Gamma-ray Observations of Explosive Nucleosynthesis Products

0.43 MB·English

by  Jacco Vink

1
Gamma-Ray Observations of Explosive Nucleosynthesis Products
Jacco Vink a
a SRONNationalInstituteforSpaceResearch,Sorbonnelaan2,3584CAUtrecht,TheNetherlands
February 2, 2008
5 Abstract
0
0 InthisreviewIdiscussthevariousγ-rayemissionlinesthatcanbeexpectedand, insomecaseshavebeenobserved, fromradioactive
2 explosivenucleosynthesisproducts. Themostimportantγ-raylinesresultfromthedecaychainsof56Ni,57Ni,and44Ti. 56Niistheprime
explosivenucleosynthesisproductofTypeIasupernovae,anditsdecaydeterminestoalargeextenttheTypeIalightcurves. 56Niisalsoa
n
productofcore-collapsesupernovae,andinfact,γ-raylineemissionfromitsdaughterproduct,56Co,hasbeendetectedfromSN1987Aby
a
J severalinstruments. Theearlyoccurrenceofthisemissionwassurprisingandindicatesthatsomefractionof56Ni,whichissynthesizedin
theinnermostsupernovalayers,musthavemixedwiththeoutermostsupernovaejecta.
9
2 Special attention is given to the γ-ray line emission of the decay chain of 44Ti (44Ti→ 44Sc→ 44Ca), which is accompanied by line
emission at 68 keV, 78 keV, and 1157 keV. As thedecay timeof 44Ti is ∼ 86 yr, one expects this lineemission fromyoung supernova
1 remnants. Although the 44Ti yield (typically 10−5 −10−4 M⊙) is not very high, its production is very sensitive to the energetics and
v asymmetriesofthesupernovaexplosion,andtothemasscut,whichdefinesthemassofthestellarremnant.Thismakes44Tianidealtoolto
5 studytheinnerlayersofthesupernovaexplosion. Thisisofparticularinterestinlightofobservationalevidenceforasymmetricsupernova
4 explosions.
6 Theγ-raylineemissionfrom44Tihassofaronly beendetectedfromthesupernova remnant CasA.Idiscussthesedetections, which
1 weremadebyCOMPTEL(the1157keVline)andBeppoSAX(the68keVand78keVlines),which,combined,giveafluxof(2.6±0.4±
50 0.5)×10−5 phcm−2s−1 perline, suggesting a44Tiyieldof(1.5±1.0)×10−4 M⊙. Moreover, Ipresent somepreliminaryresultsof
CasAobservationsbyINTEGRAL,whichsofarhasyieldeda3σdetectionofthe68keVlinewiththeISGRIinstrumentwithafluxthatis
0
/ consistentwiththeBeppoSAXdetections. FutureobservationsbyINTEGRAL-ISGRIwillbeabletoconstrainthecontinuumfluxabove90
h keV,astheuncertaintyaboutthecontinuumshape,isthemainsourceofsystematicerrorforthe68keVand78keVlinefluxmeasurements.
p
Moreover, with the INTEGRAL-SPIinstrument it will be possible to measure or constrain the line broadening of the 1157 keV line. A
o- preliminaryanalysisoftheavailabledataindicatesthatnarrowlineemission(i.e. ∆v < 1000kms−1)canbealmostexcludedatthe2σ
r level,foranassumedlinefluxof1.9×10−5phcm−2s−1.
t
s PACS:26.30.+k;95.85.Pw;97.60.Bw;98.38.Mz
a Keywords: Nucleosynthesis;Supernovae;Supernovaremnants;Gamma-rays;X-rays;individual(CasA);individual(SN1987A)
:
v
i
X
r 1. Introduction terial are the lengths and shapes of supernova light curves.
a
Although the expanding plasma cools adiabatically, radio-
Nature’s heaviest elements are the products of explosive
activity heats the plasma making the supernova visible for
nucleosynthesis in supernovae. A large fraction of the ex-
anextendedperiod. Theamountofradioactivematerialcan
plosive nucleosynthesis products begin as radioactive ele-
beestimatedfrommodelingthelightcurves,takingintoac-
ments. ThisisinparticularthecaseforTypeIasupernovae,
count radiative transfer in the expanding atmosphere, the
which, according to the standard scenario, are the result of
possibleescapeofpositrons,andmixing(Milneetal.,2001;
thermo-nuclear disruptions of carbon-oxygen white dwarfs
FranssonandKozma,2002).
withamassclosetotheChandrasekharlimit. Theenergyfor
The γ-ray emission from the radioactive elements can in
the disruptioncomesfromthe thermo-nuclearburningof C
principlebeobserveddirectly. Althoughintheearlyphases
andO into∼ 0.6M⊙ of56Ni, a radioactiveelementthatis ofthesupernovatheatmosphereisopaquetoγ-rays,atlater
the sourceof56Fe, the mostabundantironisotope(see e.g.
phases γ-ray emission should emerge. In fact the phase at
Arnett,1996).
which γ-rays emerge is an important diagnostic tool in it-
For core-collapse supernovae, explosive nucleosynthesis
self(Hoeflichetal.,1998). Thespectralinformationcanbe
isnotthedriverbehindtheexplosion(theenergycomesfrom
used to measure the velocity distribution of the radioactive
thegravitationalcollapseofthestellarcore). Thehot,inner
material. Thegoalofγ-rayobservationsoffreshradioactive
regionsofthesupernovatypicallyproduce0.1M of56Ni,
⊙ materialistoprovidenewinsightsintotheyields,expansion
but with a larger variation in yield from one core collapse
velocitiesandasymmetriesoftheexplosivenucleosynthesis
eventtoanother.
products. Moreover,thelineemissionfromthelongerlived
Themosttelltalesignsofthesynthesisofradioactivema-
2
radioactive element 44Ti is a tracer of recent supernovaac-
tivity.1
For Type Ia supernovaeγ-rayobservationcan be used to
distinguishbetweenvariousexplosionmodels, suchas“de-
layed detonation” and He-detonationof sub-Chandrasekhar
mass white dwarfs (WoosleyandWeaver, 1994). Thisis of
particularinterestforcosmology,asTypeIasupernovaeare
usedasstandardcandles(e.g.Riessetal.,2004),butthevari-
ationinabsolutepeakbrightnessanditscorrelationwithlight
curve properties(Phillips, 1993), which is used to calibrate
theabsolutebrightnessofthesupernovae,isnotwellunder-
stood. Thebrightnessvariationislikelytoreflectavariation
in56Niyield,butthecauseofthisvariationisstillunderde-
bate(e.g.Mazzalietal.,2001;Ro¨pkeetal.,2004).
For core-collapse supernovae the prime interest is to un-
derstand the nature of the explosion mechanism. This has
become more important now that gamma-ray bursts ap-
Figure 1. Image based on a very deep, 1 Ms,
pear to be related to core-collapse supernovae, which chal-
Chandra observation of CasA, which has been pro-
lenges our view of the core-collapse explosion mechanism
cessed to bring out the jet/counter-jet structure. (Credit:
in general. There are two main hypothesesaboutthe likely
NASA/CXC/GSFC/U.Hwangetal.)
bipolar nature of gamma-ray bursts, the collapsar model
(MacFadyenetal.,2001),or,alternatively,jet-formationdue
to magneto-rotational instabilities (Akiyamaetal., 2003a).
In particular the latter hypothesis may, to some extent, be The most abundant radioactive nucleosynthesis prod-
relevantforallcollapseexplosions.Acaseinpointisthedis- uct, 56Ni, is the product of Si-burning and α-rich freeze-
coveryof a jet/counterjet system in the supernovaremnant out, whereas 44Ti is almost exclusively the product of α-
CasA (Fig. 1, Vink, 2004; Hwangetal., 2004; Hinesetal., rich freeze-out in core-collapse supernovae (Arnett, 1996;
2004), and the non-sphericalexpansionof its brightSi-rich Theetal., 1998), although sub-Chandrasekhar mass Type
shell (Markertetal., 1983; Willingaleetal., 2002). On the Ia supernovae may be an additional source of 44Ti
other hand, recent simulations of more conventional core- (WoosleyandWeaver, 1994). Alpha-rich freeze-out occurs
collapsesupernovaeindicateahighlyturbulentnatureofthe when material that is initially in nuclear statistical equilib-
innerregions,whichmaygiveriseto56Ni-richhigh-velocity riumcoolsadiabaticallyinthepresenceofanαparticleex-
material. Thishighvelocitymaterialmayfinallybeejected cess caused by the triple-alpha reaction bottleneck. This
withahighvelocity,ifthestarhasalowenvelopemass,e.g. makes the 44Ti yield very sensitive to the explosionenergy
due tostellar wind masslosses, as isthoughtto be thecase and asymmetries, as faster expansion causes a more rapid
for TypeIb/c supernovae(Kifonidisetal., 2003). Thismay freezeout.Theyieldisalsoverysensitivetotheplaceofthe
bethecauseoftheFe-rich“bullets”inCasAthathaveover- mass cut2, as 44Ti is synthesized in the deepest supernova
takentheSi-richshellofejecta(HwangandLaming,2003). layers,closetothemasscut.
CasA, a likely Type Ib remnant, will feature prominently Finally, γ-rays observed from the inner Galaxy from
here, as it is the only confirmed source of 44Ti γ-ray emis- electron-positron annihilation, consisting of 511 keV line
sion, and an important target of the INTEGRAL γ-ray mis- emission, and three photon continuum decay of positron-
sion. ium, may also find its origin in explosive nucleosynthe-
sis. The reason is that the β-decays of 56Co and 44Sc pro-
2. Gamma-raylinesfromsupernovae ducepositrons. Duetoitslonglifetimealmostallpositrons
from 44Sc will escape the supernova, but this is less clear
Table 1 list the most importantradioactiveelements pro- for 56Co, as its decay takes place during the earlier phases
duced by explosive nucleosynthesis (for more general re-
of the supernova, when the density is high (Colgate, 1970;
views onγ-rayline emission, see DiehlandTimmes, 1998;
ChanandLingenfelter, 1993; Milneetal., 1999). However,
Prantzos, 2004). In the case of electron capture, addi- more 56Co positrons will escape if the expansion is fast
tional lines in the X-ray band will arise from characteris-
(Casse´ etal.,2004;Vink,2004). ThisledCasse´ etal.(2004)
tic de-excitationsof the atomic shellsof a few keyisotopes
to the suggestion that a hypernova in the Galactic cen-
(Leising,2001).
ter region may be responsible for the 511 keV electron-
1Another long lived element that traces recent stellar nucleosynthesis is positronlineemissionfromthatregion(Kinzeretal.,2001;
26Alat1809keV,withandecaytimeof1.1×106yr. However,themost
likelydominantsourceof26Alisnotsupernovae,butthewindsofmassive 2Themasscutdefinestheboundaryofmaterialthatwillbeejected,versus
stars(Prantzos,2004). whatwillfallbackonthestellarremnant(neutronstarorblackhole).
3
Table1
Decaychainsandγ-raysignaturesofshortlivedradioactiveproductsfromexplosiveSNnucleosynthesis.
Decaytime Process Lines(keV)
56Ni→ 8.8d EC 158,812
56Co→ 56Fe 111.3d EC,e+(19%) 847,1238
57Ni→ 52hr EC 1370
57Co→ 57Fe 390d EC 122
44Ti→ 86.0yr EC 67.9,78.4
44Sc→ 44Ca 5.7hr e+,EC(1%) 1157
linesbecameobservableasearlyas160daysaftertheexplo-
sion(Matzetal.,1988). 56Cowasexpectedtobeburiedtoo
deepintotheejectatobeobservable,buttheearlydetection
indicatesthatatleastsomeofthe56Comusthavemixedout
tolargerradii. Invelocitycoordinatesthismeansthat∼5%
ofiron-grouprichmaterialmusthavemixedouttovelocities
of ∼ 3000 kms−1 (Arnettetal., 1989; LeisingandShare,
1990, and reference therein), while most of the iron-group
materialisthoughttohaveavelocitylessthan1000kms−1.
The cause for this mixing may be Rayleigh-Taylor insta-
bilities, and, in addition, the expansion of 56Ni/56Co-rich
bubbles caused by radioactive heating (e.g. Basko, 1994;
Kifonidisetal.,2003).
Modelsforγ-rayemissionthatincludeejectamixingpre-
Figure2. Theγ-rayspectrumofSN1987A,asaccumulated
dict that the γ-ray lines should be blue-shifted, as only
bytheSMM-GRSovertheperiodfromAugust1987toMay
the γ-rays from the expanding sphere facing us should
1988 (LeisingandShare, 1990). Note that apart from the
be observable (PintoandWoosley, 1988). However, the
prominent 847 keV and 1238 keV lines, there is also evi-
high spectral resolution observations of SN 1987A showed
dence for the weaker 2599 keV and 3250 keV lines from
56Co. (CourtesyM.Leising) that the 847 keV and 1238 keV lines were broadened by
∼ 3000kms−1 (FWHM) and redshifted by ∼ 400kms−1
(Sandieetal., 1988; Teegardenetal., 1989; Tuelleretal.,
1990). Thisdiscrepancyisstillnotunderstood,butitclearly
Kno¨dlsederetal.,2003, Kno¨dlsederetal. 2005,inprepara-
pointstoanasymmetricexplosion.Theγ-rayemissionfrom
tion)
SN 1987A thus adds another piece to the puzzle of under-
2.1. Detectionsof56ColinesfromSN1987A. standingthenatureofcore-collapsesupernovae.
The importance of future γ-ray observations of super-
novae can be easily justified by the impact of the detec- 2.2. Gamma-rayobservationsofTypeIasupernovae
tion of γ-ray lines from SN 1987A. The 847 keV and As explained above, nucleosynthesis in Type Ia super-
1238 keV 56Co lines were detected by a number of bal- novae, is the cause for the explosion, and not merely a by-
loon experiments (Cooketal., 1988; Mahoneyetal., 1988; product of the explosion, as for core collapse supernovae.
Sandieetal.,1988;Teegardenetal.,1989),andthegamma- Gamma-ray observations of Type Ia therefore promise to
ray spectrometer(GRS) onboardthe Solar MaximumMis- givenewinsightsintothelightcurvevariationamongType
sion (SMM) satellite (Matzetal., 1988; LeisingandShare, Iasupernovae.Unfortunately,nosoliddetectionsof56Niand
1990, seeFig.2). Theobservedfluxesweretypicallyinthe 56Colineemissionhavebeenmade,astherehasbeenalack
range(0.5−1)×10−3 phcm−2s−1, indicatingthatonlya ofsufficientlynearbyTypeIaevents. Thebestcandidatefor
fewpercentofthetotal∼ 0.075M⊙ of56Co wasexposed. γ-rayobservationswasSN1991T,arelativelybrightTypeI
The detection of 122 keV line emission from 57Co by the supernova(Phillips,1993)atadistanceof∼13.5Mpc.
Oriented Scintillation Spectrometer Experiment (OSSE) on Both OSSE and the Compton Telescope (COMPTEL) on
board the Compton Gamma Ray Observatory (CGRO) was board CGRO observed this supernova remnant. An initial
laterreportedbyKurfessetal.(1992). analysisoftheCOMPTELdataandtheOSSEdataresultedin
Althoughtheemergenceof56Coγ-raylineshadbeenpre- upperlimitson56Colinesfluxesof3−4×10−5phcm−2s−1
dicted (ClaytonandSilk, 1969), it was a surprise that the (resp. Lichtietal., 1994; Leisingetal., 1995). However, a
4
stringent flux upper limit (< 4 × 10−5 phcm−2s−1), ob-
tainedwiththeBeppoSAX-PDS,wasinfactbelowtheorigi-
nalCOMPTELfluxmeasurement(Vinketal.,2000).
This discrepancywas finally solved with a 500 ks obser-
vation of CasA by BeppoSAX which resulted in the first
detection of the 67.9 keV and 78.4 keV lines (Vinketal.,
2001, see Fig. 3). A complication for measuring the line
fluxes of the low energy lines is the presence of contin-
uum emission from CasA. The nature of the continuum
emission is unknown, and interesting in its own right. It
could be either due to synchrotron radiation from elec-
trons with energies in the TeV range (Allenetal., 1997;
Reynolds, 1998), or it could arise from bremsstrahlung by
supra-thermalelectrons,whichhavebeenacceleratedbyin-
Figure 3. BeppoSAX-PDS spectrum of CasA (Vinketal.,
ternal shocks (Laming, 2001a,b), or by Fermi acceleration
2001; VinkandLaming, 2003). Thecontinuummodelcon-
at the forward shock (Asvarovetal., 1990). Both the syn-
sists ofa thermal(dash-tripleddottedline)andnon-thermal
chrotronandthenon-thermalbremsstrahlungmodelbyLam-
bremsstrahlung model (dashed), the latter based on elec-
ing predict a gradualsteepening at high energies. Unfortu-
tron acceleration by lower hybrid wave (Laming, 2001b).
natelytherearenoreliablecontinuumfluxmeasurementsat
The 67.9keV and78.4keV line contributionsare indicated
energies just above the 78.4 keV line. As a result, assum-
jointlybyadottedline. Inaddition,apotentialinstrumental
ingthatthecontinuumisasimplepower-lawspectrumgives
backgroundlineofTa-Kαisindicatedbyadash-dottedline. a line flux foreach line of (1.9±0.4)×10−5 phcm−2s−1
The observed count-rate spectrum has been divided by the
(68%confidence range), whereasthe gradualsteepening of
instrumentseffectiveareainordertoobtainanapproximate
synchrotron and specific non-thermalbremsstrahlung mod-
fluxdensityestimate. elsresults in a line flux of (3.2±0.3)×10−5 phcm−2s−1
(Vinketal., 2001; VinkandLaming, 2003). Within the er-
rorsbothvaluesareconsistentwiththelatestCOMPTELre-
sults(Scho¨nfelderetal.,2000). WecanaveragetheCOMP-
reanalysis of the COMPTEL observation showed evidence TEL and BeppoSAX results to obtain a 44Ti line flux of
forlineemissionatthe∼ 3σlevel(Morrisetal.,1997). Al- (2.6±0.4± 0.5)×10−5 phcm−2s−1 per line, where the
thoughitisuncertainwhethertheCOMPTELdetectionholds
firsterroristhe1σstatisticalerrorandtheseconderroristhe
up, thepredicted56Co line fluxesforSN 1991Twere close
systematicerror,duetouncertaintyinthecontinuummodel-
to the COMPTEL and OSSE upper limits. This gives some ing.3
hopethatINTEGRALwillbeabletodetectanottoodistant
TypeIasupernovainthenearfuture. 2.4. The44TiyieldofCasA
Thedetectionof44TiinCasAledtoarenewedinterestin
2.3. 44TidetectionsbyCOMPTEL&BeppoSAX measuringthelifetimeof44Ti,whichwashighlyuncertain,
One of the highlights of the COMPTEL experiment is
with published life times varying between 67 yr and 96 yr
the detection of the 1157 keV line associated with the de-
(DiehlandTimmes,1998). In1998,however,accuratemea-
cay of 44Ti (Iyudinetal., 1994). CasA is the youngest
surements by three independent groups give a life time of
known Galactic supernovaremnant, and is probablythe re-
85yr(Ahmadetal.,1998;Go¨rresetal.,1998;Normanetal.,
sult of a Type Ib or IIb supernova (Vink, 2004, for a re-
1998). Meanwhile more measurements have been pub-
view). So ofallremnantsCasA wasthemostlikely source
lishedinagreementwiththeseresults(Wietfeldtetal.,1999;
of 44Ti. On the other hand, CasA exploded around 1672
Hashimotoetal.,2001).Theerrorweightedaverageofthese
and given the expected 44Ti yields of around 10−5 M⊙ resultsgivesalifetimeof86.0±0.5yr.
(e.g. WoosleyandWeaver, 1995), the 1157 keV line flux The distance to CasA is 3.4+0.3 kpc (Reedetal., 1995),
−0.1
shouldhavebeenbelowtheCOMPTELdetectionlimit. Sur-
andthemostrecentestimateofthesupernovaeventisA.D.
prisingly, COMPTEL detected the 1157 keV line emission, 1671.3±0.9(Thorstensenetal.,2001).4 Theγ-rayemission
withthelatest,mostreliablefluxestimatebasedonCOMP- implies an initial 44Ti mass of M0(44Ti) = (1.6 ± 0.3 ±
TEL observations being (3.4 ± 0.9) × 10−5 phcm−2s−1
(Scho¨nfelderetal.,2000). 3NotethatVinketal.(2001)list90%errors,whereasforweighingthere-
The 44Ti decay chain also produces line emission at sultsandassessingthetotalstatisticalerror1σ(68%)errorsareused(c.f.
67.9 keV and 78.4 keV. Several observation with OSSE, VinkandLaming,2003).
RXTE-HEXTE, and BeppoSAX-PDSwere made in order to 4Thisisclosetotheputative dataofA.D.1680ofthedetection ofapu-
tative supernova by Flamsteed (Ashworth, 1980), but the connection of
confirm the high 44Ti flux, but without success (Theetal.,
Flamsteed’s observation with thesupernova is debatable, and rejected by
1996; Rothschildetal., 1999; Vinketal., 2000). The most StephensonandGreen(2002).
5
can be argued that for CasA all those effects play a role.
After all, CasA was probably a Type Ib supernova, and
the ejecta mass as measured in X-rays is quite low, in the
range 2-4M . Furthermore, the kinetic energy, as derived
⊙
fromthesupernovaremnant’skinematics(Vinketal.,1998;
DeLaneyetal.,2002)hasbeenestimatedtobe∼2×1051erg
(LamingandHwang,2003). Finally,thereisampleevidence
forasymmetricejectaexpansion(HwangandLaming,2003;
Vink,2004;Hwangetal.,2004).
The 44Ti yield, therefore, strengthens our current ideas
aboutCasA,butoneshouldkeepinmindthatthepredicted
44Ti yields are uncertain. Most simulations are based on
one-dimensionalmodels,inwhichartificiallyenergyormo-
mentum is deposited at the base of the ejecta. Multidi-
mensionalmodelsofcore-collapsesupernovaeshowthatthe
Figure 4. 44Ti yield as a function of main sequence ejectakinematicsishighlyturbulent,whichislikelytohave
an effect on the 44Ti yield. Moreover, there is still uncer-
mass. The standard models are shown as crosses (based
taintywhethermagneticfieldsandrotationareimportantfor
onWoosleyandWeaver,1995)andtriangles(Nomotoetal.,
theexplosionmechanism(Akiyamaetal.,2003b).
1997). Also shown are the modelsby WoosleyandWeaver
Itisworthwhiletonotethatthehighfluxesassociatedwith
(1995)forwhichtheexplosionenergywasincreased(1foe
= 1051 erg) and the Type Ibc model of a single massive 44TidecayfromCasA,mightalsobetheresultofachangein
effectivelife-timeof44Ti,aspointedoutbyMochizukietal.
starthatexplodedwithafinalmassof4M (Woosleyetal.,
⊙
(1999); MotizukiandKumagai (2004). The reason is that
1993).
44Ti decays through electron capture, which in most cases
involvesthecaptureofoneoftheinnermost,K-shell, elec-
trons. If the44Tiissufficientlyionized,i.e. oneorbothK-
0.3)×10−4M⊙. Thisiscomparabletothe44Tiyieldfrom shellelectronsareabsent,theelectroncaptureratedropssig-
SN1987A,which,basedonthelightcurve,isestimatedtobe nificantly. However,to have an effectthe ionizationshould
M0(44Ti)=(1.5±1.0)×10−4M⊙(MotizukiandKumagai, have taken place early in the life of the remnant, in order
2004). AnestimatebasedontheforbiddenFeIandFeIIin- to inhibit the decay of a substantial amount of 44Ti. This
fraredlineemissionsuggeststhatthe44Tiyieldwasslightly is unlikely to be the case for CasA, given the fact that X-
lowerthanforCasA,M0 .1.1×104M⊙(Lundqvistetal., rayspectroscopyindicatesthatbothCa andFe, which have
2001). similar ionization cross sections, are still ionizing5 and are
presently in the He-like state (e.g. Vinketal., 1996). This
2.5. Implicationsofthe44Tiyield
meansthattheK-shellisfullypopulated,andonlyamodest
A comparison of the 44Ti yield of CasA with model effectof∼10%onthelifetimeistobeexpected.Moreover,
calculations (Fig.4) shows that nucleosynthesis models a substantial fraction of 44Ti may not yet have been heated
(WoosleyandWeaver, 1995; Nomotoetal., 1997) under-
bythereverseshock. Findingthisoutisoneofthegoalsof
predict the observed yield. Taken at face value this im-
INTEGRAL observations of CasA. More detailed hydrody-
plies that the supernova event must have been a more en-
namicalcalculationsalsoindicatethattheionizationhistory
ergetic event (e.g. 2 × 1051 erg instead of the canonical hasonlyaminoreffectonthe44Tilineflux(Laming,2001c).
1051 erg), anasymmetricsupernova(Nagatakietal., 1998),
or the result of the explosion of a star that lost most of its 2.6. Potentialothersourcesof44Tiγ-rayemission
mass (Woosleyetal., 1995). As for the effectsof asymme- The Galactic supernova rate is thought to be approxi-
try, MaedaandNomoto(2003) consider variousbipolar su- mately 2 per century. However, the latest observed su-
pernovamodels,whichindeedgiveahigher44Tiyield.How- pernova was observed in A.D. 1604 (Kepler’s supernova
ever,thehigher44Tiyieldsoftheirmodelsseemtobelargely StephensonandGreen, 2002), and the youngest remnant
causedbythehighkineticenergy,∼ 1052 erg,oftheexplo- known,CasA,wasalsoa17thcenturyevent. Itisunlikely6
sion modelsrather than due to the bipolar natureof the ex- thatthesetwosupernovaeweretrulythelast,soothersuper-
plosions,asacomparisonofthebipolarandsphericalmodels novae must have been too dim to be noted, and their rem-
(resp.models40Aand40SHa)shows.Ontheotherhand,the
bipolarmodelshaveahigher44Ti/56Niratio,whichisinbet- 5Evenwheninstantlyheatedittakessometimebeforeanatomisionized
teragreementwiththe44Tiand56NiestimatesofSN1987A. uptoequilibrium ionization. Thisisgoverned bytheproductofelectron
Both mass loss and a higherexplosion energyresult in a densityandtime,net. Ahotplasmaisroughlyinequilibriumionizationif
net∼1012cm−3s,whereasinCasAitistypicallynet∼1011cm−3s.
reduced fall back of ejecta on the proto-neutron star/black 6ThechancethatnosupernovaoccurredintheGalaxysince1672,givena
hole, since both affect the final velocity of the ejecta. It rateof2percentury,is0.17%.
6
nants may still escape our attention. The reason could be fromtheGalacticplane(Diehletal.,2004),andthe511keV
that Galactic supernovae have been heavily obscured, and annihilationlinefromtheGalacticCenter(Jeanetal.,2003;
their remnants are faint because they evolve in low density Kno¨dlsederetal., 2003). Moreover,a comprehensivestudy
regions. of the available data puts new constraints on the 511 keV
Theγ-rayemissionfrom44Ti, however,doesnotdepend emissionfromtheGalacticplane(Teegardenetal.,2004).
ontheenvironmentofthesupernova,andγ-rayscanuninhib- OneofthemainmissiongoalsofINTEGRAListodetect
itedlypenetratetheinterstellarmedium. Gamma-rayobser- andmeasure44Tiemissionfromyoungknown,orstilltobe
vationsof 44Tiline emissionis thereforea possiblemethod discovered, supernovaremnants. There are several projects
fordiscoveringyoungremnants. toobserve44Tilineemission.Thepointedobservationscon-
A search for 44Ti in the Galactic plane was made with cern the (potential) 44Ti sources “Vela Jr” (section ), SN
COMPTEL, initially without finding any obviouscandidate 1987A,TychoandCasA.ForpreliminaryINTEGRALresults
sources (Duprazetal., 1997; Iyudinetal., 1999), but later on “Vela Jr” and supernova remnants searches in the 44Ti
a potential 44Ti source, GRO J0852-4642, was discovered lines,seevonKienlinetal.(2004)andRenaudetal.(2004).
close to the Vela supernova remnant (Iyudinetal., 1998).
TheVelasupernovaremnantistoooldtocontainasignificant 3.1. CasA
amountof 44Ti, but ROSATobservationsof the Vela region Asaknown44TisourceCasAisanimportantINTEGRAL
showed the presence of a supernova remnant, RX J0852.0- target. The main goalof observing CasA with INTEGRAL
4622/G266.2-1.2, in projection to the Vela supernova rem- is to obtain an independent measurement of the 67.9 keV,
nant (Aschenbach, 1998), which is often affectionately re- 78.4keV and1157keV linefluxes, and,moreover,to mea-
ferredtoas“VelaJr”. sure,orputstronglimits, onthe44Tikinematics. Thekine-
Although, the COMPTEL observation triggered the dis- matics is of particular interest, given the evidence that the
covery of the new remnant, it is not clear whether it is explosion of CasA was asymmetric, and some fraction of
truly a source of 44Ti. First of all, the X-ray absorption the Fe-rich ejecta must have been ejected with a velocities
is relatively high (Slaneetal., 2001), indicating a distance of 4500-7800kms−1(e.g. Vink, 2004). This Fe is likely to
beyond the Vela (“Sr”) remnant, but it is also rather ex- bethe decayproductof56Ni, whichwas, like 44Ti, synthe-
tended, 2˚. Together this suggests a remnant with an age sizedinthehottestregions,deepinsidethesupernova.How-
ofmorethan1000yr,whichishardtoreconcilewiththeob- ever,onlytheshockedFecanbeobservedinX-rays,soitis
served1157keVlinefluxof(3.8±0.7)×10−5phcm−2s−1 possible that most Fe, and thereforemost 44Ti, moveswith
(Aschenbachetal., 1999; Slaneetal., 2001). Also the de- velocities below 1000 kms−1 and has not yet been heated
tectionitself,althoughformallyatthe5σlevelislesssecure by the reverse shock. Note that the γ-ray line emission of
thanthedetectionof44TiinCasA(Scho¨nfelderetal.,2000). 44Ti,unlikeX-raylineemission,doesnotdependonthelo-
Nevertheless, Vela Jr is an interesting object and an impor- calconditions(exceptifthe44Tihasbeenalmostcompletely
tantINTEGRALtarget(vonKienlinetal.,2004). ionized,seesection2.5).
Measuringthe44Tivelocitymaythereforecastnewlight
on the explosion mechanism of core-collapse supernovae.
3. INTEGRAL
Unfortunately,the linesensitivity ofSPI decreasessubstan-
The International Gamma-Ray Astrophysics Laboratory tially as a function of line broadening. For example, for a
(INTEGRAL)isESA’sγ-rayimagingandspectroscopymis- narrowlineonecanobtaina3.4σdetectionofthe1157keV
sion and was launched in October 2002 (Winkleretal., linewith SPI witha 2Ms exposure,butthisis only1.9σ if
2003). The two main γ-ray instruments are the spec- thelinesarebroadenedwith2000kms−1FWHM.8Onecan
trometer SPI (Vedrenneetal., 2003) and the imager IBIS increase the sensitivity, however,by using notonly the sin-
(Ubertinietal.,2003),bothofwhichusecodedmaskstoob- gledetectorevents,butalsoeventsthatareregisteredintwo
tainpositionalinformation. detectors,duetoaComptoninteraction.
IBIS has a fully coded field of view of 5˚×5˚, and an Up to January 2005, INTEGRAL has completed the first
angular resolution of 3′. Its principaldetector plane instru- two cycles of observations of CasA, with a total exposure
mentisISGRI(Lebrunetal.,2003),consistingof128×128 of 3 Ms.9 The December 2004 observations have not yet
CdTe detectors. SPI, on the other hand, uses 19 Ge detec- beenprocessedatthetimeofthiswriting,butthefirst1.6Ms
tors7, giving a limited spatial resolution of 2.5˚ (FHWM), of INTEGRAL observations have resulted in a detection of
butaspectralresolutionof∆E =3keVoverawideenergy CasA by the IBIS-ISGRI instrument. The data were ana-
rangefrom20keV-8MeV.Thisresultsinaveryhighline lyzed with the standard INTEGRAL software package OSA
resolvingpower,inparticularforMeVenergies.
SPIisthemaininstrumentforγ-ray-lineemission,andhas 8This is based on the INTEGRAL “Observation Time Checker”
(http://astro.estec.esa.nl/Integral/isoc/operations/html/OTE.html
alreadyobtainedresultsonthe1809keV26Allineemission 9TheINTEGRALCasAobservationsareamalgamatedwithobservationsof
Tycho’ssupernovaremnant,whichisinthesamefieldofviewandmaybea
7Unfortunately, oneoftheGedetectorsfailedinDecember2003,andan- sourceof44Tiaswell,ifitistheresultofasub-Chandrasekharwhitedwarf
otherinJuly2004,decreasingthesensitivityoftheinstrumentby10%. explosion(WoosleyandWeaver,1994).
7
Figure5. Lefttoppanel: INTEGRAL-ISGRIsignificancemapoftheCasAfieldforthe20-50keVband. Thebrightestsource
inthefieldisthehighmassX-raybinary2S0114+650(e.g.Halletal.,2000). Righttoppanel: INTEGRAL-SPIsignificance
mapforthe1142-1172keVband,selectingbothsingleanddoubleevents(SEandME2). ThelocationofCasAisindicated
byacirclewitharadiusof2˚,thecoordinatesshownareGalactic. Bottompanel:INTEGRAL-ISGRIspectrumofCasA.The
BeppoSAX-PDSpointsareshowninlightgrayforcomparison(Fig.3).
8
v3and,morerecently,v4.2.Inordertominimizethefluxer- cantdetection.However,a1.8σexcessisfoundwithin2˚ of
rorsduetocalibrationuncertainties,weestimatedfluxesby thepositionofCasAwithafluxof1.8×10−5phcm−2s−1,
comparing the CasA source signal per energy bin directly similartothefluxmeasuredbyBeppoSAXandINTEGRAL-
with that of the Crab nebula. For the conversion to flux IBIS.
units we used the parametrization of the Crab spectrum by Apreliminaryanalysisshowsthatnarrowline44Tiemis-
Willingaleetal.(2001). sion (∆E = 4 keV,∆v = 1000kms−1) can almost be ex-
The IBIS-ISGRI spectrum is shown in Fig. 5 and is cludedatthe 2-3σ level, as the 3σ upperlimit foremission
fully consistent with the BeppoSAX-PDS spectrum. There is 3.1×10−5 phcm−2s−1 the flux level foundby COMP-
is a clear signature of excess emission with respect to a TEL (Scho¨nfelderetal., 2000). and the 2σ upper limit is
power-law continuum emission, and the detection signifi- 1.7×10−5phcm−2s−1. Itis,however,prudenttoallowfora
cance of the 67.9 keV line is at the 3σ level with a flux of 10%systematicuncertaintyinthefluxcalibration.Notethat
(2.3 ± 0.8) × 10−5 phcm−2s−1. The 78 keV is not de- without a detection, the exclusion of narrow line emission
tected, but fitting the 67.9 keV and 78.4 keV jointly, us- ultimately depends on the accuracy with which the overall
ingapower-lawcontinuum,boththemeasuredfluxperline 44Tiemissionisknown. TheplannedobservationsofCasA
(1.2±0.6)×10−5 phcm−2s−1, and the power law index, byINTEGRALwillhelpthereforein two ways: On the one
3.27±0.15,areconsistentwiththeBeppoSAX-PDSmeasure- hand IBIS-ISGRI will allow us to make more reliable total
ments. fluxestimates,ontheotherhandSPIwillprovideeithermore
Further results based on the full cycle 1 and 2 data are stringent upper limits, which will help to constrain the line
expected in the near future (Vink et al. 2005, in prepara- broadening, or a detection, in which case one can actually
tion). Inaddition,2.5Msofobservationtimehavebeenap- measurethelinebroadening.
provedforcycle3,whichisisexpectedtoyieldadetection
ofthecontinuumabove90keV,ifthecontinuumhasindeed 4. Concludingremarks
a power law shape. This is important, as the apparent low
Gamma-raylineemissionfromexplosivenucleosynthesis
78.4keVlinefluxmayinfactbecausedbyasteepeningof
thecontinuumabove∼60keV.Thiswouldbeofgreatinter- products provides a view of what happens deep inside su-
pernovaexplosions. Explosionasymmetriesand turbulence
est,asasteepeningispredictedforallsynchrotronandsome
duringtheexplosioncanbededucedfromthetimeofemer-
bremsstrahlungmodels(section2.3).
gence, and Doppler broadening of line emission from the
ItisclearthatINTEGRAL-IBIShasnotyetimprovedupon
shortlivedradioactiveelements56Ni and56Co. Thelonger
the BeppoSAX measurementof CasA. However, this is ex-
lived radioactive element 44Ti can be used to probe the ex-
pected to change in the near future. Moreover, IBIS is an
plosionwithinafewhundredyearsafterthesupernovaevent.
imaginginstrument, whereasBeppoSAX-PDSwas a narrow
Although, γ-ray astronomy is a specialized field, its results
fieldinstrumentusingacollimator(Fronteraetal.,1997).As
areofgeneralastrophysicalimportance,asthenatureofboth
a result IBIS can observe a large field and separate point
Type Ia and core-collapse supernovae are not well under-
sources, buttheuseofcodedmasksrequiresa largerdetec-
stood. This is even more important now that Type Ia su-
tor area, which causes a relatively large backgroundsignal.
pernovae are used to probe the history and fate of the uni-
The advantage is that we can now resolve several hard X-
verse, and gamma-ray bursts appear to be connected with
raysourcesinthefieldofCasA,whichallowsustoexclude
core-collapsesupernovae.
the possibility that a hard X-ray source was contaminating
I have shown that γ-ray-astronomy has already fulfilled
theBeppoSAX-PDSspectrumofCasA.Moreover,withIBIS
part of its promise. The detection of 56Co lines from SN
it is possible to observe simultaneously CasA and Tycho,
1987A has provided us with clues about the mixing of su-
and in addition it reveals the presence of many unresolved
pernovaejecta,althoughthetheoreticalunderstandingofthe
sources, two of which were not previously known as hard
observationsisnotyetcomplete. Thedetectionof44Tifrom
X-raysources(denHartogetal.,2004b,a).
CasAbyCOMPTELandBeppoSAX,ontheotherhand,sug-
Figure5showsanIBIS-ISGRIandaSPIsignificancemap.
geststhattheoreticalmodelsunderpredictthe44Tiyield.
The SPI map is made with the spiros imaging software us-
The work that started with SMM, COMPTEL and
ingthemeancountmodulation(MCM)methodfortheback-
BeppoSAX-PDS and other experiments is continued with
ground modeling (SkinnerandConnell, 2003). Both single
ESA’sINTEGRALmission. Forthemoredistantfuturesev-
anddoubledetectoreventswereused, excludingtimeinter-
eralsatellite missionsare envisionedrangingfromfocusing
vals with high backgroundrates. The map shown in Fig. 5
telescopes, using grazing incidence telescopes with multi-
coverstheenergyrange1142-1172keV,whichcorresponds
toatotalvelocitywidthof±7800kms−1,thevelocitywidth layer coating, von Laue lenses (i.e. using Bragg-crystals
thatcanbeexpectedifmostofthe44Tiissituatedinorisin- vonBallmoosetal., 2004), or a new Compton telescope
(Kurfess,2004).
teriortothebrightX-rayshell(Willingaleetal.,2002). Nei-
AtthetimeofthiswritingINTEGRALisinitssecondob-
therthismap,northeanalysisoftheavailabledatainanar-
serving cycle, having already produced the first results on
rowenergyrangefrom1155keVto1159keVyieldasignifi-
511 keV, 26Al, and 44Ti γ-ray line emission, which will be
9
improveduponinthenearfuture. Andwithsomeluckanot Colgate, S. A., Positrons and Low Energy Cosmic Rays
toodistantTypeIasupernovamayproducedetectableγ-ray from Supernovae, Astroph. and Space Science, 8, 457-
lineemission,or,wemightevenbeasluckyasTychoBrahe 460,1970.
andJohannesKeplerandobserveaGalacticsupernova. Cook,W.R.etal.,AnimagingobservationofSN1987Aat
gamma-rayenergies,ApJ,334,L87-L90,1988.
Acknowledgments DeLaney,T., Koralesky,B., Rudnick, L., andDickel, J. R.,
Radio Spectral Index Variations and Physical Conditions
I thank Peter den Hartog, Hans Bloemen and Ju¨rgen inKepler’sSupernovaRemnant,ApJ,580,914-927,2002.
Kno¨dlsederfortheirhelpinanalyzingtheINTEGRALdata, den Hartog, P. R., Kuiper, L., Hermsen, W., and Vink, J.,
and Matthieu Renaud and Francois Lebrun for discussing Detection of AXP 4U 0142+614 with INTEGRAL, The
INTEGRAL-IBIS data analysis. I am grateful to Roland Astronomer’sTelegram,293,1-1,2004a.
Diehl and Cara Rakowski for their careful reading of the den Hartog, P. R. et al., IGR J00370+6122 - A new high-
manuscript. This work has profited from discussions dur- massX-raybinary,TheAstronomer’sTelegram,281,1-1,
ingtwomeetingsonsupernovaremnantsattheInternational 2004b.
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