High Energy Astrophysical Phenomena (astro-ph.HE)

Abstract: Aims. Assuming fast radio bursts (FRBs) are produced by matter travelling ultra-relativistically in a localised region of a smooth bundle of streamlines, we study the constraints applied by geometry to the morphology and polarisation of the burst in time and frequency independently of the intrinsic radiative process. Methods. We express the problem only in terms of the local properties of direction and curvature of a streamline. This allows us to cast the general results to any desired geometry. We illustrate by applying this framework to two geometries inspired by pulsar and magnetar magnetospheres, namely the dipolar polar-cap region and a magnetic dipole with an additional toroidal component. Results. Geometry constrains bursts to occur within an envelope in the frequency vs. time plane (dynamic spectrum). This envelope notably characterises spectral occupancy and frequency drifts (both burst-to-burst and within an individual burst). We illustrate how one can simulate bursts by specifying some basic properties of an intrinsic emission process. In particular we show that the typical properties of one-off bursts can be produced in polar-cap geometry by a star with spin period > 1s, while bursts from repeating sources are better accounted for with an additional strong toroidal component and a sub-second spin period. Conclusions. We propose that a relationship between burst morphologies and the properties of the source, such as its spin period and magnetospheric properties, can be established at least qualitatively based on geometrical considerations. Our results favour models where repeaters are younger and faster magnetars with highly twisted magnetospheres.

Abstract: We present the results obtained from a comprehensive timing and spectral study of two high-mass X-ray binary sources using NuSTAR observations. These two sources, IGR J16320-4751 and IGR J16479-4514, were discovered by INTEGRAL and have been characterized for the first time in the hard X-ray band (beyond 10~keV) in this work. In these sources, we observe the occurrence of intense X-ray flares, with average luminosities exceeding 10$^{36}$~erg~s$^{-1}$. Our analysis reveals that these flares can be described consistently in the quasi-spherical accretion regime. The orbital phase of the first flare in NuSTAR observation of IGR J16479-4514 matches with the orbital phases of previous flares ($\phi=0.35$) in this source detected by other telescopes. We conclude that this flare occurs as a result of the periastron passage of the neutron star, rather than due to the presence of a corotating interaction region (CIR). Furthermore, from the energy-resolved pulse profile analysis of IGR J16320-4751, we find that the pulse fraction is lower in hard X-rays compared to soft X-rays. We present the hard X-ray spectral parameters of these two sources using several standard spectral model components. We do not detect a cyclotron absorption feature in either target. We provide estimates of the surface magnetic field strength of NS in IGR J16320-4751 using two indirect methods. Lastly, we observe spectral hardening during flaring segments compared to the off-flaring segments which indicates that comptonization is more effective during the flaring segments.

Abstract: The Pevatron Test Statistic (PTS) is applied to data from $\gamma$-ray observatories to test for the origin of Cosmic Rays (CRs) at energies around the knee of the CR spectrum. Several sources are analyzed within hadronic emission models. Previously derived results for RX J1713.7$-$3946, Vela Jr., and HESS J1745$-$290 are confirmed to demonstrate the concept, reliability, and advantages of the PTS. It is excluded with a significance more than $5\sigma$ that the sources RX J1713.7$-$3946 and Vela Jr. are Pevatrons, while strong indications exceeding $4\sigma$ are found for excluding HESS J1745$-$290 as a Pevatron. The importance to resolve source confusion with high angular resolution observations for Pevatrons searches is demonstrated using PTS for the region containing the SNR G106.3+2.7 and the Boomerang nebula. No statistically significant conclusion with respect to Pevatron associations could be drawn from this region, for the diffuse $\gamma$-ray emission around the Galactic Center, and the unidentified $\gamma$-ray sources LHAASO J2108$+$5157, HESS J1702$-$420A and MGRO J1908$+$06. Assuming the entire $\gamma$-ray emission from MGRO J1908+06 and the tail region of SNR G106.3+2.7 is hadronic, a statistical indication exceeding $3\sigma$ is found for the underlying proton spectrum to extend beyond 350$-$400 TeV as a power-law. This result can indicate that these sources are proton and helium Pevatrons, in which the accelerated particles contribute to the knee of proton and helium spectra observed at Earth.

Abstract: BlackHoleCam is a project funded by a European Research Council Synergy Grant to build a complete astrophysical description of nearby supermassive black holes by using a combination of radio imaging, pulsar observations, stellar astrometry and general relativistic magneto-hydrodynamic models. BlackHoleCam scientists are active partners of the Event Horizon Telescope Consortium. In this talk I will discuss the use of pulsars orbiting Sagittarius A* for tests of General Relativity, the current difficulties in detecting such sources, recent results from the Galactic Centre magnetar PSR J1745-2900 and how BlackHoleCam aims to search for undiscovered pulsars in the Galactic Centre.

Abstract: We use four observations with the European VLBI network to measure the first precise radio parallax of the Crab Pulsar. We found two in-beam extragalactic sources just outside the Crab Nebula, with one bright enough to use as a background reference source in our data. We use the Crab Pulsar's giant pulses to determine fringe and bandpass calibration solutions, which greatly improved the sensitivity and reliability of our images and allowed us to determine precise positional offsets between the pulsar and the background source. From those offsets, we determine a parallax of $\pi=0.53\pm0.06\rm{\;mas}$ and proper motion of $(\mu_{\alpha},\mu_{\delta})=(-11.34\pm0.06,2.65\pm0.14)\rm{\;mas\;yr^{-1}}$, yielding a distance of $d=1.90^{+0.22}_{-0.18}\rm{\;kpc}$ and transverse velocity of $v_{\perp}=104^{+13}_{-11}\rm{\;km\;s^{-1}}$. These results are consistent with the Gaia 3 measurements, and open up the possibility of far more accurate astrometry with further VLBI observations.