Tag Archives: conference

Extreme-Gravity Stars

The lives of massive stars are characterized by companionship: these stars are almost always found in gravitationally bound pairs. As such massive binaries evolve further, their cores run out of nuclear fuel and the stars can explode as supernovae, leaving behind in their centers either a neutron star or a black hole. In most cases such an explosion would be fatal for the binary, and disrupt it. In some cases, however, the final phases of binary stellar evolution can produce two compact objects -either white dwarfs, neutron stars or black holes– in tight binary systems.

Compact objects in binary pairs are driven closer and closer together as their system loses energy through interactions and gravitational radiation. The resulting merger of these types of objects is thought to be responsible for some of the most energetic events in the Universe including Type Ia supernovae and short duration gamma ray bursts.

The Laser Interferometer Gravitational-wave Observatory (LIGO) has made it possible to observe the gravitational waves emitted during mergers of compact objects. Since the beginning of data collection with the advanced instrument the LIGO observatory has detected gravitational waves from three different mergers of black hole pairs. Each of these detections was surprising because they involved a population of black holes that had not been observed before : black holes with masses of a few tens of solar masses. Scientists are also anticipating that LIGO will detect merging neutron stars.

A photo of Stephan Rosswog
Stephan Rosswog, Oskar Klein Centre, Stockholm University

Compact binary systems are really a cornerstone of modern astrophysics. Once the merger of a neutron star binary is detected in gravitational waves and electromagnetically, this will tell us about General Relativity, give us hints on how and where such binary systems form and —maybe most surprisingly— it may answer questions about nuclear physics that cannot be answered otherwise. This is maybe the most fascinating part of this rich story. — Stephan Rosswog

Connecting the sources of gravitational waves with phenomena that scientists are already familiar with, like supernovae and gamma ray bursts, requires that we observe the electromagnetic counterpart to the gravitational wave event. This is a challenging task with LIGO in its current state because the detector isn’t able to localize an event very precisely so follow-up searches with optical (and other wavelength) telescopes must search large areas of the sky for a new transient source.

A conference on The Physics of Extreme-Gravity Stars took place in Stockholm in June 2017.

Header image is from a simulation of two neutron stars merging, credit to Stephan Rosswog.

The 13th Marcel Grossmann Meeting at AlbaNova

Einstein with his friend and mathematical collaborator Marcel Grossmann.
This week Stockholm University and AlbaNova are hosting the 13th Marcel Grossmann Meeting. The triennial Grossmann meetings are organized by the ICRANet research institute, this year with Stockholm University and KTH as local partners.

The general theme of the Grossmann meetings is applications and theory relating to Einstein’s general relativity.

The meeting has a broad spectrum with 20 parallel sessions devoted to theory and experiments/observations including quantum aspects of gravity, precision tests of general relativity, relativistic astrophysics, cosmology and astroparticle physics.

There will also be two special lectures on Saturday about the latest discovery of the Higgs(-like) particle by Chiara Mariotti from CMS and Domizia Orestano from ATLAS.
Continue reading The 13th Marcel Grossmann Meeting at AlbaNova

Fermi/Swift GRB Symposium 2012: Polarisation and thermal emission in GRBs

The Fermi/Swift gamma-ray burst Symposium 2012 was held in Munich 7-11 May 2012.

Recent results on the prompt and afterglow emissions in gamma-ray burst were discussed at the Fermi/Swift gamma-ray burst Symposium 2012 which was held in Munich 7-11 May 2012.

Among the most important issues presented was the recent gamma-polarisation measurement with IKAROS-GAP. Significant degrees of polarisation in several bursts have now been detected. In particular, the change in polarisation angle was significantly detected. It was speculated that this is due to variation in emission patches in very narrowly collimated jets.

Another point which gained a lot of attention was on the existence of thermal components in GRB spectra. Previously the leading model for the prompt emission has been optically thin synchrotron emission. However, amounting observational evidence is showing that the photosphere in the relativistic flow is responsible for, at least a part of, the observed emission. Also recent progress in the theory and numerical simulations of relativistic jet was presented, and again thermal emission seems to be unavoidable.

Links:
Featured on www.nature.com/
Conference website

Gamma ray spectrum of GRB110721A
Gamma-ray spectrum of GRB110721A which will soon be presented in a Fermi publication led by Magnus Axelsson. In addition to the dominant broad component, there is a bump at lower energies which is likely emission from the photosphere. The spectrum cannot be explained by synchrotron emission and thus disproves the long-held view that synchrotron emission alone can explain GRB spectra.

The 7th TeV Particle Astrophysics conference in Stockholm

The Oskar Klein Centre will host the 7th TeV Particle Astrophysics conference at the Albanova Centre in Stockholm August 1-5, 2011.

The fields of high energy astrophysics, particle astrophysics and particle physics at colliders are dealing with some of the most pressing subjects in modern physics, in trying to understand the nature of dark matter and the origin of cosmic rays.

The scientific program includes gamma rays, neutrinos, charged cosmic rays, cosmic rays above the knee, direct dark-matter searches, distribution of dark matter, and particle physics.

Invited speakers will present and discuss the status of different experiments Continue reading The 7th TeV Particle Astrophysics conference in Stockholm

The Fermi symposium 2011: AGNs, pulsars and gamma ray bursts

The results presented at the III Fermi symposium in Rome reflected, in particular, what a magnificent instrument the Fermi LAT is for observing active galactic nuclei and pulsars. The 2 source catalogue 2FGL was presented and will soon be released with 1888 sources. Much attention was given to the blazar 3C454.3 which has been monitored since the launch and has undergone a series of very bright outbursts. The multiwavelength analysis by Stefan Larsson revealed a far more complex behaviour than expected in the simple picture we had of AGN jets before the launch of Fermi. The discovery of spectral breaks at GeV energies was nicely interpreted by the former Stockholm astronomer Juri Poutanen and collaborators as a result of gamma-ray absorption via photon-photon pair production on He II Lyman recombination continuum and lines within the broad-line region.

The Aula Magna at La Sapienza, Rome

It was also made clear that all models we have for description of the high energy emission around pulsars are, more or less, wrong. Fermi has told us for certain that the emission is from high altitudes in the outer magnetosphere; Fermi has killed the polar cap model and the classical TPC, while the other models are in need of modifications. Continue reading The Fermi symposium 2011: AGNs, pulsars and gamma ray bursts

The Fermi Symposium 2011: a dark matter perspective

The Fermi Symposium of 2011 in Rome has now reached its last day and we have heard many interesting talks, ranging all the way from dark matter to various astrophysical sources and observations. The OKC has been very well represented with participants both from the Department of Physics and the Department of Astronomy at Stockholm University and by the KTH group.

Maja Llena Garde presents the Fermi-LAT results on dark matter searches from a stacked dwarf analysis.
Maja Llena Garde presents the Fermi-LAT results on dark matter searches from a stacked dwarf analysis at the 2011 Fermi Symposium.

Science-wise, and from my personal dark matter oriented perspective, I think one of the most interesting talks was Maja Llena Garde’s talk on placing limits on dark matter models from dwarf galaxies using Fermi data in a stacked likelihood analysis. The limits are really approaching the vanilla WIMP expectations.   Continue reading The Fermi Symposium 2011: a dark matter perspective

The Return of de Sitter

The return of De Sitter During the last three weeks, we were visited by an impressive list of cosmologists trying to make sense of what is driving the present accelerated expansion of the Universe. The return of De Sitter, this is the name of the NORDITA workshop organized by OKC members Fawad Hassan and Ariel Goobar, together with Stefan Hofmann from LMU in Munich.
I asked Ariel Goobar, professor at OKC, and Stefan Sjörs, a PhD student in the Cosmology, Astroparticle Physics and String Theory group, to tell us about the conference.

Continue reading The Return of de Sitter

News from the Venice Neutrino Workshop

Here is just a very brief summary of results that have been presented so far. (For an extensive blog coverage about the event, see this link.)
Unfortunately, Elena Aprile did not present the new results from Xenon100, but she said that they will be presented at a press conference in Gran Sasso in April. It seems that they have new accurate measurements of the efficiency L_eff over a substantial energy range, that of course will be crucial when interpreting the data.
In neutrino physics, the present buzz concerns the possibility of sterile neutrinos, as seems to be mildly preferred by cosmological data. Continue reading News from the Venice Neutrino Workshop