Pub Date : 2008-11-25DOI: 10.1017/cbo9781139193627.003
R. Durrer, R. Maartens
Explanations of the late-time cosmic acceleration within the framework of general relativity are plagued by difficulties. General relativistic models are mostly based on a dark energy field with fine-tuned, unnatural properties. There is a great variety of models, but all share one feature in common -- an inability to account for the gravitational properties of the vacuum energy, and a failure to solve the so-called coincidence problem. Two broad alternatives to dark energy have emerged as candidate models: these typically address only the coincidence problem and not the vacuum energy problem. The first is based on general relativity and attempts to describe the acceleration as an effect of inhomogeneity in the universe. If this alternative could be shown to work, then it would provide a dramatic resolution of the coincidence problem; however, a convincing demonstration of viability has not yet emerged. The second alternative is based on infra-red modifications to general relativity, leading to a weakening of gravity on the largest scales and thus to acceleration. Most examples investigated so far are scalar-tensor or brane-world models, and we focus on the simplest candidates of each type: $f(R)$ models and DGP models respectively. Both of these provide a new angle on the problem, but they also face serious difficulties. However, investigation of these models does lead to valuable insights into the properties of gravity and structure formation, and it also leads to new strategies for testing the validity of General Relativity itself on cosmological scales.
{"title":"Dark Energy and Modified Gravity","authors":"R. Durrer, R. Maartens","doi":"10.1017/cbo9781139193627.003","DOIUrl":"https://doi.org/10.1017/cbo9781139193627.003","url":null,"abstract":"Explanations of the late-time cosmic acceleration within the framework of general relativity are plagued by difficulties. General relativistic models are mostly based on a dark energy field with fine-tuned, unnatural properties. There is a great variety of models, but all share one feature in common -- an inability to account for the gravitational properties of the vacuum energy, and a failure to solve the so-called coincidence problem. Two broad alternatives to dark energy have emerged as candidate models: these typically address only the coincidence problem and not the vacuum energy problem. The first is based on general relativity and attempts to describe the acceleration as an effect of inhomogeneity in the universe. If this alternative could be shown to work, then it would provide a dramatic resolution of the coincidence problem; however, a convincing demonstration of viability has not yet emerged. The second alternative is based on infra-red modifications to general relativity, leading to a weakening of gravity on the largest scales and thus to acceleration. Most examples investigated so far are scalar-tensor or brane-world models, and we focus on the simplest candidates of each type: $f(R)$ models and DGP models respectively. Both of these provide a new angle on the problem, but they also face serious difficulties. However, investigation of these models does lead to valuable insights into the properties of gravity and structure formation, and it also leads to new strategies for testing the validity of General Relativity itself on cosmological scales.","PeriodicalId":8453,"journal":{"name":"arXiv: Astrophysics","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2008-11-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"77323947","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A. Fraisse, Jack Brown, G. Dobler, J. Dotson, B. Draine, P. Frisch, M. Haverkorn, C. Hirata, R. Jansson, A. Lazarian, A. M. Magalhães, A. Waelkens, M. Wolleben
We report on our knowledge of Galactic foregrounds, as well as on how a CMB satellite mission aiming at detecting a primordial B-mode signal (CMBPol) will contribute to improving it. We review the observational and analysis techniques used to constrain the structure of the Galactic magnetic field, whose presence is responsible for the polarization of Galactic emissions. Although our current understanding of the magnetized interstellar medium is somewhat limited, dramatic improvements in our knowledge of its properties are expected by the time CMBPol flies. Thanks to high resolution and high sensitivity instruments observing the whole sky at frequencies between 30 GHz and 850 GHz, CMBPol will not only improve this picture by observing the synchrotron emission from our galaxy, but also help constrain dust models. Polarized emission from interstellar dust indeed dominates over any other signal in CMBPol's highest frequency channels. Observations at these wavelengths, combined with ground-based studies of starlight polarization, will therefore enable us to improve our understanding of dust properties and of the mechanism(s) responsible for the alignment of dust grains with the Galactic magnetic field. CMBPol will also shed new light on observations that are presently not well understood. Morphological studies of anomalous dust and synchrotron emissions will indeed constrain their natures and properties, while searching for fluctuations in the emission from heliospheric dust will test our understanding of the circumheliospheric interstellar medium. Finally, acquiring more information on the properties of extra-Galactic sources will be necessary in order to maximize the cosmological constraints extracted from CMBPol's observations of CMB lensing. (abridged)
{"title":"CMBPol Mission Concept Study: Foreground Science Knowledge and Prospects","authors":"A. Fraisse, Jack Brown, G. Dobler, J. Dotson, B. Draine, P. Frisch, M. Haverkorn, C. Hirata, R. Jansson, A. Lazarian, A. M. Magalhães, A. Waelkens, M. Wolleben","doi":"10.1063/1.3160889","DOIUrl":"https://doi.org/10.1063/1.3160889","url":null,"abstract":"We report on our knowledge of Galactic foregrounds, as well as on how a CMB satellite mission aiming at detecting a primordial B-mode signal (CMBPol) will contribute to improving it. We review the observational and analysis techniques used to constrain the structure of the Galactic magnetic field, whose presence is responsible for the polarization of Galactic emissions. Although our current understanding of the magnetized interstellar medium is somewhat limited, dramatic improvements in our knowledge of its properties are expected by the time CMBPol flies. Thanks to high resolution and high sensitivity instruments observing the whole sky at frequencies between 30 GHz and 850 GHz, CMBPol will not only improve this picture by observing the synchrotron emission from our galaxy, but also help constrain dust models. Polarized emission from interstellar dust indeed dominates over any other signal in CMBPol's highest frequency channels. Observations at these wavelengths, combined with ground-based studies of starlight polarization, will therefore enable us to improve our understanding of dust properties and of the mechanism(s) responsible for the alignment of dust grains with the Galactic magnetic field. CMBPol will also shed new light on observations that are presently not well understood. Morphological studies of anomalous dust and synchrotron emissions will indeed constrain their natures and properties, while searching for fluctuations in the emission from heliospheric dust will test our understanding of the circumheliospheric interstellar medium. Finally, acquiring more information on the properties of extra-Galactic sources will be necessary in order to maximize the cosmological constraints extracted from CMBPol's observations of CMB lensing. (abridged)","PeriodicalId":8453,"journal":{"name":"arXiv: Astrophysics","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2008-11-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"89483152","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The cosmological constant problem is seen as a symptom of the ambiguity of the Riemann curvature in general relativity. The solution of that ambiguity provided by Nash’s theorem on gravitational perturbations along extra dimensions eliminate the direct comparison between the vacuum energy density and Einstein’s cosmological constant, besides being compatible with the formation of structures and the accelerated expansion of the universe.
{"title":"Applications of Nash’s Theorem to Cosmology","authors":"A. Capistrano, M. D. Maia","doi":"10.5772/23526","DOIUrl":"https://doi.org/10.5772/23526","url":null,"abstract":"The cosmological constant problem is seen as a symptom of the ambiguity of the Riemann curvature in general relativity. The solution of that ambiguity provided by Nash’s theorem on gravitational perturbations along extra dimensions eliminate the direct comparison between the vacuum energy density and Einstein’s cosmological constant, besides being compatible with the formation of structures and the accelerated expansion of the universe.","PeriodicalId":8453,"journal":{"name":"arXiv: Astrophysics","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2008-11-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"90938538","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2008-11-23DOI: 10.1007/978-3-540-49535-2_14
F. Schunck, A. Liddle
{"title":"Boson stars in the centre of galaxies","authors":"F. Schunck, A. Liddle","doi":"10.1007/978-3-540-49535-2_14","DOIUrl":"https://doi.org/10.1007/978-3-540-49535-2_14","url":null,"abstract":"","PeriodicalId":8453,"journal":{"name":"arXiv: Astrophysics","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2008-11-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"79503987","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A large fraction of the recently discovered Galactic Very High Energy (VHE) source population remains unidentified to date. VHE γ‐ray emission traces high energy particles in these sources, but for example in case of hadronic processes also the gas density at the emission site. Moreover, the particles have sufficiently long lifetimes to be able to escape from their acceleration sites. Therefore, the γ‐ray sources or at least the areas of maximum surface brightness are in many cases spatially offset from the actual accelerators. A promising way to identify the objects in which the particles are accelerated seems to be to search for emission signatures of the acceleration process (like emission from shock‐heated plasma). Also the particles themselves (through primary or secondary synchrotron emission) can be traced in lower wavebands. Those signatures are best visible in the X‐ray band, and current X‐ray observatories are well suited to conduct such follow‐up observations. Some aspects of the current status...
{"title":"X‐ray follow‐up observations of unidentified VHE γ‐ray sources","authors":"G. Pühlhofer","doi":"10.1063/1.3076636","DOIUrl":"https://doi.org/10.1063/1.3076636","url":null,"abstract":"A large fraction of the recently discovered Galactic Very High Energy (VHE) source population remains unidentified to date. VHE γ‐ray emission traces high energy particles in these sources, but for example in case of hadronic processes also the gas density at the emission site. Moreover, the particles have sufficiently long lifetimes to be able to escape from their acceleration sites. Therefore, the γ‐ray sources or at least the areas of maximum surface brightness are in many cases spatially offset from the actual accelerators. A promising way to identify the objects in which the particles are accelerated seems to be to search for emission signatures of the acceleration process (like emission from shock‐heated plasma). Also the particles themselves (through primary or secondary synchrotron emission) can be traced in lower wavebands. Those signatures are best visible in the X‐ray band, and current X‐ray observatories are well suited to conduct such follow‐up observations. Some aspects of the current status...","PeriodicalId":8453,"journal":{"name":"arXiv: Astrophysics","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2008-11-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"77428115","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2008-11-21DOI: 10.1088/1475-7516/2009/03/015
J. B. Jiménez, A. L. Maroto
Large-scale matter bulk flows with respect to the cosmic microwave background have very recently been detected on scales 100 Mpc/h and 300 Mpc/h by using two different techniques showing an excellent agreement in the motion direction. However, the unexpectedly large measured amplitudes are difficult to understand within the context of standard LCDM cosmology. In this work we show that the existence of such a flow could be signaling the presence of moving dark energy at the time when photons decoupled from matter. We also comment on the relation between the direction of the CMB dipole and the preferred axis observed in the quadrupole in this scenario.
{"title":"Large-scale cosmic flows and moving dark energy","authors":"J. B. Jiménez, A. L. Maroto","doi":"10.1088/1475-7516/2009/03/015","DOIUrl":"https://doi.org/10.1088/1475-7516/2009/03/015","url":null,"abstract":"Large-scale matter bulk flows with respect to the cosmic microwave background have very recently been detected on scales 100 Mpc/h and 300 Mpc/h by using two different techniques showing an excellent agreement in the motion direction. However, the unexpectedly large measured amplitudes are difficult to understand within the context of standard LCDM cosmology. In this work we show that the existence of such a flow could be signaling the presence of moving dark energy at the time when photons decoupled from matter. We also comment on the relation between the direction of the CMB dipole and the preferred axis observed in the quadrupole in this scenario.","PeriodicalId":8453,"journal":{"name":"arXiv: Astrophysics","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2008-11-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"74294678","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The detection of gamma-rays, antiprotons and positrons due to pair annihilation of dark matter particles in the Milky Way halo is a viable indirect technique to search for signatures of supersymmetric dark matter where the major challenge is the discrimination of the signal from the background generated by standard production mechanisms. The new PAMELA antiproton data are consistent with the standard secondary production and this allows us to constrain exotic contribution to the spectrum due to neutralino annihilations. In particular, we show that in the framework of minimal supergravity (mSUGRA), in a clumpy halo scenario (with clumpiness factor > 10) and for large values of tan(beta)> 55, almost all the parameter space allowed by WMAP is excluded. Instead, the PAMELA positron fraction data exhibit an excess that cannot be explained by secondary production. PPB-BETS and ATIC reported a feature in electron spectrum at a few hundred GeV. The excesses seem to be consistent and imply a source, conventional or exotic, of additional leptonic component. Here we discuss the status of indirect dark matter searches and a perspective forPAMELA and Fermi gamma-ray space telescope (Fermi) experiments.
{"title":"Status of indirect searches in the PAMELA and Fermi era","authors":"A. Morselli, Infn Roma, I. Moskalenko","doi":"10.22323/1.064.0025","DOIUrl":"https://doi.org/10.22323/1.064.0025","url":null,"abstract":"The detection of gamma-rays, antiprotons and positrons due to pair annihilation of dark matter particles in the Milky Way halo is a viable indirect technique to search for signatures of supersymmetric dark matter where the major challenge is the discrimination of the signal from the background generated by standard production mechanisms. The new PAMELA antiproton data are consistent with the standard secondary production and this allows us to constrain exotic contribution to the spectrum due to neutralino annihilations. In particular, we show that in the framework of minimal supergravity (mSUGRA), in a clumpy halo scenario (with clumpiness factor > 10) and for large values of tan(beta)> 55, almost all the parameter space allowed by WMAP is excluded. \u0000Instead, the PAMELA positron fraction data exhibit an excess that cannot be explained by secondary production. PPB-BETS and ATIC reported a feature in electron spectrum at a few hundred GeV. The excesses seem to be consistent and imply a source, conventional or exotic, of additional leptonic component. Here we discuss the status of indirect dark matter searches and a perspective forPAMELA and Fermi gamma-ray space telescope (Fermi) experiments.","PeriodicalId":8453,"journal":{"name":"arXiv: Astrophysics","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2008-11-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"84366612","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
LS 5039 is among the most interesting VHE sources in the Galaxy. Two scenarios have been put forward to explain the observed TeV radiation: jets vs pulsar winds. The source has been detected during the superior conjunction of the compact object, when very large gamma‐ray opacities are expected. In addition, electromagnetic cascades, which may make the system more transparent to gamma‐rays, are hardly efficient for realistic magnetic fields in massive star surroundings. All this makes unlikely the standard pulsar scenario for LS 5039, in which the emitter is the region located between the star and the compact object, where the opacities are the largest. Otherwise, a jet‐like flow can transport energy to regions where the photon‐photon absorption is much lower and the TeV radiation is not so severely absorbed.
LS 5039是银河系中最有趣的VHE源之一。人们提出了两种情况来解释观测到的TeV辐射:喷流vs脉冲星风。该源是在致密天体的超合期间探测到的，届时伽马射线的不透明度将非常大。此外，电磁级联可能使系统对伽马射线更加透明，但对大质量恒星环境中的实际磁场几乎没有效率。所有这些都使LS 5039的标准脉冲星场景变得不太可能，在标准脉冲星场景中，发射器位于恒星和致密物体之间的区域，那里的不透明度最大。否则，像射流一样的流可以将能量输送到光子-光子吸收低得多、TeV辐射吸收不那么严重的区域。
{"title":"Clues to unveil the emitter in LS 5039: powerful jets vs colliding winds","authors":"V. Bosch-Ramon, D. Khangulyan, F. Aharonian","doi":"10.1063/1.3076645","DOIUrl":"https://doi.org/10.1063/1.3076645","url":null,"abstract":"LS 5039 is among the most interesting VHE sources in the Galaxy. Two scenarios have been put forward to explain the observed TeV radiation: jets vs pulsar winds. The source has been detected during the superior conjunction of the compact object, when very large gamma‐ray opacities are expected. In addition, electromagnetic cascades, which may make the system more transparent to gamma‐rays, are hardly efficient for realistic magnetic fields in massive star surroundings. All this makes unlikely the standard pulsar scenario for LS 5039, in which the emitter is the region located between the star and the compact object, where the opacities are the largest. Otherwise, a jet‐like flow can transport energy to regions where the photon‐photon absorption is much lower and the TeV radiation is not so severely absorbed.","PeriodicalId":8453,"journal":{"name":"arXiv: Astrophysics","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2008-11-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"74549910","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Infrared interferometry is currently in a rapid development phase, with new instrumentation soon achieving milliarcsecond spatial resolutions for faint sources and astrometry on the order of 10 microarcseconds. For jet studies in particular, the next generation of instruments will bring us closer to the event horizon of supermassive black holes such as Sgr A*, and the region where jet launching must occur. But a new possibility to study microquasars in general and jet physics in particular may also arise, using techniques similar to those employed for finding faint exoplanets around stars. The compact, steady jets observed in the hard state of X-ray binaries display a flat/inverted spectrum from the lower radio wavelengths up through at least the far-IR band. Somewhere above this band, a turnover is predicted where the jets become optically thin, revealing a power-law spectrum. This break may have been observed directly in GX339-4, but in most sources such a feature is likely hidden under bright emission from the stellar companion or accretion disk components. Detecting the exact location of this break would provide a new constraint on our models of jet physics, since the break frequency is dependent on the total power, as well as internal density and magnetic field. Furthermore, knowing the break location combined with the spectral index of the power-law would help constrain the amount of synchrotron emission contributed by the jets to the X-ray bands. Along with a summary of some potential observations requiring less optimal instrumental specifications, I will discuss a technique which may enable us to discern the jet break, and the chances of success based on theoretical models applied to some potential target sources.
{"title":"Looking to the future: using IR interferometry to study microquasars","authors":"S. Markoff","doi":"10.22323/1.062.0056","DOIUrl":"https://doi.org/10.22323/1.062.0056","url":null,"abstract":"Infrared interferometry is currently in a rapid development phase, with new instrumentation soon achieving milliarcsecond spatial resolutions for faint sources and astrometry on the order of 10 microarcseconds. For jet studies in particular, the next generation of instruments will bring us closer to the event horizon of supermassive black holes such as Sgr A*, and the region where jet launching must occur. But a new possibility to study microquasars in general and jet physics in particular may also arise, using techniques similar to those employed for finding faint exoplanets around stars. The compact, steady jets observed in the hard state of X-ray binaries display a flat/inverted spectrum from the lower radio wavelengths up through at least the far-IR band. Somewhere above this band, a turnover is predicted where the jets become optically thin, revealing a power-law spectrum. This break may have been observed directly in GX339-4, but in most sources such a feature is likely hidden under bright emission from the stellar companion or accretion disk components. Detecting the exact location of this break would provide a new constraint on our models of jet physics, since the break frequency is dependent on the total power, as well as internal density and magnetic field. Furthermore, knowing the break location combined with the spectral index of the power-law would help constrain the amount of synchrotron emission contributed by the jets to the X-ray bands. Along with a summary of some potential observations requiring less optimal instrumental specifications, I will discuss a technique which may enable us to discern the jet break, and the chances of success based on theoretical models applied to some potential target sources.","PeriodicalId":8453,"journal":{"name":"arXiv: Astrophysics","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2008-11-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"89776988","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
We present an internal shocks model to investigate particle acceleration and radiation production in microquasar jets. The jet is modelled with discrete ejecta at various time intervals. These ejecta (or 'shells') may have different properties including the bulk velocity. Faster shells can catch up and collide with the slower ones, thus giving rise to shocks. The particles are accelerated inside the shocked plasma. Each collision results in a new shell, which may take part in any subsequent collisions as well as radiate due to synchrotron radiation. Almost continuous energy dissipation along the jet can be obtained with a large number of shell collisions. We investigate the spectral energy distribution of such jets as well as the physical significance of various parameters (e.g. the time interval between ejections and the shell size).
{"title":"Internal shocks model for microquasar jets","authors":"Omar Jamil, R. Fender, C. Kaiser","doi":"10.22323/1.062.0026","DOIUrl":"https://doi.org/10.22323/1.062.0026","url":null,"abstract":"We present an internal shocks model to investigate particle acceleration and radiation production in microquasar jets. The jet is modelled with discrete ejecta at various time intervals. These ejecta (or 'shells') may have different properties including the bulk velocity. Faster shells can catch up and collide with the slower ones, thus giving rise to shocks. The particles are accelerated inside the shocked plasma. Each collision results in a new shell, which may take part in any subsequent collisions as well as radiate due to synchrotron radiation. Almost continuous energy dissipation along the jet can be obtained with a large number of shell collisions. We investigate the spectral energy distribution of such jets as well as the physical significance of various parameters (e.g. the time interval between ejections and the shell size).","PeriodicalId":8453,"journal":{"name":"arXiv: Astrophysics","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2008-11-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"80973418","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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