Tag Archives: GRBs

Interview with Michael Burgess

Michael Burgess
OKC fellow Michael Burgess
Michael Burgess joined the Oskar Klein Centre in mid-January as OKC fellow, after finishing his PhD in the US. His specialty is GRBs and that is why he joined with Felix Ryde in the KTH group.

Why did you choose the Oskar Klein Centre for doing a postdoc?
When I was working on my PhD I studied a lot from the research that was occurring within the gamma-ray burst team of the OKC. When the possibility opened that I could come work with them I jumped to take it. It was really my first choice after graduate school. In addition, I think Sweden has a generally forward looking view when it comes to science compared with the US. It is appreciated here and I wanted to work in that kind of environment. There are so many projects going on both within and outside my field here that I’m sure I will be able to expand upon my current skill set and perhaps become involved in other fields.

How would you describe the experience of working in the OKC so far?
It has been a great experience so far. The variety of research that is going on both within the OKC and the institute in general has provided me with lots of resources to learn new subjects and improve my skills within my own field. The wealth of knowledge here is fresh and exciting.

What is your field of research and the project you are involved with?
I study the gamma-ray spectra of gamma-ray bursts (GRBs). My main project has been taking the theoretically predicted spectral models that have been around for some time and directly comparing them to the data instead of using the standard empirical models. The project has been quite fruitful and really builds upon work that has been occurring at the OKC for years. From this approach of physical modeling, we are extending the work done in the past with mainly empirical models and digging deeper into the structure of GRB jets and beginning to piece together the relatively unknown processes occurring in these star-shattereing events.

What do you find unique for your field of research about the OKC?
The experience with spectral analysis and physical modeling is uniquely high at OKC. In addition, the researchers such as Felix Ryde and the rest of the group, here have a very keen way of connecting the complicated nature of spectral analysis in GRBs to the bigger picture of the jet. It’s very close to storytelling and makes the science they do very accessible. The competence level of the team is also quite attractive. It’s nice to see that even graduate students here are involved in the decision making for new projects and have a high level of expertise.

Tell us something about you.
I grew up in Atlanta, Georgia back in the states. When I was in school, there was a very good science program in that city and I wanted to study black holes from a very early age.
I went graduate school at the University of A with the Fermi Gamma-Ray Bursts Monitor (GBM) team. While doing research on GRBs, I also had lots of access to the instrument team and gained a lot from their nearly 3 decades of experience working on GRB detection. The GBM team is great with a vibrant history and I made lots of great friends within the team. However, I always wanted to move to Scandinavia for the warm weather and sandy beaches.

And that was indeed a good reason for moving to Sweden 🙂
Thank you Michael!

The first gravitational lens seen in Gamma-rays

The sky seen by the Fermi Large Area Telescope (LAT). The gamma-ray sky is dominated by blazars and diffuse radiation from our galaxy. Credit: NASA/DOE/Fermi LAT Collaboration

The Fermi satellite was launched in 2008 and since then it has continuously monitored the sky at gamma-ray energies above 100 MeV. Most of the sources detected at these energies are blazars, Active Galactic Nuclei in which the accretion onto a supermassive black hole also leads to the launching of two opposite relativistic jets. If a jet is pointing close to our line of sight we will see intense high energy emission due to strong Doppler boosting.

Fermi has so far detected well over 1000 blazars. One of these is B0218+357, which is known from optical and radio observations to be gravitationally lensed by a foreground spiral galaxy. The lens forms two closely spaced images of the blazar. The Fermi Large Area Telescope (LAT) can not spatially separate the two images so it can only measure the sum of both. However,
with timing analysis it is still possible to separate the signal of the individual components. This is because the path length from the blazar to us is different for the two images so we measure all blazar variability twice, with some time separation. Continue reading The first gravitational lens seen in Gamma-rays

GRB130427A – a challenge to our models

This image illustrates the ingredients of the most common type of gamma-ray burst. The core of a massive star (left) has collapsed, forming a black hole that sends a jet moving through the collapsing star and out into space at near the speed of light. Radiation across the spectrum arises from hot ionized gas (plasma) in the vicinity of the newborn black hole, collisions among shells of fast-moving gas within the jet (internal shock waves), and from the leading edge of the jet as it sweeps up and interacts with its surroundings (external shock). Credit: NASA's Goddard Space Flight Center
About once a day, a gamma-ray burst is detected. When this happens, e-mails get sent around and scientists scramble to detect whatever few photons might have been sent our way. But sometimes things are different…

On April 27th this year, an e-mail alert was sent around signifying the detection of yet another GRB. Yet this event was like no other. Rather than fighting to catch photons, there were suddenly too many to detect! The main emission episode was so bright that the GBM instrument on Fermi became saturated. And not only that – the GeV emission lasted for more than a day!
Continue reading GRB130427A – a challenge to our models

Large grant for supernova research at OKC with the iPTF

It is the Knut och Alice Wallenbergs foundation that grants a 5-year long project for finding and studying supernovae. The OKC are already since the beginning of this year members of the intermediate Palomar
Transient Factory (iPTF) – a supernova search aimed at finding supernovae soon after explosion. This is a pathfinder for the next generation of this project – the Zwicky Transient Facility (ZTF). The 30 million grant from KAW will now enable OKC astronomers and physicists to play a leading role in that project.

Jesper Sollerman, from the department of astronomy, is leading the application:
– Previous supernova surveys have often discovered supernovae days or weeks after explosion. We want to find them on the very first night. In this way we hope to learn more about their progenitors, the stars that actually exploded.

The deaths of massive stars is the focus for the supernova astronomers at the department of astronomy, including both observers such as Sollerman and modelers as co-applicant Claes Fransson.

The supernova group at the department of physics are more interested in thermonuclear supernovae and their use for cosmology. Co-applicant Ariel Goobar:
Continue reading Large grant for supernova research at OKC with the iPTF

Radioactive glow as smoking gun: cosmic explosions, heavy elements and gravitational waves

On June 3rd 2013 at 15:49 UT NASA’s Swift satellite detected an intense flash of γ -rays known as a short γ-ray burst. Follow-up observations by the Hubble Space Telescope revealed infrared emission that was present 9 days after the burst, but had faded away after 30 days. This infrared transient is likely the first ever observed example of a “macro-nova”, emission that is produced by the radioactive decay of very heavy nuclei that have been freshly synthesized in the merger of a compact binary system consisting of either two neutron stars or a neutron star with a black hole. If this interpretation is correct, the observation could have profound consequences for high-energy astrophysics, cosmic nucleosynthesis and detections of gravitational waves.

γ-ray bursts (GRBs) come in two flavors of different duration. Long bursts (longer than about 2 seconds) are produced in the death of a rare breed of massive stars, whereas short bursts (shorter than 2 seconds) are thought to result from compact-binary mergers. To date, we know 10 systems containing two neutron stars— extremely densely packed objects with masses around 1.4 times the mass of the Sun, but only about 12 kilometres in radius, and that consist predominantly of neutrons. As the stars orbit around each other they emit gravitational waves and therefore slowly spiral in towards one another until they finally merge. Such orbital decays have actually been observed2, and they agree remarkably well with the predictions from Einstein’s theory of general relativity.
Continue reading Radioactive glow as smoking gun: cosmic explosions, heavy elements and gravitational waves

GRB 130427A: a chance not to miss

Credit NASA, DOE, Fermi LAT Collaboration (click to enlarge and make the GRB explode)
On 27 April, an incredible opportunity was given to GRB science detectives. As the spring was outbursting here in Stockholm the explosion of a distant star almost blinded the Gamma ray Burst Monitor (GBM) detectors on board the Fermi satellite. GRB130427 is the brightest GRB ever detected in the keV – MeV band and the longest lasting in the GeV energy range: Fermi Large Area Telescope (LAT) could detect it for hours after the trigger.

Gamma ray bursts (GRBs) are cosmological flashes of which the prompt emission, lasting for 0.01-100s, is in the gamma ray band. Their late emission can be detected at lower energy ranges like optical and radio. One or two GRBs per day are typically observed, but their origin and the particle acceleration mechanisms involved remain nowadays unknown. The favourite hypothesis on their origin is the collapse of a supermassive star, while there is not a leading hypothesis for the acceleration mechanisms involved in the outflow responsible for the prompt emission.

This burst was also detected by other experiments such as Swift and Integral which allowed a rapid and precise localization which enabled optical, infrared and radio follow-up observations. The redshift was measured within hours from the original trigger and revealed that the outbursting star was quite close (for this kind of objects): z= 0.34.

With nearly 1000 photons per second and square centimeter in the 10-1000 keV band and 14 photons per second per square meter in the 100 MeV – 10 GeV band (see attached figure), this burst is a unique occasion for the scientific community to probe models for particle acceleration and photon emission in the outflow.

source: arXiv:1303.2908
Soon the Fermi and Swift collaborations will publish their papers and hopefully more news and more papers will follow. We expect to be able to take a step further in understanding the physics of the GRB thanks to this record breaker burst. Keep an eye on it!

The figure (source: arXiv:1303.2908) shows the fluence in two energy band of the Fermi LAT detected burst, the star indicate the position this burst would have in this plot.
The event fluence in the first 20 seconds in the 10-1000 keV band is (1.975 +/- 0.003) E-03 erg/cm^2, while in the fluence in the first 140s in the 100 MeV – 10 GeV band is (1.1 +/- 0.1)E-4 erg/cm^2.

For more information: http://www.nasa.gov/topics/universe/features/shocking-burst.html
http://edition.cnn.com/2013/05/06/opinion/urry-gamma-ray-burst/index.html

– Elena Moretti, (OKC fellow) – moretti@particle.kth.se

Interview with a new Oskar Klein Fellow

Elena Moretti is the first of the about 300 applicants who was selected to become an Oskar Klein Fellow this year. She comes from a little country-side town, called Cartura, on the south of Padua in Italy, where she graduated in physics in 2006. She got her PhD in Trieste where she worked with the AGILE and Fermi experiments on GRBs. She developed a method that was used to calculate the flux upper limits on the GRB emission that was used in both experiments. In 2010 she moved to Stockholm working as a postdoc at the KTH. We ask her to tell us more about herself and the work she will be doing at the Oskar Klein Centre.

Congratulations Elena! You have been offered an Oskar Klein Fellowship. How does it feel?
It feels good! It gives me the opportunity to develop my newborn interest in the polarimetry field. Wen I came here 2 years ago I was working only in the high energy astrophysics field with the 2 gamma-ray experiments Fermi and AGILE. After one year a new interest was tickling me: PoGOLite. I started to work on it as a “side job” on my spare time….well I guess that would change soon.
Continue reading Interview with a new Oskar Klein Fellow

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

Dissipative photospheres in gamma-ray bursts

The initial stage of a gamma-ray burst. The core of the star has collapsed deep inside the star. A black hole has formed within the star, and within a few seconds launches a jet of matter away. (Larger image) (Credit: NASA / SkyWorks Digital)
The origin of the emission during the prompt phase in gamma-ray bursts is still a mystery. One suggestion is that the photosphere of the relativistic jet plays an important role. Indeed, recently the Fermi gamma-ray space telescope has made interesting observations of the gamma-ray spectra of several GRBs which show a clear signature of a photospheric emission component.

Another recent development in the field is the realisation that energy dissipation naturally should occur close to the jet photosphere. Such theoretical predictions for kinetic outflow as well as for Poynting flux dominated outflows are confirmed by numerical jet simulations.

In a recent paper, that has just been accepted for publication in the Monthly Notices of the Royal Astronomical Society, we present observational evidence for the onset of such subphotospheric dissipation. This is clearly seen during the prompt phase in the exceptionally bright burst GRB090902B. Initially the main spectral emission component is close to a Planck function, expected for a photosphere. Later this component broadens into a spectral shape that is typical for GRBs. This illustrates that the photosphere emission can have a variety shapes. This is indeed what is expected if the dissipation pattern in the jet changes and gives rise to subphotospheric heating. This we show through numerical simulation of the dissipation processes and argue that the change in spectral behaviour as being due to a decrease in the outflow Lorentz factor. This leads to a substantial part of the kinetic energy being dissipated at optical depth of approximately 10. This causes the change in spectral shape since the photons do not have time to thermalise into a Planck function.

These observations show that the photosphere emission indeed is important in GRBs and can even be a common feature.
The spectral shape of the photospheric emission can have a variety of shapes and not only a Planck function shape.
The identification of the photosphere as cause of the main emission in GRBs provides us a way to study the physics of the relativistic jet. This allows us to learn more about these enigmatic events that are the largest explosions in the Universe.

If you want to read the original paper, check this out: http://arxiv.org/abs/1103.0708