The Extreme Sky - Seven Years of INTEGRAL
TThe Extreme Sky - Seven Years of INTEGRAL
Roland Diehl ∗ Max Planck Institut für extraterrestrische Physik, D-85748 Garching, GermanyE-mail: [email protected]
Seven years of successful observations of the sky have been completed within the INTEGRALmission, in the transition regime between X-rays and γ -rays from ∼ Fe radioactivitylines; these stimulated both theoretical and observational studies, and now make INTEGRAL avaluable asset for the astronomical survey of high-energy sources across the sky. This contributionsummarizes the situation after seven years of the mission, and concludes the 7-year anniversaryworkshop
The extreme sky held in Otranto, Italy, in Oct 2009.
INTEGRAL WorkshopThe Extreme UniverseOctober 13-17, 2009Otranto, Italy ∗ Speaker. c (cid:13) Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/ a r X i v : . [ a s t r o - ph . H E ] M a y orkshop Summary Roland Diehl
1. INTEGRAL’s Origins and Expectations
When the INTEGRAL mission was first discussed among scientists, the NASA ComptonGamma-Ray Observatory (CGRO) had just been launched in April 1991 [11], beginning its surveyof the γ -ray sky. Developments had led to a change of original plans for CGRO: the fifth originally-planned instrument, a γ -ray spectroscopy experiment (GRSE), had been abandoned for cost andcomplexity reasons[10]. The Compton Observatory performed its all-sky survey for γ -ray sourcesin the 100 keV to few GeV range with degree-sized angular resolution and 10-% sized spectralresolution during a 9-year mission. Simultaneously, and specifically during 1994, proposals wereprepared for a next advance in this field, aiming at imaging and spectral resolution improvements byan order of magnitude for this energy range: a coded-mask imaging instrument, and a Ge-detectorbased spectrometer; the INTEGRAL mission was given shape [46]. The SIGMA instrument [43] onthe GRANAT mission had shown the usefulness of a coded mask for such purpose [27], and thatsame concept was optimized for INTEGRAL’s
Imager [49, 50]. Ge spectrometers had success-fully been applied to observations of γ -ray lines following the spectacular supernova SN1987A inthe Large Magellanic Cloud [47], which led the way to high-resolution γ -ray spectroscopy. Thescience community welcomed these European efforts for a high-resolution spectrometer, and theUS Nuclear-Astrophysics Explorer mission concept (NAE) [32] eventually merged with Europeanideas to form INTEGRAL’s Spectrometer [28, 51, 52].INTEGRAL’s mission goals were stated as “ ... spectral γ -ray features to be uniquely identifiedand line profiles to be measured for physical studies of the source region, .... and accurate loca-tion and hence identification of the γ -ray emitting objects with counterparts at other wavelengths" [60] through “ ...fine spectroscopy with imaging and accurate positioning” [60]. The objects thenlisted as astrophysical targets were nucleosynthesis, nova and supernova explosions, the interstel-lar medium, cosmic-ray interactions and sources, neutron stars, black holes, γ -ray bursts, activegalactic nuclei, and the cosmic γ -ray background [60]. Expectations were great, the quantum leap for γ -ray astronomy after the Compton Observatory survey was announced widely. Seven years ofsuccessful operations are behind us, and hopefully many more to come. The instruments are all infine shape and performance in spite of some minor defects, and the healthy spacecraft still has fuelfor many more mission years [59].
2. Scientific Delivery
The first mission years saw impressive statements of scientific delivery – the variety of papersof this conference reflect latest insights and status summaries. We now have in hand the 4 th IBIScatalogue of sources [1], which shows a total of almost 700 clearly-significant ( ∼ σ ) sources inthe 17–100 keV range of hard X-rays. With its source location capability of better than 1 arcmin,comparisons to data at other wavelengths helped to identify the nature of the γ -ray object in 71%of cases, to tell us about their thermal versus non-thermal emission. The γ -ray line spectroscopymeasurements with SPI have obtained clearly-resolved line shape measurements for the strongest γ -ray line in the sky, the annihilation line of positrons at 511 keV [17, 3], and Al line spectraresolved for different source regions along the plane of the Galaxy [5, 56]. Line shapes and lo-cations have been interpreted in terms of astrophysical processes of nucleosynthesis sources and2 orkshop Summary
Roland Diehl
No. 1, 2010 THE FOURTH IBIS / ISGRI SOFT GAMMA-RAY SURVEY CATALOG 7
Figure 5.
Evolution of source type and number through the four IBIS / ISGRIcatalogs produced to date.(A color version of this figure is available in the online journal.)
X-ray observations are no longer always able to provide asso-ciations for the new sources. Combined with the variability of the Galactic sources, this is a clear indication that further ob-servations of the Galaxy will continue to uncover new sources,and follow up of these new sources is of critical importance.However, we should also point out that many of the new sourcesfound in the Galactic Plane by
INTEGRAL have been identifiedas active galactic nuclei (AGNs), so this separation of Galacticand extragalactic sources is not a straightforward one.With regards to the “unknown” sources that now constitutenearly 30% of the source list, one of the main values ofthis catalog will be to provide hard X-ray sources that willneed follow up at X-ray wavelengths in order to reach a firmidentification. To this end, we expect a large fraction of themto be identified in the coming year, as part of an ongoingmulti-wavelength campaign. In the third IBIS catalog, 113sources were not firmly classified. Many of these sources havebeen followed up at other wavelength starting with an X-rayobservation to provide more precise location, allowing for morediagnostic optical or infrared observations. As a result of theseobservations, 24 previously unidentified sources now have afirm identification and 16 have a tentative but unconfirmedidentification. The firm classifications comprise 10 AGNs,five CVs, five HMXB, three LMXB, and an XB, while tentative
Figure 6.
Classifications of sources in the four IBIS / ISGRI catalogs produced to date.(A color version of this figure is available in the online journal.)
Figure 7.
Map of incremental exposure since the third catalog, showing the locations of the new sources found. Key: green circles = AGN; cyan squares = HMXB;magenta diamonds = LMXB; yellow boxes = CVs; red crosses = unknown.(A color version of this figure is available in the online journal.) Figure 1:
The IBIS source catalogue of over 700 sources demonstrates the richness of the sky in γ -raysources [1]. The fine source location capability of 1’ or better has helped to identify the majority of thesesources (71%) with known objects, comparing to observations at other wavelengths. Galactic Plane
Cygnus -5 o Figure 2: The SPI γ -ray spectrometer resolves astrophysical γ -ray lines. Left: Positron annihilation γ -raysshape the line at 511 keV, as annihilation occurs in cold neutral, warm and partially-ionized, or hot plasma,with expected line widths increasing in that order. (from [16]). Right: Al γ -rays have now been seen fromdifferent regions along the plane of the Galaxy, and allow to constrain the recent nucleosynthetic history fordifferent massive-star groups in our Galaxy. (from [56]). interstellar-medium parameters. This clearly demonstrated the delivery from both of INTEGRAL’smain instruments, the Imager and the Spectrometer, keeping up with the promises as cited above. 3. Some Hard Lessons We had to learn that some things are not as easy as hoped for. SPI’s instrumental backgroundturned out to be a big challenge, the hope for efficient rejection through the pulse-shape discrimina-tion system was not fulfilled [44]. As a result, sensitivity remains below simulated performances.Nuclear de-excitation lines from the interstellar medium ( C and O at 4434 and 6129 keV, re-spectively) could not (yet) be detected. Detections of less-bright emissions in annihilation γ -rays,in Al, and the discovery of Fe γ -rays [55] had to await accumulation of data from five or more3 orkshop Summary Roland Diehl mission years. This is beyond the originally-planned duration of even the extended mission; butevaluation committees recognized the issues of properly estimating such a dominating and time-variable instrumental background, and generously supported deviations from the original missionplans. The time variations of the spectral response, incurred from detector degradation resultingfrom cosmic-ray bombardment, and rectified periodically through annealings , lead to additionalefforts in maintaining the spectroscopic precision as required for γ -ray line shape analyses.Gamma-ray lines from supernovae remain a matter of (bad) luck: No sufficiently-nearby( < Ti γ -rays at 68 and 78 keV were seenwith the IBIS instrument and thus confirmed earlier measurements [39]; but SPI’s spectrometerfails to obtain a new measurement of the high-energy line from Ti decay at 1157 keV. Althoughdisappointing at first glance, this can be understood from Doppler-broadening of still-fastly-movingejecta, causing a broader line to drown in SPI’s instrumental background [30]. But the hopes formeasuring (and interpreting) the Ti line shape from young supernova remnants could not be ful-filled. Similarly, also INTEGRAL does not detect more of the supernovae that are expected fromthe Galaxy’s current rate of core-collapse supernovae, and only confirms earlier constraints fromCOMPTEL; no new Cas A-like study objects are found [40].Also the Imager instrument encountered difficulties: its spectral response shows a snake-like nonlinearity which is difficult to control, signal rise-time differences over the large dynamic rangeof signal amplitudes for the > 4. Scientific Surprises and New Challenges In spite of these difficulties, INTEGRAL data also presented a number of surprises and newdiscoveries from the γ -ray sky:New and unexpected sources were detected in the Galaxy, and identified as sources deeply em-bedded in surrounding molecular clouds [53, 54, 2]. Their intense high-energy emission came as asurprise. Understanding such emission presents a novel challenge. On the other hand, INTEGRALis able to constrain the contributions from such sources with (non-thermal emission) high-energytails to the high-energy emission from galaxy clusters, as shown in the Coma cluster [24, 41].Binaries with γ -ray emission resulting from the interaction of a massive-star’s intense windwith the orbiting compact companion star were discovered [19, 13]. Interestingly, γ -ray emissionwas detected from binary sources seen also in TeV high-energy γ -rays [25, 13], and led to newstudies of cosmic-ray acceleration [48, 31].Gamma-ray emission from highly-magnetized neutron stars were discovered, and helped toclassify the anomalous X-ray pulsars [20]. Understanding the magnetosphere as the now-plausiblesource of high-energy emission remains a challenge, and INTEGRAL data extending well beyond100 keV in energy are crucial in such study [4]. 4 orkshop Summary Roland Diehl The transient γ -ray sky also led to surprises [21, 36]. Magnetar flares such as the INTEGRAL-discovered SGR1806-20 event in 2004 [35, 9, 12] are one example. Other examples arose from γ -ray burst opportunities [34]. For GRBs within INTEGRAL’s instrument field of views, whichoccur at a rate of one a month, a few GRBs were seen with harder spectra than expected fromthe internal-shock synchrotron model, indicating that the GRB engine is not yet understood. OtherGRBs, on the contrary, exhibit spectra which are unusually soft , and helped to explore the transitionrange to the phenomena of X-ray flashes . Moreover, the anticoincidence system (ACS) of SPIturned out to be useful in GRB studies [14], adding an important GRB monitor with nearly all-skysensitivity to the interplanetary network, and thus helping GRB locations.The inner region of our Galaxy is rich in transient sources, presumably mostly accreting X-ray binaries. INTEGRAL’s monitoring program [21] identified and tracked many new such bina-ries. With now a considerable statistical sample, the spatial and luminosity distributions of thesetransient sources are being studied [23, 29, 37]. It has been recognized that the type of sourcechanges as one steps up into INTEGRAL’s γ -ray domain, with cataclysmic variables and coronally-active stars dominating the X-ray domain up to several tens of keV, and low-mass X-ray binaries(LMXB) dominating above [42]. The sample of high-mass X-ray binaries could be substantiallyincreased from INTEGRAL’s observation of the hard X-ray regime, and now holds nearly hundredobjects; prospects for determining their Galactic distribution are realistic now [23]. Within thissample, more sources with unexpected properties are found, such as neutron-star companions withunexpectedly-low rotation periods, or with surprisingly-intense stellar winds [8].Among these HMXBs, a new source class emerged, called superfast X-ray transients [38, 45].In these, the wind interaction with the orbiting compact neutron star apparently sets conditions forintense flaring activity of typically ∼ hour durations. Constraining the nature of such outbursts withdetailed observations of the temporal and spectral changes promises to provide new and unexpectedinsights into the phenomena and process of accretion of matter onto a compact neutron star, andthe nature of the companion star.Active galaxies were confirmed to mostly drop sharply in their intensities as one reaches be-yond X-ray energies of several keV, as a dusty torus around the active nucleus is believed to absorbradiation. But probing rare active galaxies on the high side of the distribution of absorbing-toruscolumn densities, much fewer such sources were found than had been extrapolated from X-rayobservations and theories [26]. As a result, the diffuse cosmic X-ray background with its emissionmaximum at 30–40 keV cannot be inferred by extrapolation of X-ray emission properties, and suchextrapolation explains only ∼ 10% of the peak emission.The emission from positron annihilations in interstellar space has been mapped across the sky,and consolidated earlier hints for the bulge region of the Galaxy being by far the brightest emissionregion on the sky [18, 57, 58]. These maps revealed a surprisingly-symmetric bulge emission,and barely were able to detect annihilation emission from the Galaxy’s disk, where most of thecandidate sources are located (see [7] for a review, and Churazov et al., this volume).The Al line could be seen to vary in centroid position, as would be expected from theGalaxy’s large-scale rotation [5]. But such variation of line position is small at tenths of a keV,and it was surprising that INTEGRAL’s spectrometer turned out to be able to measure interstellar-medium velocities down to the 100 km s − range. This is helped by INTEGRAL’s finding that theintrinsic Doppler broadening from ISM kinematics is not as broad as reported before, and instead5 orkshop Summary Roland Diehl rather narrow and below instrumental line widths [6]. Additionally, Fe radioactivity was firstclearly measured by INTEGRAL’s Spectrometer [55]. 5. Prospects for INTEGRAL in its Late Years The INTEGRAL mission is now part of a fleet of astronomy space missions. Coming of age,one may wonder how valuable continued observations with INTEGRAL would be. It is useful toconsider the special strengths of this mission for new astrophysical insights.INTEGRAL’s spectrometer SPI features spectral resolution of ∼ 600 ( E δ E ). This capabilitywill remain unrivaled and unique for many years to come: Since many astrophysical lines will bekinematically broadened, specifically from the target objects of supernova explosions (see Cas A’sexperience, as discussed above), the future instrumental developments targeting γ -ray line studiesfrom supernovae will not need such high resolution. Experiments thus can be optimized towardslarge collecting areas with less-costly detectors such as CdZn, with adequate spectral resolution forthat purpose.INTEGRAL targets radiation originating in atomic nuclei from cosmic objects. It is likely thatobservations in this energy regime have only revealed the tip of the iceberg of cosmic sources, evenwithin our Galaxy. It is worth reminding that most astronomical observations from radio throughIR, optical, UV and X-ray bands address emission processes of thermal origins, and spectral infor-mation arises from line transitions in the atomic shells. This implies that the state of the atomicshell is part of the emission process, or, stated otherwise, this state must be determined for a properinterpretation of the observed emission. Nuclear emission processes, on the other hand, are oftenrather independent of the thermodynamic parameters of the emission region, such as in lines origi-nating in radioactive decay, or in high-energy collisions from cosmic rays. This window is uniquelyaddressed by INTEGRAL and its large field-of-view instruments. With luck, nuclear explosionssufficiently nearby may show the usefulness of such observations of radiation of primarily-nuclearorigins.Polarization of light encodes processes of magnetic-field origins in the cosmic source of radia-tion. For high-energy radiation, the differential Compton scattering process shows angular patternswhich encode polarization, and becomes observable at γ -ray energies with a sufficiently-large cam-era. First results obtained with INTEGRAL on a γ -ray burst and on the Crab pulsar emission arepromising [33]. Future observations may further exploit this unique potential.The INTEGRAL sky survey has emphasized exposure along the plane of the Galaxy. As aresult, now deep exposure has been accumulated over regions of the sky which are not as easilyaccessible by low-orbit missions such as SWIFT. This allows complementarity of sky surveys foractive galaxies at high energies (see Krivonos et al., this volume), which can address the issue ofwhich sources are responsible for the cosmic X-ray background in its peak region and above.Science issues which appear interesting and hot and can be addressed by future INTEGRALobservations are, for example:– What is the Galactic population of high-energy emitting accreting binaries?– How does the accretion process occur in close or interacting binaries?– What is the nature of high-energy emission in high-field magnetospheres near neutron stars?– What does the morphology of positron annihilation emission teach us about positron sources,6 orkshop Summary Roland Diehl what about positron escape and interstellar propagation?– What are the state and conditions of hot interstellar medium around massive-star groups?– How can models of massive-star and supernova structure be aligned with nucleosynthesis of Aland Fe from those objects?– Which active galaxies and their subtypes are responsible for accumulating to the observed diffuseX-ray background?– Is emission from γ -ray bursts, or from γ -ray pulsars, polarized?The way an observatory such as INTEGRAL is managed, some tension arises typically in latermission years from the fact that additional observations suggested in competitive proposals onlyadd incremental data to a large and growing database. Therefore, new insights are not expected, atleast from persistent sources of high-energy emission. This appears to argue for an observing pro-gram being widely-open for targets of opportunity and tracking of source variabilities, in particularfavored by INTEGRAL’s large field of view which allow some amalgamation and serendipidity .On the other hand, the unique strengths of INTEGRAL should be utilized to build and consolidatethe legacy of results specific to this type of instrumentation, so that astrophysical and deep analysisof these findings (but also future mission proposals to take up and deepen such studies) can besupported with best-achievable precision.INTEGRAL is now managed by a Users Group composed of instrument experts and a broadspectrum of scientists involved in high-energy astrophysics (see also [59]). This group is very effec-tive in discussing the different aspects of observing program alternatives. It turns out a very usefulmoderator to the diversity of proposed satellite pointings through the annual observing opportu-nities. Considering the limited gain of such individual and necessarily short (by comparison withexisting) exposures, a Key Program concept has successfully been implemented. Here, two classesof proposal opportunities were installed: Long Key Program observations set the general programof where INTEGRAL’s survey of the γ -ray sky will be directed to, and a second category of pro-posals requests data rights from such prior-selected pointings for analysis of specific astrophysicalquestions or sources. In a Gedanken-Experiment , this author also presented a more extreme versionof this concept, where exposures are directed by the key program and user group such that syner-gies are maximized. [One scenario would be to point INTEGRAL at intermediate latitudes alongthe plane of the Galaxy. This could complete and complement the Galactic-plane survey with thebrightest plane regions being in outer field-of-view regions. Simultaneously, exposure would beadded to constrain latitudinal extents of diffuse high-energy emission of special interest, such aspositron annihilation emission. Active Galaxies could be surveyed at those intermediate latitudes,deepening existing exposures sufficiently for meaningful constraints.] It will remain a challengeto ensure community interest in INTEGRAL observations, especially if the common scheme ofregular and frequent observation opportunities would be deviated from. But a fresh look at theoptions best-suitable for INTEGRAL could be worthwhile. In any case, INTEGRAL’s next sevenyears could be fruitful to harvest and consolidate the excitements of the extreme universe from itshigh-energy emission. Acknowledgements This review was stimulated from many papers by the active communityof high-energy astrophysicists gathered in parts at this nice location at the beautiful but temporarilyunusually-cold coastline of Puglia. We all appreciated the hosting institutions and generous supportof the conference from ESA, Italy’s research institutions, and a very energetic organization team.7 orkshop Summary Roland Diehl It was a pleasure to celebrate seven years of INTEGRAL in Otranto! References [1] A. J. Bird, A. Bazzano, L. Bassani, F. Capitanio, M. Fiocchi, A. B. Hill, A. Malizia, V. A. McBride,S. Scaringi, V. Sguera, J. B. Stephen, P. Ubertini, A. J. Dean, F. Lebrun, R. Terrier, M. Renaud,F. Mattana, D. Götz, J. Rodriguez, G. Belanger, R. Walter, and C. Winkler. The Fourth IBIS/ISGRISoft Gamma-ray Survey Catalog. ApJS , 186:1–9, January 2010.[2] S. Chaty, F. Rahoui, C. Foellmi, J. A. Tomsick, J. Rodriguez, and R. Walter. 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