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New Astronomy Reviews 50 (2006) 530–533

Scientific highlights from INTEGRAL

Christoph Winkler

ESA-ESTEC, Research and Science Support Department, Astrophysics Division, Keplerlaan 1, 2201 AZ, Noordwijk, The Netherlands

Available online 26 July 2006

Abstract

The gamma-ray observatory INTEGRAL was launched in October 2002 and produces since then a wealth of discoveries and impor-tant new results. I will present a selection of scientific highlights obtained during the first 2.5 years of the mission.� 2006 Elsevier B.V. All rights reserved.

Keywords: INTEGRAL; Gamma-ray astronomy; Nucleosynthesis; Compact galactic objects

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5302. Origin of soft Galactic gamma-ray emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5313. A new class of high-mass X-ray binaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5314. Accreting ms-pulsar IGR J531291 + 5934 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5315. Electron–positron annihilation (511 keV) emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5316. Gamma-ray line emission from Cas A, the inner Galaxy and Cygnus region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5327. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 533

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 533

1. Introduction

INTEGRAL was launched on 17th October 2002, fromBaikonur Cosmodrome (Kazakhstan). The four-stagePROTON launch vehicle injected the spacecraft into itshigh elliptical 72hr orbit with utmost precision. INTE-GRAL carries two main gamma-ray instruments(20 keV–10 MeV): the spectrometer SPI – optimized forthe high-resolution gamma-ray line spectroscopy (2 keVFHWM at 1.3 MeV), and the imager IBIS – optimizedfor high-angular resolution imaging (12 0 FWHM). Twomonitors, JEM-X in the 3–35 keV band, and OMC in theoptical band (V) complement the payload. A detailed

1387-6473/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.newar.2006.06.044

E-mail address: christoph.winkler@rssd.esa.int.

description of the INTEGRAL observatory and instru-ments can be found in Winkler et al. (2003) and referencestherein. As of today (07 September 2005) the spacecraft isin excellent shape, all subsystems are fully functional with ahigh level of performance and no loss of redundancy. Highmargins on on-board fuel and power with full redundancylead to lifetime expectations well above the 5 years ofdesign lifetime. The instruments are performing nominally.However, two out of the 19 Ge detectors for SPI are notoperational, and one JEM-X unit is currently being oper-ated due to slow degradation in performance. In the fol-lowing sections, an overview of important scientificachievements obtained during the first 2.5 years of the mis-sion will be presented. Undoubtedly – also in view of thelimited space available – the author will focus on a selectedsubset of interesting results, only.

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2. Origin of soft Galactic gamma-ray emission

Below 10 keV it is believed that there is a truly diffusecomponent that accounts for 80% of the unresolved emis-sion detected in small sky regions that do not containbright sources. Above 10 keV, previous observations(SIGMA, CGRO/OSSE) had difficulties in separatingpoint sources from diffuse emission: while 50% of the emis-sion in the 50–500 keV band was attributed to pointsources, it was difficult to explain the remaining half whichwas attributed to diffuse emission, as inverse Comptonscattering of GeV CR electrons would also produce a sub-stantial radio-synchrotron emission at much higher levelthan observed. Also an alternative explanation (electronBremsstrahlung in the ISM) was difficult to reconcile withthose observations as the associated power of 1043 erg/swould affect the ionization equilibrium and dissociationof molecules in the ISM.

INTEGRAL was able to solve (Lebrun et al., 2004) thislong-standing problem by combining accurate imagingwith good sensitivity in the 20–220 keV range. Fig. 1 andFig. 2 show, as an example, the results for the 40–60 keVrange: The energy output from 91 individual point sourcesaccounts for �90% of the diffuse continuum emission in theinner Galaxy. The truly diffuse component contributes onlyto 10–25% (depending on energy) of the total ridgeemission.

Fig. 1. Galactic latitude profile of the inner Galaxy. Triangles: estimatedtotal count rate from 91 point sources, circles: total ISGRI backgroundcorrected count rate (Lebrun et al., 2004).

Fig. 2. Galactic longitude profile of the inner Galaxy. Triangles: estimatedtotal count rate from 91 point sources, circles: total ISGRI backgroundcorrected count rate (Lebrun et al., 2004).

3. A new class of high-mass X-ray binaries

The 2nd INTEGRAL source catalogue (Bird et al., 2006)identified 55 INTEGRAL gamma-ray sources (IGR) out ofwhich only 20% had been classified. These new sources arelocated around the Galactic Centre and in the Norma andScutum spiral arms. Most of these sources are highlyabsorbed below 5 keV, many are X-ray pulsars, and theintrinsic absorption is very high (NH > 1023 cm�2) (Kuul-kers, 2005). A ‘‘typical’’ IGR-source can be characterizedas a ‘‘cocooned’’ compact object residing in a binary systemwith accretion/wind from a massive companion (Walteret al., 2003). These sources escaped previous detection insoft X-rays. INTEGRAL detected a new class of super-giant high mass X-ray binaries (HMXB) and their popula-tion in the inner Galaxy has more than doubled: this opensnew insight into HMXB, their relation to star formationand Galactic structure (Lutovinov et al., 2005).

4. Accreting ms-pulsar IGR J00291 + 5934

The high energy sky is highly variable and Targets ofOpportunity (TOO) observations are crucial for every highenergy mission. INTEGRAL has performed (until August2005) 23 TOO observations (spending about 5 Ms). One ofthe most interesting is the new millisecond pulsar IGRJ00291 + 5934 (Falanga et al., 2005). The source wasdetected by INTEGRAL in December 2004 and it turnedout to be the fastest accreting X-ray pulsar (period:1.67 ms), considered as the missing link between normalradio pulsars and isolated radio ms-pulsars. Accurateimaging with IBIS isolated the pulsar from the close-by(20 0) CV V709 Cas and allowed the observation of the highenergy emission during the entire outburst. For the firsttime, pulsed emission up to 150 keV could be detected byIBIS. The total spectrum is characterized by thermal emis-sion from the disk at soft X-rays (from contemporaneousRXTE data) while at energies above 20 keV the spectrumis dominated by a 40 keV thermally Comptonized spec-trum, considerably larger than from previous X-ray obser-vations alone: Matter accretes along the magnetic fieldlines toward the pole of the neutron star and is heated byan accretion shock. Thermal (1 keV) seed photons fromthe hot spot are upscattered in the 40 keV plasma. As alsopulsed emission is observed, a large part of the hard tailemission must come from the poles, not from the disk.The INTEGRAL observations further showed for the firsttime, that the pulsed emission becomes (much) harder thanthe total spectrum with increasing energy – consistent withDoppler boosting from the emitting polar region – and alsothe pulsed fraction of the emission increases with energy:up to 20% at 100 keV (Falanga et al., 2005).

5. Electron–positron annihilation (511 keV) emission

The 511 keV line, resulting from annihilation of electronswith their antimatter particles (positrons), is the brightest

Fig. 4. Spectrum of electron-positron 511 keV annihilation emissionshowing the narrow line component and continuum emission below 511keV due to decay of ortho-Positronium (Churazov et al., 2005).

532 C. Winkler / New Astronomy Reviews 50 (2006) 530–533

gamma-ray line in the Galaxy. However, the principalsource of positrons is unknown. Current theories arewide-ranging (explosive nucleosynthesis products fromSNe, Novae, WR stars; CR interaction with ISM; blackholes and pulsars; GRB; hypernovae and light dark matterparticles). On theoretical grounds it is difficult to disentan-gle the primary positron source due to the highly uncertainyields and the uncertain source distribution and frequency.INTEGRAL mapping of a large part of the entire sky(Knodlseder et al., 2005) showed for the first time that thediffuse 511 keV emission is located towards the GalacticCentre only (Fig. 3). The emission is almost symmetricalwith a FWHM of 8�. The flux from the bulge is10�3 cm�2 s�1. Data are compatible with a weak disk(70% of the bulge flux) which can be fully explained bythe b+-decay of 26Al and 44Ti. However, the bulge/diskluminosity ratio is between 3 and 9 and imposes severe con-straints on the principal positron source (Knodlseder et al.,2005): the 511 keV sky map has a unique morphology notfully matched by any counterpart tracer map. Likely coun-terparts could be the old stellar population (SNIa, Novae,LMXB) or light dark matter particles, all having a weakor no disk component. No point sources contribute to alevel of 10�4 cm�2 s�1 and the local annihilation fountainof 511 keV emission reported by CGRO/OSSE from aregion about 8� north of the Galactic Centre (Purcellet al., 1997) could not be confirmed (Knodlseder et al.,2005). The spectral line (Fig. 4) with E = 510.954 ±0.075 keV and width (FWHM) = 2.37 ± 0.25 keV is unshif-ted and no fast expansion can be seen. The narrow line iscompatible with a single-phase warm (8000 K), weakly ion-ized (0.1) ISM with electron density ne � 0.3 cm�3 (Chur-azov et al., 2005). Very recently, the existence of a broad(FWHM = 5.4 ± 1.2 keV component, resulting from in-flight annihilation in warm neutral ISM, could be identified(Jean et al., 2006) with a flux of �50% of the narrow lineflux. The observed continuum emission due to the decayof ortho-Positronium at energies below 511 keV results ina Positronium fraction of about 96% (Churazov et al.,2005; Jean et al., 2006).

Fig. 3. All-sky map of the electron-positron 511 keV annihilation emis-sion in galactic co-ordinates (Knodlseder et al., 2005).

6. Gamma-ray line emission from Cas A, the inner Galaxy

and Cygnus region

Cas A, the youngest (320 years) known galactic SNR (@3.4 kpc), has been observed and IBIS detected a line at67.8 keV associated with 44Ti (Vink et al., 2005), a key tra-cer for core collapse supernovae. The flux is (2.3 ± 0.8) ·10�5 cm�2 s�1. The 78.4 keV line 3r upper limit is 2.3 ·10�5 cm�2 s�1. Accurate imaging excludes possible sourcecontamination of the signal during earlier BeppoSax obser-vations. The uncertainty in the determination of the contin-uum below 100 keV needs to be further removed by futureobservations. With SPI, the 3r upper limit is 3.1 · 10�5

cm�2 s�1 for a 4 keV (1000 km/s) wide line, compatiblewith the Comptel detection at 1.1 MeV, and a narrow linecan almost be excluded at this point in time (Vink et al.,2005). This may create a conflict with theoretical models(Diehl and Timmes, 1998) as 44Ti is thought to be formedclose to the mass cut, hence small velocities (61000 km/s)should be expected.

1809 keV emission from 26Al is a key tracer for nucleo-synthesis from massive stars. Accurate spectroscopy usingSPI has revealed (Diehl et al., 2006) that the emission fromthe longitude range 10�–40� is red-shifted by about 0.2 keVwhile emission from longitude range �10� to �40� is blue-shifted by about 0.4 keV with respect to the 1809 keV emis-sion as observed from the Galactic Centre (Fig. 5). Theseresults confirm the global nature of the observed 26Al emis-sion and are compatible with expected shifts due to galacticrotation in the inner Galaxy. Interestingly, we are observ-ing the entire Al-emission throughout the Galaxy (with ahalf life of about 106 years, covering many SN events), soan independent method exists to estimate the currentcore-collapse SN rate in the Galaxy to 1.9 ± 1.1 per cen-

Fig. 5. INTEGRAL measurements of the 26Al line emission at 1809 keVin three longitude intervals superposed on the modelled 3-D distributionof 26Al sources combined with the Galactic rotation curve (Diehl et al.,2006).

C. Winkler / New Astronomy Reviews 50 (2006) 530–533 533

tury (Diehl et al., 2006; Hartmann, 2006). This allows toderive a star forming rate of �4Mx per year or �7.5 starsper year in the Galaxy (Diehl et al., 2006). The Cygnusregion is the most active nearby star-forming region inthe Galaxy and a large number of massive stars shouldenrich the ISM with nucleosynthesis products. INTE-GRAL observations of the 1809 keV line (26Al) show thatthe line is much broader (FWHM = 3.3 keV) than in theinner Galaxy (1.6 keV) which can possibly be attributedto the observation of turbulent motions in hot superbub-bles (Knodlseder et al., 2004).

Gamma-ray lines at 1173 keV and 1333 keV are pro-duced by 60Fe, another tracer of nucleosynthesis in massivestars. As the lifetime is similar to that of 26Al, a steady stateabundance along the Galactic plane is expected as the timedifference between Galactic SN events is much smaller.INTEGRAL detected a flux (combined for the two lines)of 3.7 · 10�5 cm�2 s�1 from the inner Galaxy (Harriset al., 2005). The flux ratio 60Fe/26Al is 0.11 ± 0.03, sub-stantially below predictions (P0.40). Present uncertaintiesin theoretical modelling include insufficient knowledge of

the cross-sections involved, and possibly also the need foranother large source for 26Al emission other than core-col-lapse SN – possibly massive winds during the Wolf-Rayetphase. Future mapping should cast a light on this as thespatial distribution of WR stars might be quite different(mass, metallicity) from the spatial distribution of the aver-age SN progenitor, see also Woosley (2006) for a detaileddiscussion.

7. Conclusions

INTEGRAL is a mature high-energy mission, it workswell and delivers the science it was built to do. INTE-GRAL is the only gamma-ray observatory in space servingan increasing science community. Many science topicsrequire long exposures and will strongly benefit from fur-ther mission extension due to the large field of view.

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