C19: Astrophysics

ASTROPHYSICS HIGHLIGHTS 2020

Contributions from:

Astronomy, although one of the oldest sciences, is currently in a phase of remarkable discovery and growth. Nobel physics prizes in 2019 (Jim Peebles – cosmology, Michel Mayor and Didier Queloz – exoplanets), 2017 (Rainer Weiss, Kip Thorne and Barry Barish – gravitational waves), 2011 (Saul Perlmutter, Brian Schmidt and Adam Reiss – observational cosmology), 2006 (John Mather and George Smoot – observational cosmology) and 2002 (Riccardo Giacconi – X-ray astronomy, Ray Davis Jr and Masatoshi Koshiba – astrophysical neutrinos) are a very public recognition of this impact. Although the whole field of astrophysics is very broad indeed, the members of IUPAP Commission C19, motivated by Pietro Ubertini, have agreed to provide a top-level overview of the some of the recent progress in a form widely accessible to a non-expert. Although the selection of topics is personal, the impressive range of progress is manifest.

Gravitational waves (GWs), predicted as a natural consequence of general relativity in 1916 have been only recently detected with the discovery of GW150914 during the first science run of the Advanced LIGO-VIRGO interferometer, followed by the observation of one more event, namely GW151226. The detection of these events has opened a new view on the universe, first of all with the discovery of the existence of a large number of heavy stellar mass Black-Hole Binaries (BHB) mergers with mass as high as 50-60 time our sun.

The discovery of BHB and NS-NS mergers has extended the traditional multi-wavelength astronomy to a new discipline: the multi-messenger astrophysics. The discovery of the counterpart of the NS-NS binary merger GW170817 has immediately triggered most of the existing space and ground based observatories in a global effort that, in turn, has provided unprecedented insights of the physical processes taking place in the close-by Universe.

Listen to the sound of two Black Holes colliding at: https://www.ligo.caltech.edu/video/ligo20160211v2

On August 17, 2017, after a long quest, Fermi and INTEGRAL detected a short γ-ray burst (GRB 170817A) linked to GW70817 caused by the merger of two neutron stars. It led to an intense and extensive follow-up campaign by a large number of ground and space-based telescopes across the electromagnetic spectrum. The time lag (1.7 s) between the GW event and the prompt γ-ray observation after 120My travel time, imposed constraints on the difference between the speeds of light and gravity, placed new bounds on the violation of Lorentz invariance, and presented a new test of the equivalence principle. The observations also constrained the size and bulk Lorentz factor of the γ-ray emitting region.

To date, this observation remains the only firm detection of the so-called Kilonova generated by the NS-NS merging process with spill-over of nuclear dense matter.

Figure 1. Joint, multi-messenger, detection of GW170817 and GRB 170817A. Top panel: Fermi/GBM light curve for GB 170817A at 10– 300 keV, respectively. Middle panel: time- frequency map of GW170817 obtained by LIGO. The time is referenced to the GW170817 trigger time. Bottom panel: INTEGRAL/SPI-ACS light curve of GRB170817A with the energy range starting at ~100 keV and a high energy limit of >80 MeV. The GRB occurred 1.7 seconds after the binary neutron star merger. Figure adapted from Abbott et al. 2017, ApJ 848, L13 .

The initiation in 2014 of the operation of the Atacama Large Millimeter Array, with unprecedented angular resolution, stimulated research on the structure and evolution of protoplanetary disks around young stars. These disks give birth to planetary systems and provide the initial conditions for planet formation. Together with high angular resolution Very Large Array millimeter observations, astronomers have found evidence of grain growth to centimeter sizes. The recent ALMA survey of 20 nearby protoplanetary disks (DSHARP) shown in Figure 2 discovered the surprising pervasiveness of structures like gaps, rings, vortices, and spiral arms, resolved at scales of a few astronomical units (au)*. Several processes have been invoked to produce them, including the presence of protoplanets. These substructures also influence the evolution of the dust and gas. They can stop the dust migration towards the central star and act as dust traps that promote the growth of the dust particles into planetesimals of kilometer sizes, the building blocks of planets.

To date there are more than 4400 confirmed exoplanets, many of them in multiplanetary systems. The diversity of properties like mass, size, and orbital characteristics provides evidence of the exoplanet structure and processes of formation. The first exoplanets were found using the radial velocity method that detects the gravitational influence of an unseen planet on the stellar motion. This method mainly finds massive exoplanets close to the star (like the so-called Hot Jupiters). For the discovery of the first exoplanet in 1995, Michel Major y Didier Queloz shared the Nobel Prize in Physics in 2019, together with the cosmologist James Peebles. Another method, capable of detecting Earth-like planets is the transit method used in the Kepler mission (2009-2018). This method detects the shadow that planets cast on their host star as they orbit around it. Currently, telescopes on the Earth and space look for transits around nearby stars. In particular, the Transiting Exoplanet Survey Satellite launched in 2018 will monitor 200,000 stars looking for exoplanet transits.

Recently, our Solar System has had two interstellar visitors: the asteroid Oumuamua (2017) and the interestellar comet 2l/Borisov (2019) which crossed the plane of the Solar System in unbound hyperbolic orbits. With the continuous monitoring of Near Earth Objects, one expects to detect more visitors in the coming years.

*1 au is the distance from the Earth to the Sun.

Figure 2. Millimeter ALMA images of 20 nearby protoplanetary disks.
(DSHARP survey, https://almascience.eso.org/almadata/lp/DSHARP/)

Fast Radio Bursts (FRBs) are a new kind of transient objects that have been discovered during the last ten years. These events are bright extra-galactic radio flashes of unknown nature that last for only a few milliseconds. Despite hundreds of them have been discovered to date, only a handful of FRBs have been precisely localized and associated to host galaxies. Additionally, only a few percent of them are known to repeat, and it is yet unclear if they belong to a different type of objects. FRBs have been found in different host galaxies and local environments, thus their unclear origin. Current scenarios assume the presence of very young magnetars either powering up superluminous supernovae or interacting with massive black holes to explain their astonishing burst luminosities.

On April 28, 2020 a very bright and short radio pulse, resembling an FRB, has been detected from the Galactic magnetar SGR 1935+2154 by the CHIME and STARE2 radio telescopes at 400-800 MHz and 1.4 GHz. Magnetars are isolated neutron stars powered by their extremely high magnetic fields and characterized by the emission of powerful bursts of hard X-rays, but they were never seen to emit FRB-like bursts, up to now. At the time of the radio pulse, several X/gamma-ray satellites detected a particularly hard burst from SGR 1935+2154. Figure 3 shows its light curve, as measured with the INTEGRAL observatory in the 20-200 keV energy range.

Figure 3. Light curve showing the GRB, detected by INTETGRAL in the 20-200 keV range (to=14:34:24 UTC of April 28, 2020), black line, with overlapped the two FRB detected by CHIME and Stare2 (Figure courtesy of Mereghetti et al, ApJL 2020 in press and CHIME/FRB Collaboration et al. 2020, arXiv e-prints, arXiv:2005.10324).

Gamma-ray bursts (GRBs) are brief and extremely powerful cosmic explosions, suddenly appearing in the sky, about once per day. They are thought to result from the collapse of massive stars or the merging of neutron stars in distant galaxies. They commence with an initial, very bright flash, called the prompt emission, with a duration ranging from a fraction of a second to hundreds of seconds. The prompt emission is accompanied by the so-called afterglow, a less bright but longer-lasting emission over a broad range of wavelengths that fades with time.

On January 14th, 2019, a GRB was discovered independently by two space satellites: the Neil Gehrels Swift Observatory and the Fermi Gamma-ray Space Telescope. The event was named GRB 190114C, and within 22 seconds, its coordinates in the sky were distributed as an electronic alert to astronomers worldwide. The MAGIC telescopes pointed just 50 seconds after the beginning of the GRB to its sky position, detecting for the very first time TeV photons from a GRB. The TeV data, together with the very comprehensive multiwave-length data gathered by other instruments, provide the first unequivocal evidence for an additional, distinct emission process in the afterglow.

The H.E.S.S. telescopes detected very high-energy photons from the afterglows of two GRBs, namely GRB 180720B (on July 20th, 2018) and GRB 190829A (on August 29th, 2019). In the case of GRB 180720B, the detection succeeded 10 hours after the prompt GRB, thus unexpectedly deep in the afterglow phase. Successful observations of GRB 190829A commenced about 4 hours after the prompt GRB.

Figure 4: Artistic representation of a Gamma Ray Burst overlaid on a picture showing the MAGIC telescopes and the Milky Way. Credits: Superbossa.com and C. Righi.

On September 22, 2017, IceCube detected a neutrino with an energy of about 290 TeV, which indicated that the particle could have been produced in a faraway astrophysical object. Scientists could also determine its direction with high precision (see Figure 5).

Fermi-LAT, a space observatory which studies the whole sky, noted that the direction of the neutrino was coincident with an active gamma ray source (high energy photons): the TXS 0506+056 blazar. Also, the 17-meter MAGIC telescopes, which detect high-energy gamma rays from Earth, found that radiation of the blazar reached energies of at least 0,5 TeV. Measurements obtained with H.E.S.S. and VERITAS were consistent with the observed flux from MAGIC.

These findings, together with the direction of the neutrino, make the blazar a potential candidate for the neutrino source. TXS 0506+056 is a galaxy active nucleus, at a 4,500-million-year light distance from the Earth. It has a supermassive black hole emitting a collimated flow of particles and energetic radiation that moves at the speed of light, known as jets.

The creation of neutrinos is always related to proton interactions, and these observations can help solving the origins of cosmic radiation, found by the physicist Victor Hess in 1912.

Figure 5: IceCube detection of a neutrino o On September 22, 2017.

The Gaia satellite, sent to space in 2013 by the European Space Agency, is creating the most accurate three-dimensional map of the local Universe. The mission, with almost 6 years of successful scientific operations, has been extended to 2020 with an additional “indicative extension” until 2022. Observations are planned to continue until the exhaustion of the fuel required for precision pointing corrections, currently predicted to be early 2025, with 6-month uncertainty. As of June 2020 some 1.5 trillion astrometric observations have been recorded. Nowadays, with astrometric and photometric data with unprecedented accuracy already published for more than 1.7 billion stars, solar system bodies and quasars, Gaia is revolutionizing many areas of astrophysics. Our understanding of the origin and evolution of the Milky Way is being transformed. Gaia is revealing new and exciting features which unveil an amazing history of its past evolution. Some 3,500 scientific articles using Gaia data have been published to date, making Gaia the most productive (using that metric) space astrophysics mission ever, although only the first 22 months of data have been published.

Figure 6. Gaia’s view of the sky. Credit: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO

Aside from their crucial input for the study of exoplanets, the 4-year µmag-precision photometric light curves assembled by the above-mentioned Kepler satellite brought us into the renaissance of stellar astrophysics. This is thanks to the method of asteroseismology, which exploits the frequencies of non-radial oscillations of stars detected in the light curves (see Figure 7). These oscillation frequencies allow to turn the theory of stellar interiors into observational astronomy. Asteroseismology delivers the mass, size, and age of stars with unprecedented levels of precision better than ~5%, ~2%, and ~15%, respectively. This has been put into practice for thousands of stars. Aside from the derivation of these global stellar parameters, asteroseismology applied to 4-year long time-series data assembled with Kepler also brought major new insights into how stars rotate in their deep interior, revealing major shortcomings in the theory of angular momentum transport. In particular, asteroseismology of low-and intermediate-mass stars reveals that stars rotate almost rigidly across their evolution, unlike what is predicted from the theory of stellar evolution that has been used for decades in astrophysics.

Figure 7: Asteroseismic light curve excerpts of seven stars with nonradial oscillations (sometimes also referred to as starquakes) observed with the Kepler space telescope (black). The oscillation frequencies of these stars are overplotted (red). Deciphering and modelling of these frequencies allows to probe the interior physics deep inside the stars. This has been achieved meanwhile for tens of thousands of stars with space photometric light curves similar to those shown here. Credit: Péter Pápics.

The Kepler data reveal that numerous intermediate-mass stars rotate at a high fraction of their critical rotation rate, demanding the development of a novel theory of stellar structure in 2D to understand the evolution of those rapid rotators. Altair, the brightest member of the Eagle Constellation, is such a star. While it was too bright to be observed by Kepler, it has been a prime target for interferometric observations, thus leading to a detailed picture of its surface. Altair’s polar radius is 20% smaller than its equatorial radius. Hence, both asteroseismology and interferometry point to the need of 2D stellar models. Such models have recently been developed with the computer code ESTER developed at Toulouse University. A 2D ESTER model is shown in Figure 8; it is in concordance with all the observational constraints and reveal Altair to have a mass of 1.86 times the mass of the Sun and an age of ~100 million years, which is about ten times younger than previous estimates based on 1D models. These results are a first steps to improve our understanding of stellar evolution of fast rotators of intermediate and high mass, which are the main producers of metals in the Universe and lie at the basis of computations for the chemical evolution of galaxies.

Figure 8: Altair and the Sun for comparison. The surface of Altair is brighter at the poles than at the equator due to centrifugal flattening. In the meridional section we see the differential rotation: the green part is basically the radiative envelope which is rotating more slowly than the convective core (in red). The equatorial velocity of Altair is now estimated to 313 km/s, thus more than 150 times that of the Sun. Credit: Pierre Kervella.