Tải bản đầy đủ - 0 (trang)
II. Observations: X-Rays, Cosmic Rays and Meteoritic Anomalies

II. Observations: X-Rays, Cosmic Rays and Meteoritic Anomalies

Tải bản đầy đủ - 0trang

This page intentionally left blank




Ma-Plunck-hstitut fur eztmterrestrische Physik

Postfuch 1312, 0-85741 Gurchin.g, Germmy

E-marl: rod@mpe.mpg. de

With gamma-ray measurements from the decay of radioactive isotopes of interme

diate lifetimes, nucleosynthesis in stars, supernovae, and novae may be constrained.

ESA's INTEGRAL observatory with its Ge spectrometer SPI is now in orbit for

one yeac, and at least four more years are scheduled. The INTEGRAL satellite and

instruments show excellent performance. Instrumental background, higher than,

estimated, limits current gamma-ray line results. Nevertheless, first results on annihilation radiation and "A1 demonstrate the potential of this mission for nuclear

astrophysics. The "A1 line is narrow, casting doubt on previously-reported broadening which would require typical "A1 decay at r y 500 km sP1; the annihilation

of e+ appears concentrated in the inner Galaxy, more extended than what would

correspond to the Galactic bulge, but without a significant Galactic-disk component. After its first successful mission year with emphasis on hard-X-ray sources,

the second mission year is aimed more at the study of nucleosynthesis sources, with

their required long exposures.

1. Introduction

Radioactive isotopes are common by-products of nucleosynthesis in cosmic

sources. Gamma-rays from their decay can be measured with satellite-borne

telescopes, and thus provide direct constraints to the physics of nuclear

burning regions inside these sources. In comparison, other measurements

of cosmic nucleosynthesis are less direct (e.g. the isotopic analysis of presolar grains found in meteorites, or the analysis of X-ray line emission from

ionized-gas portions of supernova remnants and galactic-halo gas). Yet,

the technique of gamma-ray telescopes is complex, and less precise than

such alternatives for abundance measurements, with spatial resolutions of

N degrees and signal-to-background ratios of N 9%. Only nearby sources in

the Galaxy (up to 10 Mpc for SNIa 56Ni sources) are sufficiently bright for

such isotopic measurements; but these are unaffected by physical conditions

inlaround the source such as temperature or density, and not attenuated



along the line-of-sight due t o the penetrating nature of gamma-rays (attenuation length = few g c n r 2 ) .

Candidate sources are supernovae and novae, but also the winds from

massive stars. Freshly-produced nuclei from explosive layers near the surface of compact stars and from stellar interiors are ejected into interstellar

space, where gamma-rays from their decay can be observed directly. Radioactivity which is still embedded within the source will lead to X-ray/lowenergy gamma-ray continuum emission due t o Comptonization, and early

radioactive energy is completely thermalized in novae and supernovae. This

allows indirect measurements of radioactivity, where bolometric measurements are converted t o original radioactive energy through radiation transport models. In outer envelopes, where densities are still sufficient,ly high

for collisions, and temperatures are low enough for dust formation from refractory species, characterisic isotopic samples of the nuclosynthesis source

may be conserved in ” presolar dust grains” ; some fraction of those survives

interstellar t,ransport and processings by interstellar shocks, and have been

found inside meteorites. Even though the transport and processing history

of such grains is uncertain, isotopic anomalies detected in precision laboratory measurements are sufficiently large (factors 10-1000) t o allow significant nucleosynthesis inferences, in particular on AGB stars, but more

recently even on supernovae and novae.

Before INTEGRAL, gamma-ray studies established this new window

for the study of cosmic nucleosy~itliesis~:(1) Interstellar 26A1 has been

mapped along the plane of the Galaxy, confirming t h a t nucleosynthesis is

a n ongoing process 7 , 3 5 , 3 3 . (’2) Characteristic Ni decay ga.mma-rays have

been observed from SN1987A 44,27,24,directly confirming supernova production of fresh isotopes up to iron group nuclei. ( 3 ) 44Ti gamma-rays have

been d i ~ c o v e r e dfrom

~ ~ the


young supernova remnant Cas A, confirming

models of a-rich freeze-out for core collapse supernovae. (4) A diffuse glow

of positron annihilation gamma-rays has been recognized from the direction of the inner G a l a ~ y ~ ~consistent


with nucleosynthetic production

of Pi-decaying radioactive isotopes from supernovae, novae and massive


But these positive results are accompanied by new questions and open

issues, which should be addressed through bet’ter new measurements and

through theoretical studies: (1) Which fraction of radioactive energy is converted into other forms of energy in supernovae? This adresses the absolute

normalization of indirectly-inferred radioactive amounts ( 56Ni in SNIa, and

44Ti in core-collapse SNe22, and the positron leakage from ~ u p e r n o v a e ~ ~ ) ,


and the morphology of expanding supernova envelopes4 (”bullets”, filaments, jets). (2) How good are our (basically one-dimensional) models for

nova and supernova nucleosynthesis, in view of important 3D effects such as

rotation and convective mixing’? This adresses the amount of 44Ti ejected

from regions near the mass cut between compact remnant and ejected supernova envelope43, and also the seed compositions for explosive hydrogen

burning in novae, leading t o predicted production of 22Nain novae which is

variety of physical conditions is

yet t o be directly o b ~ e r v e d ’ ~ (3)

? ~ Which


expected for nucleosynthesis events, from above effects, but also from clustering of events in space and time? This adresses stellar mass distributions

and supernova rates in massive-star clusters, self-enrichment, triggered star

formation in dense, active nucleosynthesis regions, but also the very different stellar evolution of the first stellar generations when metallicity was

extremely low. (4)How are ejecta and energy from nucleosynthesis events

fed back into the interstellar medium on all scales and over the time of

the chemical evolution of interstellar gas, i.e., how is cosmic nucleosynthesis related to the morphology of the interstellar medium, t o the spatial

pattern of star formation, to nucleation of dust and its processing by interstellar shocks, and to the acceleration of cosmic rays? Different methods of

measuring cosmic nucleosynthesis can be related, in particular connecting

models for chemical evolution on different scales of time and space.

With ESA’s INTEGRAL*’, now a new step is taken with two codedmask telescopes, improving sensitivities by = an order of magnitude, and

resolutions in spatial and spectral domains significantly over previous experiments. INTEGRAL’S spectrometer substantially improves the measurement of characteristic gamma-ray lines through their unique identification in energy, and through the prospect of observing kinematic signatures

from Doppler-shifted energy values in expanding/accelerated radioactive

material from sources of cosmic nucleosynthesis.

2. INTEGRAL and its Spectrometer

The INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL)

of ESA has been launched into a highly-excentric 72-h orbit by a Russian

Proton rocket on Oct,ober 17, 2002. With an apogee of = 150000 km and a

perigee of = 8000 km, most of the time is spent above the radiation belts,

thus minimizing local background generation from charged-particle interactions with spacecraft material. Main instruments are the Imager and

Spectrometer coded-mask telescopes; these are supplemented by two moni-


tor detectors, one microstrip gas scintillation detector coded mask telescope

for soft X-rays, and one CCD camera for optical emission, both with larger

fields of views. INTEGRAL data are collected by ESA ground stations in

Belgium and in Australia, and pre-processed in Versiox/Switzerland at the

1NTEGR.AL Science Data Center (ISDC) for distribution to observers.

The INTEGRAL mission has a nominal duration of 2 years, but at this

time ESA has approved already the envisaged 3-year extension because of

the high-quality performance and success of the mission. The Core Program

is conducted by the INTEGRAL Science Working Team (which consists of

the instrument t.earns and data center scientistrs, space agency representatives from the US and Russia, and the ESA mission and project scientists).

The percentage of Core Program reduces from initially 35% through 30 t o

25% at and beyond the 3rd year, leaving most of the observing t,ime t.o the

”Open Program”, which is open t o the international scientific community

with proposal rounds at one-year intervals (AO-2 for Dec 2003 Dec 2004

just approved at the time of this conference).

The spectrometer SPI OKI INTEGRAL45,38 is based on 19 hexagonal

Ge detector modules, each one 7 cm thick and 5.5 cm wide (flat-t’o-flat

along their hexagonal shape), arranged in a densely packed detector plane

of 500 cni’ total. It is illuminated through a 127-element coded mask made

of tungsten 3 cm thick and 2.5 times dimensions of the camera, about half

the area as open pixels, with a mask/camera separation of 171 cm. This

geo1net)rydefines a field-of-view of 16 degrees opening angle (” fully-coded” ),

beyond which the shadow pattern of the mask cast by a point source at

infinite distance falls outside the camera elements (” partially-coded fieldof-view”, out to 35 degrees, beyond which there is no coding at all). The

camera and mask are surrounded by BGO scintillation detectors to shield it

from low-energy photons, and to detect and veto charged-particle induced

events. Exposures are typically days to weeks. In order t o improve the mask

pattern recognition also for extended and diffuse sources, the INTEGRAL

satellite pointing is varied during such exposures in steps of two degrees

on a regular ”dithering” pattern around the target position, typically every

2200 s. The Ge detector temperature is maintained a t ‘v 90 K by stirling

coolers. The accumulated darnage from cosmic-ray bombardement in the

detector crystals leads to Y 10% degradation of spectral resolution and

an asymmetric response; this is cured every ‘v 6 months by heating up

the camera to = 105 C for ‘v 1.5 days (”annealing”); this procedure has

been exercised successfully in orbit two times already, and re-establishes

the original spectral response and resolution of ‘v 3 keV at 2 MeV.



Figure 1. The SPI Spectrometer on INTEGRAL

3. First Science Results

In its first mission year, INTEGRAL observations have already demonstrated the useful complement of such measurements of high-energy photons:

Many gamma-ray bursts have been measured, helping to constrain,

in particular, the spectrum at the high-energy end, and the time

profile towards short-time variability and s t r ~ c t u r e ~ * ~ ~ ~ ~ ~ .

High-energy sources such as accreting binaries including black-hole

candidates have been observed, where several outbursts and transient events provide rich material to constrain the accretion process

near black holes (e.g. Cyg X-134, GRS1915+10510).

New transient gamma-ray sources have been found in the inner

Galaxy, and are suggested t o be accreting binaries deeply embed-


ded into interstellar clouds which absorb lower-energy radiation

and thus have hidden these sources up to now (e.g. IGR J16318484847).


The gamma-ray line sources of 26A1and positron annihilation have

been detected (details are presented in the following Sections).

3.1. Interstellar 26A1

The precision followup measurements of 1809 keV emission from Galactic

26A1has been one of the design goals of the INTEGRAL mission48, after the

COMPTEL sky survey

had demonstrated that structured emission

extended along the plane of the Galaxy. Modelling of 26A1emission from the

Galaxy and specific source regions based on knowledge about the massivestar populations suggests that such stars dominate 26A1production in the

Galaxy 35,19,20



Figure 2.

SPI measurement of the "A1 gamma-ray line from the inner Galaxy

The high spectral resolution of Ge detectors of 3 keV (FWHM) at the

26A1line energy of 1808.7 keV is expected to reveal more information about

the sources and their location through Doppler broadenings and shifts, from

Galactic rotation and from dynamics of the 26A1gas ejected into the interstellar m e d i ~ m ~In

, ~particular


after the GRIS balloon experiment and

their report of a significantly-broadened line31, alternative measurements

of the 26Al line shape were of great interest. GRIS's value translates into


an intrinsic line width of 5.4 keV, equivalent to a Doppler broadening of

540 km s-'. Considering the 1.04 x lo6 y decay time of 26A1such a large

line width is hard t o ~ n d e r s t a n d ~ > ~ ~ .





Figure 3.




Line width measurements for 26A1

From spectral analysis of a subset of the first-year's inner-Galaxy deep

exposure ("GCDE"), the SPI team obtained a clear detection of celestial

26A1emission at the level of 557a, through fitting of adopted models for

the 26A1 skymap over an energy range around the 26Al line5. Values derived for 26A1 flux, as well as details of the spectral signature, however,

vary significantly with parameters of the analysis, and thus indicate the

levels of uncertainty at this initial stage of these observations and their

analysis; statistical uncertainties are negligable, in comparison. Figure 2

shows a spectrum derived from all event types (single and multiple detector

hits), using the COMPTEL Maximum-Entropy map from 9 years of measurements as a model for the spatial distribution of the sky emission 33.

Given the rather modest spatial resolution of SPI, the particular choice of

such distribution is not critical, as long as the dynamic range of fluxes and

spatial distribution are approximately correct; any choice of good source

tracers, such as the warm dust or free electron distribution^'^, produce very


similar results.

Derived sky intensity values from the inner 2~30"of the Galaxy are (35) x l o r 4 ph cm-2s-1. These are within the range suggested by previous

ph cIn-2s-1, COMPTEL": 2.8 f

observations: SMM4': 4.0 Z!C 0.z x

0.15 x

ph cm-2s-1, RHESS141: 5.7 & 0.54 x lop4 ph cin-2s-1.

Line width results are consistent with SPI's instrumental resolut,ion of

3 keV (FWHM). Therefore, these initial and preliminary SPI results already

support RHESSI's recent finding4l that the broad line reported by GRIS31

probably cannot be confirmed (Figure 3 ) .

3 . 2 . Positron Annihilation









































Energy (keV)

Figure 4. SPI measurement of the positron annihilation gamma-ray line from the inner

Galaxy. The line is confirmed t o be instrillsically broadened.

Positrons are produced upon @+-decayof radioactive isotopes with excess protons, hence may be expected t o trace nucleosynthesis of such isotopes. Other positron sources have been proposed, however, so that the nucleosynthesis connection is not unique: Plasma jets ejected from pulsar^^^^^^

or microquasars3' as a consequence of rotat,ional magnetosphere discharges

and accretion, respectively, will produce positron beams, and annihilation of

dark-matter particles attracted by the gravitational potential of the Galaxy

may produce distributed e-e+ pairs2. The fractional contribution from

nucleosynthesis sources t o the positron budget within the iriner Galaxy regions is estimated t o range from = 30% t o 100%, most probably from novae

and SNIa (19Fand "Co being the dominating radioactive sources, respectively). Substantial uncertainty arises from the unknown escape fractions of


positrons from these sources. A lower limit is placed from the contributions

of the observed "A1 , at Y 25%. Positrons with high energies have a low

probability for annihilation. Ejected by their source processes with typically

MeV energies, they will thermalize along their trajectories in the ISM, and

annihilate preferentially through the formation of intermediate positronium

atoms. The two possible spin orient,atioiis in positronium atoms (antiparallellparallel; para/ortho positronium) result in annihilation either from a

singlet or from a triplet state, t,hereby producing either two annihilation

photoils at 511 keV energy, or a photon coiitiiiuum made up from three

annihilation photons, with a maximum energy of 511 keV for any one of

the three photons. The rat'io between the line and continuum gamma-ray

intensities thus allows to measure t.he physical conditions in the annihilation region, i.e., its density, temperature, and ionization stateg. This ratio

has been measured to be rather low, only ru 1/4 of annihilation emission

is contained in the 511 keV line17. Nevertheless, line measurements are

an important diagnostic, (a) because its measurement is easier than constraining the spectrally-distributed continuum, and (b) the details of the

annihilation line shape will encode kinematics and thermal properties of the

annihilation regions. As a complication, the lifetime of = MeV positrons

in interstellar space can be substant,ialg, up t o 105y,so that, positrons may

travel significant (few 100 pc) distances between their sources and tlie locations of their annihilation. A diffuse nature of the source is expected

froin radioactive (and from dark matter) sources, while localized emission

/ hot, spots would be expected if annihilation near compact sources (microquasars, pulsars) is significant. Therefore, models which have been used t o

int.erpret previous measurements with instruments of rather modest (few

degrees) imaging resolut,ioii have been composed from disk contributions

(diffuse radioactivity from bulge and disk novae and other sources, or latitudinally more-extended warm or hot ISM gas models) and from point

sources for candidate e-e+ producers such as 1E 1740.7-2942.

From OSSE scans of the inner Galaxy with its field-of-view of 11.4 x 3 . 8 O ,

tlie spatial distribution was found t o be best, represented by a Gaussian with



an extent of E 5' (FWHM) in longitude and l a t i t ~ d e l ~ , ~Surprisingly

clear disk-like component was observed, and the "bulge" component appeared rather extended; furthermore, there was indication of a s y ~ n ~ n e t

with an excess of annihilation emission in the northern hemisphere towards

latitudes of Y loo. Yet, no mapping of annihilation emission is available

along the disk of the Galaxy outside this inner region; this is one of the

projects for the SPI instrument on INTEGRAL.


The intensity in the annihilation line, derived from first analysis of SPI

data from a part of the inner-Galaxy deep exposure (GCDE) of the first mission year14 (see Fig.4 from Jean et al., 200314), is 9.9'24:;~10-~ ph cm-2s-1,

consistent with previous measurements and theoretical predictions. This

corresponds to a positron production rate in the inner Galaxy on the order

of 1043s-1 for an assumed steady state 37.

The line is found to be significantly broadened, from deconvoliition with

the instrumental resolution after subtraction of the (strong) instrumental

background. The SPI value of 2.95(&0.6) keV (FWHM) is on the high

side of values measured by previous instruments (HEAO-C26: 1.6 f 1.3;

GRS125: 2.5 i= 0.4; TGRSll: 1.81 dz 0.54).

-1 10-d





longitude (degrees)





Figure 5. The positron annihilation gamma-ray line intensity in the inner Galaxy as

observed with SPI, compared to the longitude distribution expected from a Gaussiandistributed model.

The spatial extent of the annihilation emission is not (yet) well constrained frnm t h e s e first ~ t i i d i ~ ~


l ~n r, e ~


~i n n r v i n s n e r t i n n s nf t h e

511 keV line flux as it varies for different exposures successively pointed

along the plane of the Galaxy (Fig. 5, from Jean e t al., 200314) suggests that diffuse emission extends over a large volume. A preliminary

constraint is given by a Gaussian distribution with an extent (FWHM) of

6°-18014. First imaging attempts do not show hints for asymmet,ry, but

confirm the extended and rather smoothly-distributed emission, both in

longitude and latitude".

This suggests that annihilation near localized

sources (in particular microquasars) does not dominate the positron bud-

Tài liệu bạn tìm kiếm đã sẵn sàng tải về

II. Observations: X-Rays, Cosmic Rays and Meteoritic Anomalies

Tải bản đầy đủ ngay(0 tr)