explore the Universe up to cosmological distances. Thus, we are
indeed in a very exciting period for GW astrophysics and cosmology.
ThefirstvolumeofthisbookdealtwiththetheoryofGWs,inPartI,
and with experiments, in Part II. This second volume is devoted to
what we can learn from GWs, in astrophysics (Part III) and in cosmol-
ogy (Part IV). Our main aim has been to systematize a large body of
theoreticalandobservationaldevelopmentsthathavetakenplaceinGW
physics over the last few decades. Even though we expect the field to
evolve rapidly, thanks to new experimental discoveries as well as to the-
oretical advances, we believe that most of these methods will still form
the backbone of our understanding for many years to come, and will be
xiv
thebasisforfuturedevelopments. AswithVol.1,wehavetypicallytried
to rederive afresh all theoretical results, trying to give a coherent and
consistentpictureofthefield,andclarifyingorstreamliningtheexisting
derivations whenever possible. An Errrata web page will be maintained
at http://theory.physics.unige.ch/˜maggiore/home.html
Various people have read and commented on the drafts of some chap-
ters. I am particularly grateful to Alessandra Buonanno, Emanuele
Berti, Thibault Damour, Ruth Durrer, Valeria Ferrari, Stefano Foffa,
Kostas Kokkotas, Michael Kramer, Martin Kunz, Georges Meynet, An-
drea Passamonti, Eric Poisson, Luciano Rezzolla, Alberto Sesana and
Riccardo Sturani, for their careful reading of selected chapters, and to
Camille Bonvin, Daniela Doneva, Alex Kehagias and Vittorio Tansella
for useful comments.
I also wish to thank Sonke Adlung, Harriet Konishi and the staff of
OxfordUniversityPressfortheircompetentandfriendlyhelp, andMac
Clarke for the careful and competent copyediting.
Geneva, December 2017
10
Stellar collapse
Webeginourstudyofastrophysicalsourcesofgravitationalwaves(GWs)
withachapterdevotedtostellarcollapse. FromthepointofviewofGW 10.1 Historical Supernovae 4
physics, this is interesting for at least two different reasons. First, stel- 10.2 Properties of Supernovae 10
larcollapsethroughsupernovaexplosionisitselfapotentiallyinteresting 10.3 The dynamics of core
collapse 21
source of GW production. Second, stellar collapse can leave as remnant
10.4 GW production by
a compact object, such as a white dwarf (WD), a neutron star (NS) or
self-gravitating fluids 35
astellar-massblackhole(BH).Aswewillseeindetailinlaterchapters,
10.5 GWs from stellar collapse 46
these compact objects are among the most interesting sources from the
10.6 Complements 66
point of view of GW astrophysics.
Supernovae (SNe) are among the most spectacular events in the Uni-
verse. As we will discuss in detail in this chapter, there are two very
different mechanisms leading to SN explosions: (1) The gravitational
collapse of the core of a star, once the nuclear fuel that feeds the ther-
monuclear reactions inside the core is exhausted. As we will see in Sec-
tion 10.2.1, depending on the properties of the progenitor, this leads to
events classified as type Ib, Ic or type II SNe, and leaves behind a com-
pactremnant,usuallyaneutronstar(whichissometimesobservableasa
pulsar)orpossiblyablackhole(BH).1 (2)Thethermonuclearexplosion 1Therearealsoothermechanismsthat
ofawhitedwarfthataccretesmassfromacompanion, goingbeyondits cantriggertheexplosion,asinelectron-
capture SNe, as well as a more refined
Chandrasekhar limit. This gives rise to type Ia SNe. In this case the
classificationofSNtype,aswewilldis-
starthatexplodesisdispersedinspaceanditsremnantisnotacompact
cussinmoredetaillaterinthechapter.
object. A typical core-collapse SN can release an energy 1053 erg. Of It is also in principle possible that the
this energy, 99% is emitted in neutrinos, about 1% goes i∼nto kinetic en- collapse produces no visible SN event
and all the matter is swallowed by the
ergy of the ejected material, and less than 0.01%, i.e. about 1049 erg, is
finalBH.
released in photons. The corresponding peak luminosity in photons can
be of order a few times 109 or higher. Thus, a typical core-collapse
L(cid:12)
SN atits peakhas anoptical luminosity thatrivalsthe cumulative light
emittedbyallthestarsinitshostgalaxy. Aswewillseeindetailinthis
chapter, type Ia SNe have similar electromagnetic luminosities. Such
events, when they take place in our Galaxy (barring obscuration from
dust in the Galactic plane beyond a few kpc), can lead to truly impres-
sive optical displays that have been observed by mankind since ancient
times. A number of milestones in astronomy are associated with nearby
SN explosions. Tycho’s SN in 1572 and Kepler’s in 1604 marked a new
epoch in astronomy, while recent discoveries such as the pulsar in the
remnant of the 1054 Crab SN and the detection of neutrinos from the
1987A SN in the nearby Large Magellanic Cloud rank among the mile-
stones of modern astronomy. We therefore find it appropriate to begin
this chapter with an introduction to historical SNe.
GravitationalWaves,Volume2: AstrophysicsandCosmology. MicheleMaggiore.
c MicheleMaggiore2018. Publishedin2018byOxfordUniversityPress.
(cid:13)
DOI10.1093/oso/9780198570899.001.0001
4 Stellar collapse
Table10.1SummaryofthehistoricalSNe,andthesourceoftheirrecords,adapted
fromGreenandStephenson(2003). RemnantsupdatedfromFerrandandSafi-Harb
(2012). (Theidentifieroftheremnants,asforexampleinG120.1+01.4,referstoits
Galacticcoordinates). WehaveclassifiedSN1604astypeIa,followingforexample
Blairet al. (2007),Reynoldset al. (2007)andreferencestherein. Thenatureofthe
CrabSN(corecollapseorelectroncapture)isdebated;seetheFurtherReading.
Date Lengthof Remnant Classification Historicalrecords
visibility Chinese Japanese Korean Arabic European
AD1604(Kepler’s) 12months G004.5+06.8 typeIa few – many – many
AD1572(Tycho’s) 18months G120.1+01.4 typeIa few – two – many
AD1181 6months G130.7+03.1? – few few – – –
AD1054(Crab) 21months G184.6−05.8 debated many few – one –
AD1006 3years G327.6+14.6 typeIa many many – few two
AD393 8months G347.3−00.5 – one – – – –
AD386? 3months G011.2−00.3 – one – – – –
AD369? 5months – – one – – – –
AD185 8or20months G315.4−02.3 typeIa? one – – – –
10.1 Historical Supernovae
The reader eager to plunge into more
technical issues can simply skip this As we will see in Section 10.2.1, in a galaxy like ours the estimated rate
section ofSNexplosionsisoforderoftwopercentury. However,intheGalactic
plane the extinction of visible light significantly limits our view; for
instance,theextinctionofvisiblelightfromtheGalacticcenterisabout
30 magnitudes. Thus, a Galactic SN visible to the naked eye is a very
rare event. However, when a nearby star becomes a SN, the effects are
quite spectacular. In the historical period, this has happened a handful
of times.
2Novaearisewhenawhitedwarf(WD) Evidence for the oldest SNe observed by mankind is based on his-
in a binary system accretes mass from torical records. Since a Galactic SN remains visible for months, and
acompanionatarateofabout10−9− sometimesuptoafewyears, onecanfocusonhistoricalrecordsofstars
10−8M(cid:12)/yr. The material accreted that suddenly appeared and remained visible for at least four months,
is rich in hydrogen and accumulates
on the surface of the WD, where it in order to eliminate most novae2 and the possibility of confusion with
iscompressedbyfurtheraccretionand comets (comets being eliminated also because of their evident proper
heated. Whenalayerofabout10−5− motion).
10−4M(cid:12) of hydrogen has been ac- Table 10.1 gives a summary of SNe that have been seen by the naked
creted, a runaway thermonuclear reac-
tiontakesplace,releasinganenergyof eye in historical times, and for which we have written records, in the
order 1045 erg, which is therefore sev- last 2000 years.
eralordersofmagnitudesmallerthanin
SNexplosions. TheWDthengradually
cools down and goes back to its quies- SN 185
centstate, wheretheaccretionprocess
can start again, leading to recurrent The oldest recorded SN is SN 185, which was first observed on Decem-
novaexplosions. Acleardistinctionbe- ber 7, 185 at the imperial observatory of Lo-Yang, in central China.
tween classical novae and supernovae
The Chinese astronomers recorded its appearance and its gradual fad-
was first made by Baade and Zwicky
in1934. ing, providing the oldest plausible historical account of a SN. It finally
disappeared after either 8 or 20 months (depending on the interpre-
tation of a sentence in the record to mean “next year” or “year after
next”). The identification of the recorded position of the event with
10.1 Historical Supernovae 5
the region between α and β Cen suggests that the remnant of SN 185
could be identified with the SN remnant RCW 86 (G315.4 02.3), at an
−
estimated distance of about 2.8 kpc. Recent observations with Chandra
andXMM-Newtonhaveindeedstrengthenedthecaseforthisidentifica-
tion. The study of its X-ray synchrotron emission has in fact suggested
an estimate for the age of this remnant consistent with the explosion
date, within the uncertainties of the determination. The regular shape
of the remnant shell, together with the absence of a pulsar, would point
toward a type Ia SN, which is also suggested by the Chandra X-ray
observations.
Anotherpossibleremnantcandidatehasbeenproposed,G320.4 01.2
−
(RCW 89), which contains the pulsar PSR B1509-58, and therefore
would corresponds to a core-collapse SN. The timing measurement of
this pulsar are also consistent with the age of SN 185.
SN 1006
A number of other possible SNe were recorded before AD 1000; see
Table 10.1. However, the first event for which we have extensive histor-
ical records is a SN that exploded in AD 1006 in the direction of the
Lupis constellation, and was recorded in China, Japan, Europe, Egypt
and Iraq. It disappeared a first time from view after 17 months, and
remained occasionally visible at dawn for 3 years.
The identification of the likely remnant of this SN came in 1965, by
searching radio catalogues covering part of the region of sky suggested
by the historical records. The remnant is now identified with the radio
source PKS 1450-51 (whose Galactic coordinates are G327.4+14.6). Its
distance from us, as we now infer from the remnant, was about 2.2 kpc.
From the spectrum of the remnant it is believed that it was a type Ia
SN. Using the standard luminosity of type Ia SNe together with the
known distance to the remnant (and plausible assumptions about ab-
sorption) allows us to estimate that it reached an apparent visual mag-
nitudeV 7.5.3 Bycomparison,thefullMoonhasamaximumappar- 3Thestandardastronomicalnotionsof
(cid:39)−
ent magnitude V 12.94, and the limiting brightness for an object to luminosity,magnitudesandcolorindex
(cid:39)− ofstarsarerecalledintheComplement
be visible with the naked eye when the Sun is high is V 4, dropping
(cid:39)− Section10.6.
to V 2.5 when the Sun is less than 10 degrees above the horizon.
(cid:39) −
Comparing with the visual magnitudes given in Table 10.5 on page 70,
weseethat,atitsmaximumbrightness,SN1006wasbyfarthebrightest
object in the night sky after the Moon, and easily visible during day-
time. The historical records confirm that it was indeed visible during
theday,anditwasevenbrightenoughtoreadatnightbyitslight! The
most detailed reports by far are from Chinese astronomers, who deter-
mined the position of this “guest star” to good accuracy. Two accounts
of this SN appeared also in Europe, in chronicles of the monasteries of
St. Gallen, in Switzerland, and of Benevento, in Italy.