Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Observations of nearby Galaxy Clusters with the Fermi Large Area Telescope: Towards the first Gamma Rays from Clusters
Stockholm University, Faculty of Science, Department of Physics. (CoPS)ORCID iD: 0000-0002-5735-0082
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Galaxy clusters are the most massive bound systems known in the Universe and are believed to have formed through large scale structure formation. They host relativistic cosmic-ray (CR) populations and are gravitationally bound by large amounts of Dark Matter (DM), both providing conditions in which high-energy gamma rays may be produced either via CR interactions with the intracluster medium or through the annihilation or decay of DM particles.

Prior to the launch of the Fermi satellite, predictions were optimistic that these sources would be established as γ-ray-bright objects by observations through its prime instrument, the Large Area Telescope (LAT). Yet, despite numerous efforts, even a single firm cluster detection is still pending. This thesis presents a number of studies based on data taken by the LAT over its now seven year mission aiming to discover these γ rays.

Using a joint likelihood technique, we study the γ-ray spectra of a sample of nearby clusters searching for a CR-induced signal due to hadronic interactions in the intracluster medium. While we find excesses in some individual targets, we attribute none to the cluster. Hence, we constrain the maximum injection efficiency of hadrons being accelerated in structure formation shocks and the fraction of CR-to-thermal pressure. We also perform a refined search targeting the Coma cluster specifically due to its large variety of existing observations in other wavebands. In the latter case we find weak indications of an excess which however falls below the detection threshold.

Because the cluster emission we consider is inherently extended, we need to take into account the imperfect modeling of the foreground emission, which may be particularly difficult such as is the case with the Virgo cluster. Here, we assess the systematics associated with the foreground uncertainties and derive limits based on an improved background model of the region. For the first time we derive limits on the γ-ray flux from CR and DM-interactions in which we take into account the dynamical state of the system. For DM we also include the contribution from substructure. The DM domain is further explored by searching for line-like features as they arise from the annihilation of DM into two photons in a large sample of clusters, including Virgo and Coma. Finding no evidence for γ-ray lines, we derive limits on the DM annihilation cross section that are roughly a factor 10 (100) above that derived from observations of the galactic center assuming an optimistic (conservative) scenario regarding the boost due to DM substructure. 

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University , 2015. , 80 p.
Keyword [en]
Galaxy Clusters, Fermi-LAT, γ rays, Dark Matter, Cosmic Rays
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics
Identifiers
URN: urn:nbn:se:su:diva-121592ISBN: 978-91-7649-270-3 (print)OAI: oai:DiVA.org:su-121592DiVA: diva2:860106
Public defence
2015-11-23, sal FB42, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Submitted. Paper 4: Submitted.

Available from: 2015-10-30 Created: 2015-10-10 Last updated: 2015-11-25Bibliographically approved
List of papers
1. SEARCH FOR COSMIC-RAY-INDUCED GAMMA-RAY EMISSION IN GALAXY CLUSTERS
Open this publication in new window or tab >>SEARCH FOR COSMIC-RAY-INDUCED GAMMA-RAY EMISSION IN GALAXY CLUSTERS
Show others...
2014 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 787, no 1, 18Article in journal (Refereed) Published
Abstract [en]

Current theories predict relativistic hadronic particle populations in clusters of galaxies in addition to the already observed relativistic leptons. In these scenarios hadronic interactions give rise to neutral pions which decay into gamma rays that are potentially observable with the Large Area Telescope (LAT) on board the Fermi space telescope. We present a joint likelihood analysis searching for spatially extended gamma-ray emission at the locations of 50 galaxy clusters in four years of Fermi-LAT data under the assumption of the universal cosmic-ray (CR) model proposed by Pinzke & Pfrommer. We find an excess at a significance of 2.7 sigma, which upon closer inspection, however, is correlated to individual excess emission toward three galaxy clusters: A400, A1367, and A3112. We discuss these cases in detail and conservatively attribute the emission to unmodeled background systems (for example, radio galaxies within the clusters). Through the combined analysis of 50 clusters, we exclude hadronic injection efficiencies in simple hadronic models above 21% and establish limits on the CR to thermal pressure ratio within the virial radius, R-200, to be below 1.25%-1.4% depending on the morphological classification. In addition, we derive new limits on the gamma-ray flux from individual clusters in our sample.

Keyword
galaxies: clusters: intracluster medium, gamma rays: galaxies: clusters
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-105525 (URN)10.1088/0004-637X/787/1/18 (DOI)000335924200018 ()
Funder
Knut and Alice Wallenberg Foundation
Note

AuthorCount:126;

Available from: 2014-06-25 Created: 2014-06-24 Last updated: 2017-12-05Bibliographically approved
2. Search for extended gamma-ray emission from the Virgo Galaxy Cluster with Fermi-LAT
Open this publication in new window or tab >>Search for extended gamma-ray emission from the Virgo Galaxy Cluster with Fermi-LAT
Show others...
2015 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 812, no 2, 159Article in journal (Refereed) Published
Abstract [en]

Galaxy clusters are one of the prime sites to search for dark matter (DM) annihilation signals. Depending on thesubstructure of the DM halo of a galaxy cluster and the cross sections for DM annihilation channels, these signalsmight be detectable by the latest generation of γ-ray telescopes. Here we use three years of Fermi-Large AreaTelescope data, which are the most suitable for searching for very extended emission in the vicinity of the nearbyVirgo galaxy cluster. Our analysis reveals statistically significant extended emission which can be wellcharacterized by a uniformly emitting disk profile with a radius of 3° that moreover is offset from the cluster center.We demonstrate that the significance of this extended emission strongly depends on the adopted interstellaremission model (IEM) and is most likely an artifact of our incomplete description of the IEM in this region. Wealso search for and find new point source candidates in the region. We then derive conservative upper limits on thevelocity-averaged DM pair annihilation cross section from Virgo. We take into account the potential γ-ray fluxenhancement due to DM sub-halos and its complex morphology as a merging cluster. For DM annihilating into bb,assuming a conservative sub-halo model setup, we find limits that are between 1 and 1.5 orders of magnitude abovethe expectation from the thermal cross section for mDM  100 GeV. In a more optimistic scenario, we excludev 3 10 cm s 26 3 1 ás ñ~ ´ - - for mDM  40 GeV for the same channel. Finally, we derive upper limits on the γ-ray-flux produced by hadronic cosmic-ray interactions in the inter cluster medium. We find that the volume-averagedcosmic-ray-to-thermal pressure ratio is less than ∼6%.

Keyword
Astrophysics, High Energy Astrophysical Phenomena, galaxies: clusters: individual (Virgo), gamma-rays: galaxies: clusters, gamma-rays: general
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-121583 (URN)10.1088/0004-637X/812/2/159 (DOI)000365206600071 ()
Available from: 2015-10-10 Created: 2015-10-10 Last updated: 2017-12-01Bibliographically approved
3. Search for Gamma-ray Lines Towards Galaxy Clusters with the Fermi-LAT
Open this publication in new window or tab >>Search for Gamma-ray Lines Towards Galaxy Clusters with the Fermi-LAT
Show others...
2016 (English)In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 2, 026Article in journal (Refereed) Published
Abstract [en]

We report on a search for monochromatic gamma-ray features in the spectra of galaxy clusters observed by the Fermi Large Area Telescope. Galaxy clusters are the largest structures in the Universe that are bound by dark matter (DM), making them an important testing ground for possible self-interactions or decays of the DM particles. Monochromatic gamma-ray lines provide a unique signature due to the absence of astrophysical backgrounds and are as such considered a smoking-gun signature for new physics. An unbinned joint likelihood analysis of the sixteen most promising clusters using five years of data at energies between 10 and 400 GeV revealed no significant features. For the case of self-annihilation, we set upper limits on the monochromatic velocity-averaged interaction cross section. These limits are compatible with those obtained from observations of the Galactic Center, albeit weaker due to the larger distance to the studied clusters.

Keyword
dark matter experiments, galaxy clusters, gamma ray experiments
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-121585 (URN)10.1088/1475-7516/2016/02/026 (DOI)000372467600027 ()
Available from: 2015-10-10 Created: 2015-10-10 Last updated: 2017-12-01Bibliographically approved
4. Search for gamma-ray emission from the Coma Cluster with six years of Fermi-LAT data
Open this publication in new window or tab >>Search for gamma-ray emission from the Coma Cluster with six years of Fermi-LAT data
Show others...
2016 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 819, no 2, 149Article in journal (Refereed) Published
Abstract [en]

We present results from gamma-ray observations of the Coma cluster incorporating six years of Fermi-LAT data and the newly released "Pass 8" event-level analysis. Our analysis of the region reveals low-significance residual structures within the virial radius of the cluster that are too faint for a detailed investigation with the current data. Using a likelihood approach that is free of assumptions on the spectral shape we derive upper limits on the gamma-ray flux that is expected from energetic particle interactions in the cluster. We also consider a benchmark spatial and spectral template motivated by models in which the observed radio halo is mostly emission by secondary electrons. In this case, the median expected and observed upper limits for the flux above 100MeV are 1.7 x 10(-9) ph cm(-2) s(-1) and 5.2 x 10(-9) ph cm(-2) s(-1) respectively (the latter corresponds to residual emission at the level of 1.8 sigma). These bounds are comparable to or higher than predicted levels of hadronic gamma-ray emission in cosmic-ray (CR) models with or without reacceleration of secondary electrons, although direct comparisons are sensitive to assumptions regarding the origin and propagation mode of CRs and magnetic field properties. The minimal expected.-ray flux from radio and star-forming galaxies within the Coma cluster is roughly an order of magnitude below the median sensitivity of our analysis.

Keyword
galaxies: clusters: individual (Coma), gamma rays: galaxies: clusters
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-121584 (URN)10.3847/0004-637X/819/2/149 (DOI)000372305700061 ()
Available from: 2015-10-10 Created: 2015-10-10 Last updated: 2017-12-01Bibliographically approved

Open Access in DiVA

Observations of nearby Galaxy Clusters with the Fermi Large Area Telescope(9229 kB)156 downloads
File information
File name FULLTEXT01.pdfFile size 9229 kBChecksum SHA-512
6e21f8b8af8ac0f9988766864f113fdb134f0c8d045c01bb64ae1ec8d4d2ff7f6c4cd269c2a39091fe0270583d06c7449df77545f0dd2bd9b3d7b7efa7886f7d
Type fulltextMimetype application/pdf
Observations of nearby Galaxy Clusters with the Fermi Large Area Telescope_errata(109 kB)9 downloads
File information
File name ERRATA01.pdfFile size 109 kBChecksum SHA-512
e6f32add434a62bf2d15eb774e68d88a3f0bd47c307ee93b80d108187e91bcbd0c0fd2f94feb894b7a5f9b5a67efa46126f6841927f4e6f360968b5d5cdd14bc
Type errataMimetype application/pdf

Search in DiVA

By author/editor
Zimmer, Stephan
By organisation
Department of Physics
Astronomy, Astrophysics and Cosmology

Search outside of DiVA

GoogleGoogle Scholar
Total: 156 downloads
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

isbn
urn-nbn

Altmetric score

isbn
urn-nbn
Total: 551 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf