The Oskar Klein Centre blog

Cosmology and a bit more

Do we see dark matter emission from dwarf spheroidal galaxies?

Posted on October 29, 2014 | by Serena | No Comments

From a dark matter (DM) hunter’s perspective, this year’s Fermi Symposium was highly anticipated. In the six years since the launch of the Large Area Telescope (LAT), we’ve seen our share of ups and downs. An active community, both in and outside the Fermi Collaboration (FC), works hard to fit dark matter to or explain away every deviation in excess of what we expect from the gamma-ray sky. This year’s gathering got the answer to the latest burning question: do we see dark matter emission from dwarf spheroidal galaxies (dSphs)?

Image Credit: NASA, Hubble Heritage Team, (STScI/AURA), ESA, S. Beckwith (STScI). Additional Processing: Robert Gendler

Before we hear the answer, let’s review the mileposts of the LAT DM saga. The first big stir came from the electron/positron spectrum [1], which featured a weaker version of the bump-like feature already measured by the PAMELA experiment. Theorists rushed to explain this with special flavors of ‘leptophillic’ dark matter, tuned to enhance the local flux while producing nothing other detectors would have seen. After an avalanche of 700+ citations and one even more precise measurement (AMS-02), the excitement died, along with most of those models.

Next we had the (in)famous line feature at 135 GeV [2]. With a far more clear-cut interpretation, I held my breath while this result was seen first in the galactic center (GC), then in nearly every control region, and finally at lower and lower significance [3] until its appearance at the Symposium reported a mere 0.72 sigma. A dedicated observation from H.E.S.S. II may rescue it, but for now the line looks tenuous at best.

The latest big hint also comes from the galactic center, but in the continuum emission, rather than as a sharp feature. This makes some sense; the galactic center is nearby and massive, so any dark matter signal ought to be strongest there. On the other hand, it is astrophysically complex. So much so that interpretation of the data there seems to have been saved for last (the Fermi Collaboration has yet to weigh in with a publication). Undaunted, several groups (e.g. [4]) now report that models of known sources do not account for all the emission, leaving what is blandly referred to as the galactic center excess (GCE). Despite a wealth of systematic uncertainty, all parties agree that the galactic center excess is peaked at around 1 GeV and extends fairly symmetrically to about ten degrees from the center of the Milky Way.

Plausible conventional production mechanisms for the galactic center excess include pulsars and cosmic rays, but it can also be neatly accounted for by the existence of a weakly interacting massive particle (WIMP) of about 30 GeV which self-annihilates into b quarks. Without completely ruling out alternatives, this possible dark matter explanation remains just that — possible. What made this dark matter model so exciting was that it also happened to fit a slight excess (1.4 sigma) seen in the dwarf spheroidal galaxies (dSphs) orbiting the Milky Way [5]. As dSphs represent an independent data set, and a nearly background-free one at that, the coincidence was indeed tantalizing. One well-known physicist called the galactic center excess “the most compelling signal we’ve had for dark matter particles – ever.” [6]

The dSph excess was so low that, if it was not just a fluctuation of the background, we would have to wait many years to confirm the dark matter explanation of the galactic center excess. Fortunately, the LAT’s excellent ground team just gave us a big push. A pair-conversion telescope, Fermi relies on extensively calibrated classification algorithms to reconstruct incoming gamma rays from their electronic signatures. These routines have been periodically overhauled throughout the mission as knowledge of the instrument continues to improve. The latest overhaul, known as “Pass 8,” marks the biggest advance yet, boosting the instrument’s effective area while lowering its point spread function like a pair of glasses. Upgrade in hand, the time was right to look again at the dSphs.

Dark matter searches in dwarf spheroidal galaxies have been a specialty of the Oskar Klein Centre. C. Farnier and M. Llena-Garde have both played lead roles in Collaboration papers on the subject ([7], [8]), and J. Conrad introduced the statistical technique now used to combine information from multiple targets [9]. For the latest publication, G. Martinez derived the dSph mass distributions by a nested Bayesian analysis of their hosted stellar populations [10]. Building on this success, yours truly, as part of the FC, took a peek at the Pass 8 data.

Preliminary Fermi-LAT Pass 8 constraints on the WIMP velocity-averaged cross section for one annihilation channel. Using five years of data for the combination of 15 dwarf spheroidal galaxies. Dashed line and bands represent limits expected from blank fields on the sky." Credits Fermi-LAT Collaboration.

What we found was a whole lot of nothing. The significance of the GCE model dropped drastically, along with all other WIMP annihilation masses and channels. Dropped so far, in fact, that we can now set limits which exclude the annihilation cross section WIMPs need to make up all dark matter out to masses of 100 GeV (see Figure 1). These are now the best limits in the world below 1 TeV, and represent a big bite out of the parameter space left to the indirect dark matter detection field’s favorite class of models. While these constraints do not conclusively rule out the dark matter interpretation of the galactic center excess, they lend no support. “Tension” is the colloquial term.

So at this year’s Fermi Symposium, though debate still raged over the galactic center, I had to report that dSphs had pulled their support from the dark matter interpretation. Like all of us, I was disappointed to find we still have no answer to one of the greatest physics questions of the day, but as I said, dark matter hunters are used to highs and lows. Could gamma rays from annihilating dark matter still be buried in the LAT data? Of course, but there are not many coming from dSphs.

– Brandon Anderson, OKC fellow (

[1] Fermi-LAT Collaboration. Physical Review Letters, vol. 102, Issue 18, id. 181101
[2] C. Weniger. Journal of Cosmology and Astroparticle Physics, Issue 08, article id. 007 (2012)
[3] Fermi-LAT Collaboration. Physical Review D 88, 082002 (2013)
[4] D. Hooper, L. Goodenough. Physics Letters B, Volume 697, Issue 5, p. 412-428
[5] Fermi-LAT Collaboration. Phys. Rev. D 89, 042001 (2014)
[6] New Scientist, April 2014
[7] Fermi-LAT Collaboration. Astrophysical Journal, 712, (2010), 147-158
[8] Fermi-LAT Collaboration. Phys. Rev. Lett. 107, 241302 (2011)
[9] J. Conrad. Astroparticle Physics 62 (2015) 165-177
[10] G. Martinez. eprint arXiv:1309.2641

New Wallenberg grant to OKC’s Jan Conrad

Posted on October 3, 2014 | by Lars | No Comments

Jan Conrad

The Knut & Alice Wallenberg Foundation released yesterday the list of this year’s recipients of funding for research projects with very high potential. We were very happy to see that on this list appears Jan Conrad with the project “Discovering Dark Matter Particles in the Laboratory”, with a grant of SEK 28 883 000 (around 3 MEUR) for five years.


Congratulations to Jan for this generous grant! Jan is a Member of the OKC Steering Group and has for long been one of the key researchers in the satellite experiment Fermi-LAT, and the ground-based HESS detector, searching for indirect detection signals of dark matter in gamma-rays. This is complementary to accelerator searches at CERN’s LHC in the ATLAS experiment, where we also have an important involvement from the OKC. Up until now, we have not had any activity concerning direct searches of scattering of dark matter in underground detectors, but thanks to the new grant, the OKC will through Jan Conrad have an important role to play also in this area, through the the world-leading XENON-1t experiment in the Gran Sasso tunnel.

Actually, the direct detection method was to a large extent developed in the 1980’s and onwards by Katie Freese and her colleagues. As Katie is now with us in Stockholm (she is a co-signer of the successful Wallenberg proposal, as is OKC’s Thomas Schwetz and Christian  Forssén of Chalmers, Gothenburg), we can expect this exciting new activity to flourish in Stockholm and at OKC.

Katie Freese receives big VR grant!

Posted on June 24, 2014 | by Lars | 2 Comments

The members of the Oskar Klein Centre had just noticed with satisfaction that we could keep the VR Linnaeus grant at the same level (increased by 10% in 2010) the second half of the grant period (until 2018), when even more exciting news reached us. First, our valued member of the International Advisory Board, Katie Freese from Michigan University, was announced as the new Director of Nordita, which is located in the neighbouring building to OKC.

Katie Freese, New Director of Nordita and recipient of large VR grant.

Welcome to Stockholm, Katie!

The next positive surprise came yesterday, when the Swedish Research Council, VR, announced that Katie will receive a big excellence grant for astroparticle physics, namely 101 million Swedish Crowns (around 15 million US dollars) over 10 years. This grant was suggested to the VR by the Vice-Chancellor of Stockholm University, Astrid Söderbergh-Widding (see her blog about it).

Hooray, Katie!

The future of astroparticle physics in Sweden looks brighter than ever, I am sure that Katie will find the environment here excellent (as she has seen in her work in the OKC IAB), and now she can contribute to it in a substantial way. These are exciting times for all of us!

-Lars Bergström, OKC Project Leader

Sara Strandberg new member of the Swedish Young Academy

Posted on May 31, 2014 | by Lars | No Comments

Congratulations to Sara Strandberg, member of the OKC Steering Group, who has just been elected into the Swedish Young Academy (SYA). SYA is a cross-disciplinary forum for young researchers. Although independent from the Royal Swedish Academy of Sciences, the creation of SYA has been encouraged by it. SYA has already made its voice heard in several important issues for young scientists, not the least concerning a sustainable academic career system.


Sara Strandberg

At the start of SYA in 2011, OKC:s Jan Conrad was one of the firstly elected members. I find it a great honour of OKC that we have contributed two members of this illustrious academic society, which can at most have 40 active members.

Interview with Michael Burgess

Posted on April 9, 2014 | by Serena | No Comments

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!

A close look a the nearest standard candle supernova in several decades

Posted on March 21, 2014 | by Serena | 1 Comment

Supernova 2014J in the nearby galaxy M82 -less than 12 million light-years away- exploded on January 14, 2014 and was the closest ”standard candle” supernova since (at least) 42 years. An impressive coordinated observational effort orchestrated by the intermediate Palomar Transient Factory (iPTF) team and led by Ariel Goobar from the Oskar Klein Centre at Stockholm University (Goobar et al. 2014, The Astrophysical Journal Letters, 784, L12) provides important new clues into the nature of these explosions, as well as the environments where they take place. The proximity of SN2014J allowed the iPTF team to study this important class of stellar explosions, known as Type Ia supernovae, over a very wide wavelength range, starting just hours after the deduced explosion time.

Furthermore, Goobar and collaborators used pre-explosion images of the region of M82 where the supernova went off, both from the Hubble Space Telescope and from the Palomar Oschin Telescope, to search for a star in the location of the explosion, or possible earlier nova eruptions. The lack of pre-explosion detections suggests that the supernova may have originated in the merging of compact faint objects, e.g., two white dwarf stars, i.e., the kind of Earth size stars that our sun will evolve to once it runs out of nuclear fuel.
“Until very recently, the leading model for standard candle supernovae was thought to include a companion star from which material was stripped by the white dwarf until the accumulated mass could no longer be sustained by the outwards pressure, leading to a runaway thermonuclear explosion. The observations of SN2014J are challenging for this theoretical picture”, says Goobar.

Type Ia supernovae are among the best tools to measure cosmological distances. Thanks to their consistent peak brightness, these ”standard candles” are used to map the expansion history of the Universe. In 1998 distance measurements using supernovae lead to the a paradigm shift in cosmology and fundamental physics: the expansion of the Universe is speeding up, contrary to the expectations from the attractive nature of gravitational forces: a mysterious new cosmic component, ”dark energy”, has been invoked to explain this unexpected phenomenon. This discovery was awarded the 2011 Nobel Prize in physics.

“Since Type Ia supernovae are very rare, occurring only once every several hundred years in a galaxy like ours, there have been very few opportunities to study these explosions in great detail. SN2014J in the nearby galaxy M82 is a very welcome exception”, says Rahman Amanullah a researcher at OKC.

A better understanding of the physics behind Type Ia supernovae and the material surrounding the explosion and dimming some of the light is crucial to further refine the measurements of the expansion history of the Universe. Joel Johansson, a PhD student at OKC that played an essential role in the analysis fills in “many supernovae explode in clean environments, free of dust in the line of sight. This is not the case for SN2014J, which gives us a unique opportunity to study both the properties of the supernova explosion but also of the intervening dust”.

The lessons learned by the studies of SN2014J may be very useful for the analysis of the large Type Ia SN sample that scientists have collected over decades, especially the astrophysical corrections needed to make accurate distance estimates. Only then may we be able to tell what is causing the accelerated expansion of the cosmos.

Composite image from the 2.5-meter Nordic Optical Telescope in La Palma showing SN2014J in the dusty cigar galaxy M82 (credits: J. Johansson). The right upper panel shows a detailed near-infrared image from the 10-meter Keck telecope in Hawaii used to accurately locate the site of the explosion. The bottom right panel indicates the position of the supernova on pre-explosion images from the Hubble Space Telescope (credits: A. O’Conell and M. Mountain) - Click to enlarge

The iPTF project is a scientific collaboration between Caltech; Los Alamos National Laboratory; the University of Wisconsin, Milwaukee; the Oskar Klein Centre in Sweden; the Weizmann Institute of Science in Israel; the TANGO Program of the University System of Taiwan; and the Kavli Institute for the Physics and Mathematics of the Universe in Japan.

The observations were carried out using multiple astronomical facilities. Besides the Palomar telescopes, data of SN2014J and M82 were obtained at the Nordic Optical Telescope, the Keck Telescope, the Faulkes Telescope North, the Mount Abu 1.2m Infrared telescope in India, the1.93m telescope of Haute-Provence Observatory, CNRS, France, the Spitzer Space Telescope and the Hubble Space Telescope.

Contact: Prof. Ariel Goobar (

Prof. Ariel Goobar, Dept of Physics, Stockholms universitet, tfn +46 8-55 37 86 59, e-mail
Rahman Amanullah, researcher, Dept of Physics,, Stockholms universitet, tfn +46 8-55 37 88 48 e-mail:
Joel Johansson, PhD student, Dept of Physics, Stockholms universitet, tfn +46 8-55 37 86 61, e-mail

Link to APJ article:
Read also: Hubble Space telescope images of a supernova in nearby galaxy M82

Hubble Space Telescope images of a supernova in nearby galaxy M82

Posted on February 27, 2014 | by Serena | 2 Comments

A new bright supernova exploded in the nearby galaxy M82 on January 14 this year, at a distance of approximately 11.5 million light–years from Earth, that makes it to the nearest “normal” Type Ia supernova discovered in the past 42 years. Its small distance together with the fact that the first observations were carried out only a few hours after the explosion, makes it in itself a very important astronomical object, since it allows to study the details of many aspects of these kind of objects that are so important for cosmology.
Type Ia supernovae, used as distance indicators, lead to the the discovery of the accelerated expansion of the universe in 1998, an unexpected result awarded the Nobel Prize in physics in 2011.
The nature of the accelerated expansion is attributed to a repulsive force, called dark energy.

- You might want to see our video about Dark Energy Problem -


However, though they are readily used in cosmology, the explosion mechanism behind Type Ia supernovae is still unclear, mainly due to the difficulty of catching the explosion at early stages and the ability to study these explosions over a wide range of wavelengths.

Ariel Goobar and Rahman Amanullah from the Oskar Klein Centre realized the importance of this object and applied for the Hubble Space Telescope (HST) director’s discretionary time to observe the supernova in ultraviolet (UV) wavelengths, which are otherwise absorbed by the earth’s atmosphere and not observable from ground based telescopes. Thanks to these measurements one can study the immediate surroundings of the supernova, an important part of the puzzle in understanding the progenitor system. Furthermore, the UV observations
are critical to study what it is that absorbs some of the light in the line of sight in the interstellar medium of the host galaxy. This study will have implications for the precision that can be obtained on the measurements of
the properties of dark energy.

The Hubble Space Telescope news center published today the composite image of this supernova explosion, SN2014J, in the galaxy M82.

The January 31 image shown here was taken in visible light with Hubble’s Wide Field Camera 3. This image is inset into a photo mosaic of the entire galaxy taken in 2006 with Hubble’s Wide Field Camera 2. Credit: NASA, ESA, A. Goobar (Stockholm University), and Hubble Heritage

A detailed paper about SN2014J has been written by Ariel Goobar and collaborators and accepted for publication in the Astrophysical Journal Letters, and we will soon blog again about this exceptional supernova.

Contact: Ariel Goobar
Hubble Heritage Realease

The first gravitational lens seen in Gamma-rays

Posted on January 14, 2014 | by Serena | No Comments

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. Read more

Another interesting and very successful year of the Oskar Klein Centre

Posted on December 23, 2013 | by Lars | No Comments

Francois Englert just before Nobel lecture in Stockholm University’s Aula Magna. Photo: L. Bergström.

Hello and Happy Holidays to all friends of the OKC

As the year 2013 is now nearing its end, it is time to recapitulate the main events of the year from the OKC perspective. If I temporarily put on my Nobel hat (being the scientific secretary of the Nobel Committee for Physics) the main event from the Stockholm horizon is without doubt the Nobel Prize to Francois Englert and Peter Higgs for their almost 50-year old prediction from the early 1960’s that was so spectacularly confirmed by the ATLAS and CMS experiments at CERN’s LHC accelerator last year. Hats off for Englert and Higgs, and also for the many clever and hard-working experimentalists, in particular the ATLAS people of OKC such as Jonas Strandberg, who has been directly involved in the discovery of the Higgs particle in ATLAS. Of course we now look forward to the energy upgrade of LHC, which will increase the chances substantially to find the much awaited effects beyond the Standard Model that, hopefully, could give an indication of what the dark matter may consist of. At the Oskar Klein Centre we also have been searching in gamma-ray, positron and neutrino signals, without positive results (yet), but producing some of the best limits.

Peter Higgs signing the Nobel Poster (prepared with the help of OKC's Sara Strandberg and Oscar Stål). Photo: L. Bergström.

The OKC has now been in existence for 5 years, and we will soon encounter the international mid-review panel of the Science Council (VR) of Sweden. By September 1st we had to submit our self-assessment report, containing a detailed description This meant a lot of work for me and the OKC Steering Group: Christophe Clement, Jan Conrad, Claes Fransson, Ariel Goobar, Klas Hultquist, Garrelt Mellema, Mark Pearce, Sara Strandberg and Göran Östlin (and of course our great communications manager, Serena Nobili). We hope that we managed to convey our great enthusiasm for the scientific outcome of the OKC during its first 5 years, and that the evaluation committee will agree that it has been a great success. In fact, when we meet them January 30th, we will have quite a number of recent sucesses to report:

  • A generous grant of SEK 32 million from the K&A Wallenberg Foundation was given to groups in OKC (with J. Sollerman as PI) for contributing to the Zwicky Transient Facility (with S. Kulkarni of Caltech leading the team).
  • OKC Steering Group member Sara Strandberg has obtained both a young researcher’s grant from the VR and a Wallenberg Academy Fellow (WAF) grant – both in very strong competition.
  • Jan Conrad (also OKC Steering Group member) has in addition to his previous WAF grant also been given one of the new excellent junior investigator grants from VR. Congratulations to Sara and Jan!
  • The IceCube experiment has finally detected high-energy (PeV) cosmological neutrino events, with a surprising energy distribution.  This was declared the discovery of the year of the Physics World magazine. Congratulations to the OKC IceCube group (Chad Finley, P.O. Hulth, Klas Hultquist & al.) and the Uppsala group (with present IceCube spokesperson Olga Botner and her colleagues)!
  • The Fermi satellite project with large OKC contribution continues to make important discoveries. One concerns the discovery of a gamma-ray spectrum of two supernova remnants which clearly shows a hadronic origin (from decays of neutral pions), and thus is a proof that these sources accelerate protons, and are thus the long-sought-for sources of the Galactic cosmic rays. This was one of the runner-ups for the discovery of the year of Science magazine.

To conclude, with all the interesting science produced by OKC during its first half-life, one may only anticipate with great expectations what will come out of the second half!

Merry Holidays and a Happy New Year to all in the OKC research environment and all our followers!


Interview with Timur Delahaye

Posted on December 2, 2013 | by Serena | No Comments

Timur Delahaye is one of the OKC fellows working at the Cosmology, Particle astrophysics and String theory group (CoPS) since this summer. let’s get to know him better.

Where have you studied or did research before coming to the OKC?
I did my undergraduate studies at École Polytechnique near Paris. I then completed my Mas ter degree at the theoretical physics department of École Normale Supérieure in Lyon and did my Ph.D. with the IDAPP (International Doctorate on AstroParticle Physics) program both in Annecy and Turin under the supervision of Pierre Salati and Nicolao Fornengo. Autumn 2010, I moved to Madrid to do my first post-doc at the Instituto de Física Teórica (IFT) of the Universidad Autónoma where I stayed for two years. Last year I worked at the Laboratoire d’Annecy-le-Vieux de Physique Théorique (LAPTh) and the Institut d’Astrophysique de Paris (IAP).

What is your field of research?
I work in modelling the propagation of Galactic Cosmic Rays and Dark Matter indirect detection. Cosmic rays are high energy particle that are accelerated by exploding stars, by high magnetised stars called pulsars, and maybe by the annihilation or decay of Dark Matter particles. Even though cosmic rays have been discovered more than 100 years ago we still do not understand precisely where they come from nor how they propagate in the interstellar medium. In spite of being a rather young science, cosmic ray physics are is a wonderful way to look at things in the sky that do not emit light and cannot be probed by usual astronomy.

What I really enjoy in this field is that it is at the intersection of cosmology, astrophysics and particle physics and allows to interact with many people from very different fields.
Read more

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