2013

12 July: Tsvi Piran (The Hebrew University of Jerusalem)

"Macronova and Radio Flares"

26 July: Benoit Cerutti (U. Colorado Boulder)

"Crab Gamma-ray flares: Relativistic Reconnection at work?"

6 September: David Benyamin (The Hebrew University of Jerusalem)

"Diffusion of Cosmic Rays in the Milky Way"

18 October: Ke Fang (U. Chicago)

"Newborn Pulsars as sources of Ultrahigh Energy Cosmic Rays"

The workings of the most energetic astrophysical accelerators in the Universe are encoded in the origin of ultrahigh energy cosmic rays (UHECRs). Current observations by the Auger Observatory, the largest UHECR observatory, show a spectrum that agrees with an extragalactic origin, as well as an interesting transition in chemical composition from light element to heavier element as energy increases.

Candidate sources range from young neutron stars to gamma-ray bursts and events in active galaxies. In this talk, we will discuss newborn pulsars as the sources of ultrahigh energy cosmic rays.

We will show that a newborn pulsar model naturally injects heavier elements and can fit the observed spectrum once propagation in the supernova remnant is taken into account. With the proper injection abundances, integrated cosmic rays from the extragalactic pulsar population can match observation in all aspects - energy spectrum, chemical composition, and anisotropy. We will also examine the fingerprints of their Galactic counterparts on cosmic ray spectrum.

Lastly, we will discuss the multi-messenger smoking gun of this scenario - the detectability of high energy neutrinos from pulsars and magnetars.

7 November: Juri Poutanen (University of Oulu, Finland)

"Advances in X-ray burst modelling"

Thermal emission during X-ray bursts is a powerful tool to determine neutron star masses and radii, if the Eddington flux and the apparent radius in the cooling tail can be measured accurately, and distances to the sources are known. I discuss here a novel method of determining the basic stellar parameters using the data from the cooling phase of long, photospheric radius expansion bursts covering a large range of luminosities. For this purpose, we computed a large set of atmosphere models for burst luminosities varying by two orders of magnitude and for various chemical compositions and surface gravities. We show that the variation of the inverse square root of the apparent blackbody radius with the flux, observed during the photospheric radius expansion bursts from a number of sources at low accretion rate is entirely consistent with the theoretical expectations of the color-correction factor evolution. However, for bursts happening at higher accretion rates the observed evolution is inconsistent with theory, implying that accretion strongly disturbs the neutron star atmosphere. These findings have profound implications for the recent claims on determination of the neutron star radii and masses from such bursts.

Our method allows us to determine both the Eddington flux and the ratio of the stellar apparent radius to the distance much more reliably. For 4U 1724-307, we find a lower limit on the neutron star radius of 13 km, independently of the chemical composition. These results suggest that the matter inside neutron stars is characterized by a stiff equation of state. This is consistent with the constraints coming from the existence of 2-solar-mass neutron stars in binary pulsars.

8 November: Alexandra Veledina (University of Oulu, Finland)

"Accretion in X-ray binaries: imprint of non-thermal particles"

Accreting black holes are among the brightest sources on the X-ray sky. Though being studied over 40 years these objects remain enigmatic, since the very basic questions, such as the accretion geometry, the transformation of gravitational energy into radiation and the source of variability -- are still debated. These objects are usually rather faint in the optical, yet the information contained in these wavelengths is of high importance. The shape of the optical spectrum and its evolution at the state transitions, fast optical variability and its correlation with the X-rays provide us with the irreplaceable tests for the models describing the physical conditions in the black hole vicinity. I will describe the collected multiwavelength data on the spectral and temporal properties of accreting black holes and explain how they all can be understood in terms of hybrid (thermal/non-thermal) hot accretion flow model, developed by us.