The INTErnational Gamma Ray Astrophysics Laboratory: INTEGRAL Highlights

Nuclear Instruments and Methods in Physics Research A 742 (2014) 47–55

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Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima

The INTErnational Gamma Ray Astrophysics Laboratory: INTEGRAL Highlights Pietro Ubertini n, Angela Bazzano IAPS/INAF, Via Fosso del Cavaliere 100, I-00133 Rome, Italy

on behalf of the INTEGRAL Team art ic l e i nf o

a b s t r a c t

Available online 31 December 2013

The INTEGRAL Space Observatory was selected as the second Medium size mission (M2) of the ESAs Horizon 2000 vision programme. INTEGRAL is the first high angular and spectral resolution hard X-ray and soft γ-ray observatory with a wide band spectral response ranging from 3 keV up to 10 MeV energy band. This capability is supplemented by an unprecedented sensitivity enhanced by the 3 days orbit allowing long and uninterrupted observations over very wide field of view (up to  1000 squared degrees to zero response) and sub-ms time resolution. Part of the observatory success is due to its capability to link the high energy sky with the lower energy band. The complementarity and synergy with pointing soft X-ray missions such as XMM-Newton and CHANDRA and more recently with NuSTAR is a strategic feature to link the “thermal” and the “non-thermal” Universe observed at higher energies by space missions such as Fermi and AGILE and ground based TeV observatories sensitive to extremely high energies. INTEGRAL was launched on 17 October 2002 from the Baikonur Cosmodrome (Kazakistan) aboard a Proton rocket as part of the Russian contribution to the mission, and has successfully spent almost 11 years in orbit. In view of its successful science outcome the ESA Space Programme Committee haw recently approved its scientific operation till the end of 2016. To date the spacecraft, ground segment and scientific payload are in excellent state-of-health, and INTEGRAL is continuing its scientific operations, originally planned for a 5-year technical design and scientific nominal operation plan. This paper summarizes the current INTEGRAL scientific achievements and future prospects, with particular regard to the high energy domain. & 2014 Elsevier B.V. All rights reserved.

Keywords: Space science γ-Ray astrophysics INTEGRAL Observatory

1. The INTEGRAL observatory The INTEGRAL Space Observatory, launched by a four-stage Russian PROTON rocket from Baikonur (Kazakhstan) on 17 October 2002 and, so far, has been successfully in operation for almost 11 years, being initially planned for a nominal lifetime of 2 years, and a technical life time of 5 years [1]. During the ESA Science Programme Committee meeting held on 19 June 2013 the mission, among others, has been extended till 2016 in view of the fact that was “continuing to deliver exceptional science” (for more detail see http://sci.esa.int/director-s-desk/). INTEGRAL was designed to perform fine spectroscopy and fine imaging of celestial sources in the range 15 keV–10 MeV by means of the two main instruments on board: the Gamma-Ray Spectrometer

 Corresponding author.

E-mail address: [email protected] (P. Ubertini). 0168-9002/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2013.12.027

SPI [2] and the imager IBIS [3]. The X-ray monitoring capability is supplied by Jem-X, operative in the range 3–35 keV [4], while the OMC instrument provide the optical wavelength band [5]. The orbit features a high perigee to provide long periods of uninterrupted observations and has been optimised to observe the faint and extremely variable γ-ray sky with nearly constant detectors background, away from the electron and proton radiation belts. At the beginning of the mission the orbital parameters were: 72-hour orbit duration, inclination of 52.21, perigee of 9050 km and apogee of 153,657 km. The telemetry data are transmitted to the ground in real time 24 h/day because of the absence of substantial on-board data storage, using receiving ground stations in Redu (Belgium) and Goldstone (USA). The mission is operated by the personnel of the ESA-ESOC facility in Darmstadt (Germany) that performs all standard spacecraft and payload operations and maintenance tasks. The scientific planning and operations [6] are performed at ESA-ESAC Villafranca (Spain) in close contact with the Project Scientist, so far located at ESA/ESTEC, Noordwijk

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(The Netherlands). The scientific data quick-look analysis, preprocessing and archiving are provided at the INTEGRAL Science Data Centre [7] in Versoix (Switzerland). More detailed informations on the technical characteristics of the satellite and on board scientific payload and instruments can be found in Refs. [1,8], while for a full description of the INTEGRAL mission, scientific payload, calibrations, science operations and initial first-light science results see the special issue of A&A, vol. 411, no. 1, November 2003. In Fig. 1 an artist impression of the INTEGRAL satellite is shown. In Figs. 2 and 3 the main characteristics of the SPI and IBIS main instruments and of the Jem-X and OMC monitors (data from Ref. [9]) are summarised.

2. The key role of the keV to MeV energy range INTEGRAL covers the waveband connecting the so-called ‘thermal’ sky with the ‘non-thermal’ high energy Universe. The Observatory capability to perform long and un-interrupted observations from 3 keV up to 10 MeV, with unprecedented sensitivity over a very large field of view is a unique feature that will not be available to the scientific community in the next decades. In Fig. 4 the all-sky coverage performed by INTEGRAL from AO1 to AO10,

corresponding to more than 250Ms live time and an average exposure of the Galaxy Plane of 5Ms is shown.

2.1. INTEGRAL connects low and high energies One of the unprecedented INTEGRAL feature, apart from the 11 years uninterrupted observation of the soft γ-ray sky, is the capability to connect the data obtained at low energy with X-ray missions such as XMM-Newton, Chandra, Suzaku, Swift, and more recently NuSTAR, with the high-energy detections from γ-ray observatories such as Fermi, AGILE in the MeV to GeV range, and ground-based very high energy γ-ray telescopes such as HESS, MAGIC, VERITAS investigating the very high energy Universe above GeV. As expected the 721 sources detected and reported in the 4th IBIS catalogue [10] belonged to most of the well known classes of X-ray sources. The basic information is reported in Figs. 5 and 6. Fig. 7 shows the INTEGRAL/IBIS Galactic Plane Scan (GPS) sky view: in the top panel are reported the sources emitting in the range 20–100 keV and in the bottom panel in the range 100–300 keV [11]. A more intriguing correlation was initially found between the Hard-X ray and the GeV–TeV energies [12]: most of the common sources were high energy Blazars, both Flat Spectrum Radio Quasars and BL Lac. More recently a number of PWN, isolated NS, μ QSOs and a handful of other galactic sources have been detected at high energies and wide band spectra obtained from the soft-γ to very high energies.

3. The INTEGRAL achievement

Fig. 1. The INTEGRAL Space Observatory.

To date INTEGRAL has solved several astrophysical issues that were considered a mystery since decades. A short compendium of this achievement has been recently presented in Ref. [13]. Therefore, this follow up paper, that summarizes the presentation to the RICAP13 – Roma International Conference on AstroParticle Physics, in May 2013, will be limited to the main results obtained in the framework of the INTEGRAL polarisation studies, Quantum Gravity related Lorentz Invariance implication, INTEGRAL Surveys and the diffuse Sky map in the 511 keV band. A more comprehensive review is available at: http://www.rssd.esa.int/SD/INTEGRAL/docs/ INTEGRAL/ScienceCase2012.pdf.

Fig. 2. INTEGRAL: key parameters for the SPI and IBIS main instruments.

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Fig. 3. INTEGRAL: key parameters for the Jem-X and OMC monitoring instruments.

Fig. 4. Total AO1–AO10 exposure time: about 250 ms.

3.1. INTEGRAL discovers high energy photons are polarised 3.1.1. INTEGRAL measurement of polarisation from GRBs, Crab Nebula and Cyg X-1 A further unique investigation tool is now provided by the Observatory capability to detect with good sensitivity polarisation of the γ-ray photons detected (see following references). In fact, starting from 2006 the two main instruments on board, IBIS and SPI, have been used to provide detailed information on the polarisation nature of the incoming soft γ-ray photons detected. This capability has been achieved a few years after launch, in view of the long in-flight calibration procedures coupled with the parallel development of a new complex data analysis software.

This has been necessary to implement the independent analysis of the Imager and Spectrometer data, not delivered at the launch time (2002). At the same time a new Montecarlo was developed to properly modelise and calibrate the systematic error induced by the instrument hardware configuration and the spacecraft induced particle background non-uniformity. The initial result clearly showing polarisation was obtained by IBIS and SPI data from the Gamma-ray burst GRB 041219A [14,15] and Crab pulsar [16,17]. The chance to detect the polarisation angle and intensity from (moderately strong) cosmic sources has finally opened a new observational window at high energies. More observations had been planned and performed with the specific task to look for the polarisation behavior of very well known pulsars and BHC.

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The recent IBIS detection of a polarized component above 400 keV from the X-ray binary Cyg X-1 emission, reported by Laurent et al. [18], has confirmed the existence of polarised high energy component from galactic stellar mass black holes. Taking advantage of the very long observations already performed on Cyg X-1, Jourdain et al. [19] have obtained a fully independent evidence of polarization detected with photons absorbed in the bulk SPI detector volume. The combined INTEGRAL payload result not only confirms the high energy emission from Cyg X-1, but also suggests that it originates from the jet, also active in the radio waveband (see Ref. [18] for details of the emission mechanisms).

Fig. 5. The IBIS source species evolution vs. time (source: adapted from [10]).

Fig. 6. Different IBIS sources gender at E 4100 keV (source: adapted from [11]).

3.1.2. INTEGRAL set stringent limits on the Lorentz Invariance Physical processes addressed by modern physics are based on the relativity principle, i.e. the assumption of equivalence of physical laws for all non-accelerated observers, everywhere in the Universe. This, in turn, implies the invariance of physical laws under Lorentz transformations, associated to the concept of isotropy (no preferred direction in space), homogeneity (no preferred locations) and causal ordering of space-time. This general invariance under LI relates all possible distances and therefore gives a unique chance to verify this theory by studying astrophysical phenomena that can be, and actually are, observed over all space/time scales. In particular, phenomena occurring on short (local Galaxy) to long distances (far away Universe) can be used to probe the assumption that the structure of space-time is identical on all scales. On the other hand, different models of Quantum Gravity predict that space-time looks different at very short length scales/high energies: this may imply departures from Lorentz symmetry. Specific hints of Lorentz Violations (LV) arise from various approaches to quantum gravity [20–25].

Fig. 7. The INTEGRAL/IBIS GPS view. Top: 20–100 keV range and bottom: 100–300 keV range.

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The more direct astrophysical way to check the above effects, that predict different speeds of propagation through the vacuum for left and right handed photons (vacuum birefringence), is to measure polarisation behavior of strong-distant (i.e. GRB) or moderate strongnearby (i.e. Crab Nebula) objects. Using this new technique INTEGRAL investigators have been able to place stringent limits on this effect from the discovered polarization of soft gamma-rays from the Crab Nebula [16,17]. The basic fact is that the orientation on the sky of the polarisation direction of the detected photons is the same as that of the optical photons and coincides with the rotation axis of the Crab Neutron Star. In turn, this measurement shows univocally that emitted photons with opposite helicity travel at the same speed to extremely high precision. In the above framework, a relevant result has been derived from the INTEGRAL detection of polarised soft gamma-ray photons from the Crab Nebula. An unprecedented stringent limits was set on the Lorentz Invariance [26,27]: the absence of vacuum birefringence effects constrains O(E/M) Lorentz violation in Quantum ElectroDinamic to the level ∣ξ∣ r 9  10 10 (95% confidence level). This improved by more than three orders of magnitude the previously achieved constraint. In the same paper the authors have shown the potential use of INTEGRAL observations to probe ∣ξ∣ r9  10 16 if capable to detect polarization in active galaxies at red-shift 1. In the discovery paper Dean and collaborators considered all the photons within the SPI energy band (100 keV–1 MeV). However, they pointed out that the convolution of the SPI sensitivity to polarization vs. energy was maximized in the narrower energy range between 150 and 300 keV. Accordingly, Maccione and coauthors concentrated their LI analysis in that range [26]. An even more stringent upper limit was obtained by Laurent and collaborators using the IBIS data coupled with a recent determination of the distance of GRB041219A [28]. The high degree of polarization observed [14,15] and the determination of the GRB distance allowed to increase by 4 orders of magnitude the existing constraint on Lorentz Invariance violations, arising from birefringence: ∣ξ∣ r 10 14 [29]. 3.2. The INTEGRAL Surveys 3.2.1. The INTEGRAL Galaxy and Universe View A key scientific objective of the INTEGRAL mission has been, and still is, to survey the sky in the soft γ-ray domain making use of the unique capability of the IBIS instrument. Indeed the imaging resolving power, the spectral coverage, the detection sensitivity and the large field of view of IBIS allowed the detection of about 1000 sources during the first 10 years of INTEGRAL operation changing our view of the high energy source populations. So far, the latest detailed information has been obtained with the fourth IBIS catalogue [10] and the 17–60 survey by Krivonos [30,31]. The first one listed more than 700 sources detected at the mCrab level with a typical location accuracy of 2–5 arc-minutes in the energy band 20–100 keV. For these sources light curve and averaged spectra along the whole periods of detection have been generated and exploited in detail in many different papers. Similarly, the 17–60 keV survey by Krivonos listed about 500 and 400 sources that are partially overlapping the ones by Bird et al. [10] and with very similar characteristics. A consistent (up to 30%) fraction of the reported sources do not have a counterpart so to define their nature and multi-wavelength behavior. Two different steps have been followed with the aim to associate the new detected objects: first cross-correlation with existing soft X-ray catalogs followed, in case of positive founding, by optical and near infrared spectroscopy; in the second case when no association at X-ray is possible, request of X-ray follow-up campaigns started mainly with SWIFT-XRT but also with XMM [32] and Chandra [33]. Once there is a narrower error box

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of the IGR sources positioning, many different optical campaign started and are currently still on going. Vice-versa, only in a few cases the hard-X timing properties (such as X-ray bursts or a periodicity detections) have allowed to firmly classify the detected sources. These new high energy emitters have been quite often discovered during monitoring programmes of the Galactic Plane, or specific sky area such as deep extragalactic fields and Bulge and a few also from the Small Magelanic Cloud survey [34]. Very successfully has been the classification work of these new sources initiated in 2004 mainly by Masetti and collaborators that have allowed so far to identify about 200 object making use of optical and NIR spectroscopy (for full detail see Refs. [35–47]). This activity revealed that most of these sources are of extragalactic nature followed by HMXBs while a few are LMXB. By taking into account work by other teams [48–51], this number increases to 270 with the following detailed main class separation: 62% are AGN, 25% X-ray Binaries, 11% Cataclismic Variables and 2% are possibly active stars. For the AGN class we note that, as reported in Masetti et al. [46], 55% of the identification correspond to Seyfert 1 galaxies and 44% are narrow-line AGN with a few objects corresponding to QSOs, XBONGs or BLLac. It is also worth to note that for the galactic sources 75% corresponds to HMXBs. We like to stress here that these results are not surprising as the strong absorption in the host galaxy for AGNs and intrinsic absorption characterizing sources of new type discovered by INTEGRAL [52] makes all these objects very faint in the optical and in the standard X-ray band, below 10 keV, so that only higher energies instrumentation and large optical facilities can reveal their nature. 3.2.2. INTEGRAL discovers a separate class of High Mass X-Ray Binaries (HMXB): the Supergiant Fast X-ray Transients (SFXT) One of the extreme transient phenomenon detected for the first time with INTEGRAL is the class of Supergiant Fast X-ray Transient, with emission characterized by short (100–10,000 s) bright flares and X-ray luminosities from 1032 to 1037 erg/s. This is one of the two new types of HMXB discovered by INTEGRAL and complemented by multi-wavelength observations: the highly obscured supergiant system and the SFXT. Indeed, galactic binaries account for about 26% of the sources listed in the latest IBIS catalogue [10] and are equally divided into the main class Low Mass X-ray Binaries (LMXB) and HMXB. The number of HMXB systems increased during the last 10 years and we got so far 24 Be HMXB and 19 supergiant-HMXB (for a review see Refs. [53,54]). In fact, many of the new discovered sources are HMXB with a neutron star orbiting an OB companion quite often a supergiant star. The orbital period and the spin period have been derived for a number of them showing that they have longer pulsation period and higher absorption with respect to supergiantHMXB system known before the INTEGRAL era. So far 12 SFXTs have been firmly classified while several candidates with peculiar X-ray flaring are still missing an optical identification. The main characteristic can be summarize as follows: flares rise in few minutes and last a few hours, faint quiescent emission and spectra extending to high energy. Moreover, according to duration and frequency of outburst and ratio between the minimum and maximum luminosity they are divided in to classical and intermediate SFXTs. To explain their behavior different mechanisms have been attempted: clumpy supergiant winds, accretion barrier, orbital geometries and wind anisotropy as reviewed recently by Sidoli [55]. 3.3. The Galaxy γ-ray e 7 annihilation lines 3.3.1. Galactic Bulge and Disk imaged for the first time The detection of γ-rays at 511 keV is the direct indication of the presence of positron-electron annihilation. The hint of the

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existence of such a radiation from the central part of our Galaxy was obtained since the beginning of 70s with pioneer stratospheric balloon flights [56,57]. The data were interpreted due to pair annihilation detected with a scintillation detector at an energy of about 476 keV. This result triggered the ideation and building of a number of more sophisticate experiments aimed to better identify the actual energy of the detected photons, expected to be exactly at 511 keV. The confirmation of annihilation processes, active in the central part of the Galaxy was firstly obtained, again with a balloon borne payload, by Leventhal, Mac Callum and Stang [58]. This clear evidence was obtained employing a new type of detector, making use of high purity germanium technology, flown over Australia. The authors stated that they “detect a sharp spectral features from the galactic center direction”. A 511 keV positron annihilation line was observed with suggestive evidence for the detection of the three-photon positronium continuum, and the possible origin of the positrons is discussed [58]. The line was un-ambiguosly identified with the annihilation processes in view of the high spectral resolution ( r 3:2 keV FWHM) provided by the germanium detector. However, the nature of the 511 keV photons and the origin of the electrons and positrons were still a mystery at that time and so remained for several decades. In 2002 INTEGRAL was successfully launched carrying aboard two technologically advanced main instrument based on a cooled germanium spectrometer (SPI), with high spectral resolution, and a pixellised CdTe/CsI high spatial resolution imager (IBIS). Among the main target of the spectrometer was the understanding of the nature of the annihilation line from the Galaxy centre and plane; one of the main target of the Imager IBIS was the detection of the (eventually broaden) 511 keV line from galactic isolated NS, LMXB and HMXB, with particular regard to X-ray Novae, μQSOs etc. The initial evidence of diffuse 511 keV line was reported as soon as the observation time spent on the Galaxy centre and plane was long enough to reach a good statistical signal. A strong flux was clearly imaged by the spectrometer in the Galaxy Central region (r 101 region) at the level of an integrated flux of  10 3 photon/cm2/s [59–61]. In Fig. 8 the different components of the diffuse 511 keV galactic emission are shown, as obtained from the initial INTEGRAL/SPI measurements [62]. On the contrary, a very low or negligible flux was detected from the plane (see Refs. [63,64] for more details). In Fig. 9 the spatial distribution of the 511 keV electron– positron annihilation emission in the Galactic central radian is shown. Initial lower statistics maps suggested the presence of an asymmetry in the disk [65]. The more recent result, shown in Fig. 10, is the significance map in the 508–514 keV range. The data are from six years of observation. The authors [66] have developed a new background modeling technique, suggesting that if any

Fig. 9. The origin of the positron producing this initial INTEGRAL image of the 511 keV annihilation lime from the central zone of the Galaxy has been a 40-year old [65].

asymmetry was present, it was more likely associated with an offset of the bulge. 3.3.2. No points source of 511 keV photons detected ever At the same time, no sign of “point source emission” was detected by IBIS from any known binary system nor other type of point source from the first 5 years of sky survey, in spite of the good IBIS sensitivity [3] (obtained combining the data from the low energy detection plane ISGRI [67] and the high energy detection plane PICSIT [68]). The whole sky data analysis provided an upper limit to the 511 keV flux (2 s, integration live time of about 10 ms) corresponding to 1.6  10 4 photon/cm2/s from the SGR A* position, integrating over an error box of about 10 arcminute. The same upper limit, thanks to the IBIS large Field of View, was achieved for any other sky pixel/point source in the Galactic Center region. Obviously, the long term – time resolved analysis was performed for each seasonal observation (from spring 2003 to spring 2007). The data provided an upper limit ranging from 3.2 to 9.2  10 4 photon/cm2/s in each of the observing window of SGR A*, depending on the actual live time spent on each seasonal campaign [69]. Similarly, upper limits ranging from 1.6 to 3.0  10 4 photon/cm2/s have been reported on a number of galactic sources [70]. From the beginning of the INTEGRAL science operation to date, more that 250 ms of SPI and IBIS live data have been analysed. The search for 511 keV emission from any part of the sky and any specific point source known from the available catalogues and from the newly detected INTEGRAL sources (the IGR ones) have been, and is currently, performed on all time scales, ranging from the ms for strong sources, to hours, day, orbit (3 days), months and years. So far, no single 511 keV emission has been detected from point-like sources by INTEGRAL. This analysis has been performed with particular care in case of Black Holes candidate and Neutron Stars spectral transition, particularly in coincidence with detected Jet/plasmoid emission. The search for the 511 keV signature from point sources has been looked at on different energy band-widths, to search for possible broaden 511 keV lines, and in adjacent energy band to look for red-shifted or, possibly, blue-shifted emission. To date, no positive signal has been detected. 3.4. On the nature of the positron/electron γ-ray annihilation lines and continuum Few basic facts are worth to mention, before speculating on the nature of the positrons generating the detected emission:

Fig. 8. The different component of the diffuse 511 keV galactic emission [62].

1. The total annihilated positrons are of the order of  1043/s. 2. The spectrum of the emission is well described by an overimposed combination of a Gaussian shaped narrow line ( r 3 keV width) and a 3 photon continuum [71].

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Fig. 10. The diffuse 511 keV galactic image. The strong Galaxy Centre and the Faint disk components are clearly visible (source: adapted from [66]).

3. The narrow line component does not show any variability over a year to decade time scale (see [74] and references therein). 4. The large presence of continuum below 511 keV suggests most of the positrons form positronium before annihilation [61,62]. 5. The ratio Bulge/Plane is about 4/1, very different from the amount of matter present and from any other emission at different wavelength [72]. 6. The fact that no single point source emitting at 511 keV has been detected inside the region of the diffuse emission imaged by the SPI impose a strong limits on the number of possible single (non-resolved) sources eventually contributing to the diffuse emission. In fact, if due to the sum of several HMXB, as proposed by Weidenspointer [65] and collaborators (or other point sources, i.e. NS, PWN, SNR etc.), none of them should emit more than  10 4 at any time, not to violate the upper limit continuously monitored by IBIS since the beginning of the mission operations. In spite of the quality of the INTEGRAL data, the contemporaneous observation with SPI and IBIS and the unprecedented sensitivity of the data available also due to the large amount of time spent on the GC and Plane, the primary source(s) of positrons has not yet been understood. At the same time, the INTEGRAL detailed maps and the point source upper limit have contributed to focus the issue and triggered a number of new hypothesis for the nature of the positrons and electrons, including “normal”, “new physics” or “exotic” scenarios: the combined emission of HMXB [65], hectic activity from the Super Massive Black Hole in the Galaxy center 300 thousand years ago, light dark matter annihilation [73] etc. Prantzos et al. [72] have analysed in full detail the possible progenitors of the positron responsible for the 511 keV lines. Most of the candidates are already known astrophysical sources, and most of them belong to the class of objects already detected by INTEGRAL in the γ-ray range and listed in the 4th IBIS catalogue [10]. Among them type Ia supernovae, μ QSOs or other type of X-ray binaries (LMXB, HMXB, White Dwarf), isolated NS, SNR, PWN etc., are considered as possible candidates for the production of (part of) the e 7 pairs generating the annihilation photons observed from the Bulge and possibly, from the disk [72].

Fig. 11. The 511 keV galactic Bulge component compared with the red continuum line model that refers to an ISM with an initial temperature of 6.3  105 K and initial positron energy of 250 KeV (figure from [71]).

A significative step forward in the understanding of the positron nature, has been recently provided by Churazov and collaborators. They have shown that the positrons can be generated very far in time and space from the region of the diffuse emission. In particular, they propose a model that explains very well the ratio between the ortho-positronium and 511 keV flux ratio [71]. The model discusses how positrons are initially injected a  106 K hot environment containing the inter stellar medium (ISM), which is then allowed to freely cool via radiative losses. The majority of the positrons can annihilate after the gas is cooled down to  105 K providing the necessary characteristics of annihilation in the presence of a warm and ionised medium, as observed by INTEGRAL (see Ref. [71] for a detailed analysis). The authors have demonstrated that the positrons, responsible for the annihilation and the ISM in which they were initially injected, have lost memory of the initial temperature of the ISM and energy of the positrons (cf. Fig. 11). This finding is very important: while

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Fig. 12. The table summarises the possible source of positrons contributing to the 511 keV emission. For a comprehensive discussion of the table content see [72] from which the table has been extracted.

confirmed by other measurements, the need to associate the generation of the positrons with the annihilation site which is not any more necessary, opening a new scenario. On the other hand, a deeper investigation of the new hypothesised scenario (production a annihilation) opens a new complex area of investigation, in view of the fact that the positrons can migrate (very) far away (in space and time) from their production sites before annihilation. This makes difficult to infer the proper source distribution, also in view of the large variety of new class of γ-ray emitters recently discovered by INTEGRAL and SWIFT. In addition, while the propagation of high energy particles and cosmic rays is well understood and modelised, the long range drift behavior of the low energy ( MeV) electrons and positrons in the complex environment of the ISM (turbulence, magnetic field, discontinuity in the matter distribution etc.) is not easy to understand and properly model. A recent deep analysis of the topic has been provided by Prantzos and collaborators that have critically discussed different candidate positron sources and models of positron propagation in the Galaxy. A synthetic view of their findings is shown in Fig. 12, while the full deep analysis can be found in Ref. [72].

Acknowledgments The authors are grateful to Mrs. Silvia Zampieri for the professional and careful editing of the paper. The authors acknowledge financial contribution from ASI-INAF agreement I/033/10/0. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

[11] [12]

4. Conclusion

[13]

INTEGRAL operations have just been extended by ESA Space program Committee till 2016. The outstanding performance of the astrophysical payload (including all the instruments still in the launch configuration with no redundant systems so far used) and of the S/C is very promising for the near future scientific operability of this high energy Observatory. In particular, in the future years INTEGRAL will be used to perform more observations, planned as multi-year “key programs” keened to a better understanding of hot open astrophysical problems, focussed to improve our Universe knowledge with deep “survey” type observations. Among the topics shortly depicted in this paper it is worth to mention the search for polarisation from galactic objects and GRBs, and more Galactic Plane deep observations aimed to unveil the nature of the 511 keV radiation and to provide a final clue to the origin of the emitting leptons. Most of the material presented in this paper is the outcome of the INTEGRAL User Group meetings held as a preparatory work to the 2012 mission extension process, and from the high energy scientist working on the INTEGRAL scientific data analysis. We thank all the IUG members and scientists who have contributed to this process for their invaluable contribution to the mission success ensuring future scientific operation.

[14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]

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