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LOFAR Newsletter Oct 2021 – Mar 2022
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Events

June 2022

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Daily image

Looking inside the accretion disc around a supermassive black hole

© WAB

H2O MegaMaser emission in sources such as the prominent NGC 4258 arise from a thin gas disc surrounding the supermassive nucleus. Space Very Long Baseline Interferometry (SVLBI) experiments with the Russian-built RadioAstron Observatory in Earth orbit and the large Green Bank and Effelsberg telescopes have resulted in the detection of the regularly-spaced emitting clouds inside the disc. The above figure shows a spectrum with a series of H2O emission components obtained with an Earth-space baseline of 19.5 Earth Diameters (about 250.000 km). The upper and lower frames of the above figure show the broad ground-based spectrum and the cross-correlated spectrum of one experiment. The angular resolution of this observation is 11 micro-arcsecs, which corresponds to a footprint of only 62 AU at the host galaxy. The observed molecular emission regions are found to be orbiting inside the rotating disc at a radius of about 0.126 parsec (0.38 lightyears) from the black hole nucleus of the galaxy.

The H2O emission in these regions results from maser-amplification by clouds with excited/pumped water molecules in the foreground as they drift in front of the radio continuum in the nucleus of NGC 4258. The formation of these regions, their regular velocity separation and their time-dependent emission appear consistent with the occurrence of a periodic magneto-rotational instability in the disc. This shear-instability is driven by differential rotation in the disc and is thought to regulate the momentum transfer and viscosity within an accretion disc.

The paper 'Space VLBI Observations of the H2O Megamaser in NGC 4258: evidence for periodic disc instabilities' will be published in Nature Astronomy on 30 June, 2022.

The authors are Willem Baan (XinJiang Astronomical Observatory, CHN, and ASTRON, NL), Tao An (Shanghai Astronomical Observatory, CHN), Christian Henkel (MPIfR, GER), Hiroshi Imai (Kagoshima Univ., JAP), Vladimir Kostenko (AstroSpace Center, RUS), and Andrej Sobolev (Ural Federal Univ., RUS).

Colloquia

May 1, 2022

The Commensal Radio Astronomy FAST Survey (CRAFTS)

The Five-hundred-meter Aperture Spherical radio Telescope (FAST) has released its first call for proposal and will be open to the international community next year. Based on a novel technique of high-cadence CAL injection, we have realized the world's first calibrated commensal survey mode, simultaneously taking data for pulsar search, HI galaxies, HI imaging, and FRBs. I introduce here one of the major survey plans, namely, the Commensal Radio Astronomy FAST Survey (CRAFTS, Li et al. 2018), which has discovered more than 100 new pulsars, including a few dozen MSPs, 5 new FRBs, including one new repeater. I will also briefly describe recent FAST results from CRAFTS and other dedicated programs, including new insights into the characteristic energy of FRBs, the formation process of neutron stars, the evolution of interstellar medium, etc.
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May 14, 2022

Extreme UV Emission: Bridging Galaxy Evolution Across Cosmic Time

In the last few years, our first glimpse of the spectral properties of z∼5−7 galaxies has emerged. Deep UV spectra have revealed prominent high-ionization nebular emission lines (i.e., C IV, He II, C III]) indicating that extreme radiation fields may be characteristic of reionization-era systems. While such strong high-ionization emission lines are atypical of the well-studied z∼0−3 galaxy samples, our recent UV spectral campaigns have revealed several galaxies with analogous emission-line features to reionization-era systems. I will discuss the recent detection of extremely strong UV emission in nearby galaxies and the potential sources of their very hard ionizing radiation fields. Such strong detections of high-ionization emission lines have been linked to the leakage of Lyman continuum (LyC) photons (necessary for reionization) both theoretically and observationally. These extreme UV emission-line dwarf galaxies provide a template for the extreme conditions that are important for reionization, however their features are still poorly understood. In preparation for the coming UV window onto the early universe with the advent of ELTs and JWST, I will introduce the COS Legacy Archival Spectroscopic SurveY - an upcoming large HST program designed to disentangle the stellar and nebular spectral signatures of 45 star-forming galaxies. This program will calibrate new UV diagnostics that will allow us to trace galaxy evolution to the distant universe, unveiling the properties of reionization-era galaxies.
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