Black hole X-ray binaries- accurate distances and natal kicks

Special Colloquium
Pikky Atri
ICRAR Curtin University, Perth, Australia

Even though the first black hole (BH) was detected decades ago, the mechanism by which BHs form is not well understood. Theoretical models suggest that BHs are born when a massive star dies, either with or without a supernova explosion. BHs formed following a supernova explosion are hypothesized to get strong natal kicks, whereas direct collapse BHs do not incur such a kick. In order to obtain an observationally constrained natal kick distribution for Black hole X-ray binaries (BHXBs) we use Monte Carlo simulations to determine the probability distribution of the Plane crossing velocity for 16 systems. We measure the peculiar velocity of each system by combining their radial velocities, distances, archival and Gaia proper motions and track their Galactocentric orbits back to the time of Plane crossing. We use the high resolution of Very Long Baseline Interferometry (VLBI) networks in the USA, Europe and Australia to measure the proper motions of 3 new BHXBs. We also use VLBI to obtain the model-independent distance to a BHXB, MAXI J1820+070, by measuring the parallax of the source. In this talk, I will present the implications of this newly constrained kick velocity distribution of BHXBs on the birth mechanism of BHs, on BH-BH mergers, on retention of BHs in globular clusters and on spin-orbit misalignment of BHXBs. I will also report on the parallax measurement of MAXI J1820+070.

Neutrinos from a blazar jet and the birth of non-stellar neutrino astronomy

Main Colloquium
Dr. Paolo Padovani
European Southern Observatory, Garching

IceCube has recently reported the discovery of high-energy neutrinos of astrophysical origin, opening up the PeV (10^15 eV) sky. These observations are challenging to interpret on the astronomical side and have triggered a fruitful collaboration across particle and astro-physics. Last year some very high-energy IceCube neutrinos have been associated with the blazar TXS 0506+056. I will discuss these results, show that TXS 0506+056 is not what it looks like, and conclude with an outlook. The talk is self-contained and requires no previous knowledge of neutrinos or blazars.

Probing the Interstellar Medium and Dark Matter with Pulsars

Nataliya Porayko
Max-Planck-Institut für Radioastronomie

Pulsars are rapidly rotating, highly magnetised neutron stars which emit electromagnetic radiation from their magnetic poles in the form of highly collimated beams. Pulsars are known as a powerful tool to probe the interstellar medium (ISM) and its constituents in the Miky Way. In this thesis we focus on probing the non-baryonic entities in the Milky Way, namely interstellar magnetic fields and dark matter. The first part of the thesis is dedicated to the investigation of small-scale turbulent magnetic fields in the Milky Way, which can be probed by monitoring variations in the Faraday rotation measures (RMs) of linearly polarised radiation of pulsars. For this purpose, we use high-cadence, low-frequency observations from a set of selected pulsars carried out with German LOw-Frequency ARray (LOFAR) stations. We find that measured RMs are strongly affected by the highly time-variable terrestrial ionosphere. We have mitigated the ionospheric contribution assuming a thin-layer model of the ionosphere. We conclude that within this approximation the ionospheric RM corrections are accurate to ~ 0.06 - 0.07 rad per m^2 , which defines our sensitivity towards long-term astrophysical RM variations. The second part of this thesis deals with dark matter - a matter which accounts for about a quarter of the energy density of the Universe, and the nature of which is still under debate. The ultralight scalar field dark matter is one of the compelling dark matter candidates, which leaves characteristic imprints in the times of arrival of radio pulses from pulsars. We search for traces of ultralight scalar-field dark matter in the Galaxy using the latest Parkes Pulsar Timing Array dataset that contains the times of arrival of 26 pulsars. No statistically significant signal has been detected. Therefore, we set an upper limit on the local dark matter density. We conclude by discussing the prospects of detecting the fuzzy dark matter with future radio astronomical facilities. [Referees: Prof. Dr. Michael Kramer, Prof. Dr. Frank Bertoldi, Prof. Dr. Simon Stellmer, Prof. Dr. Robert Glaum]

A cosmic collider: Was the IceCube neutrino generated in a precessing jet-jet interaction in TXS 0506+056?

Lunch Colloquium
PD Dr. Silke Britzen

The neutrino event IceCube 170922A, detected at the IceCube Neutrino Observatory at the South Pole, appears to originate from the distant active galaxy TXS 0506+056. Multiwavelength flaring, observed from the radio- to the TeV-regime, is a regular phenomenon of BL Lac objects or blazars. While many other blazers show properties similar to those of TXS 0506+056, only TXS 0506+056 has been identified as neutrino emitter so far. In the talk I will explain that the enhanced neutrino activity during the neutrino flare in 2014–2015 and the single EHE neutrino IceCube-170922A could have been generated by a cosmic collision within TXS 0506+056. Our findings indicate that this AGN might be an atypical blazar.