High-energy astrophysics missions: The X-ray spectroscopy era

SciTech Europa takes a look at future X-ray spectroscopy missions, deigned to provide us with a better understanding of the Universe.

High-energy astrophysics studies X-ray and gamma-ray photons, which are produced in some of the most extreme conditions in the Universe: temperatures of millions of degrees, high magnetic fields, extreme gravity, or massive explosions. ‘X-ray spectroscopy’ (XRS) involves X-rays being used for the analysis of the interaction between the electromagnetic spectrum and matter. XRS looks at the photons that come from a given source and the data obtained, known as ‘spectrum’, shows the plot of the intensity of energy detected versus factors of the energy such as wavelength, frequency, or mass etc.

SVOM

The satellite SVOM (space-based multiband astronomical Variable Objects Monitor) aims to detect and observe gamma-ray bursts (GRBs). The mission is due to launch in 2021 and will last for a minimum of three years and will orbit at an altitude of 600km.

Despite continuing research efforts over the last 15 years, GRBs are still poorly understood. GRBs are some of the highest-energy phenomena known in the Universe. They are linked to the cataclysmic formation of black holes, either by the merger of two compact stars (neutron star or black holes), or by the sudden explosion of a massive star, 20 to 100 times larger than our Sun. The birth of a black hole comes with the formation of material jets travelling at nearly the speed of light (300,000km per second). These material jets then slow down in the circumstellar medium, sweeping up everything in their way. Bright GRBs can be observed in the confines of the Universe, acting as real beacons lighting the dark age of its creation.

SVOM will carry four instruments, including ECLAIRS (a wide-field X-ray and gamma-ray camera) and a Microchannel X-ray Telescope (MXT). When ECLAIRs detects a GRB, the satellite will then be repointed to follow up its observations with measurements by the other instruments. Alerts will be relayed to ground in less than one minute whenever a GRB is detected to cue dedicated robotic telescopes as well as large ground telescopes.
China will be responsible for the mission, including the satellite and launch, and will share responsibility with France for the design and construction of the instruments, including the ground segment. To fulfil its objectives, the SVOM mission will use ground facilities in addition to the systems dedicated to space platform control and command tasks.

ATHENA

ATHENA, the Advanced Telescope for High-ENergy Astrophysics, is the European Space Agency’s (ESA) Cosmic Vision mission dedicated to probing the hot and energetic Universe. Planned for launch in 2031, it has three key goals:

  •  To map and study large-scale gas structures in the Universe;
  •  To survey supermassive black holes; and
  • To explore high-energy astrophysical events such as supernova explosions and energetic stellar flares.

X-ray spectroscopy

X-ray astronomy is a fundamental tool for studying ordinary matter in the Universe: that made up of baryons, like neutrons or protons. Current understanding is that the majority of this baryonic matter is trapped in hot, dispersed gas clouds, most of which make up the so-called ‘warm-hot intergalactic medium’. Since this web of galaxy-connecting gas streams has temperatures between 100,000 and 10 million degrees Kelvin, it emits mostly X-rays. As gas and dust is swallowed by black holes, their strong gravitational fields heat up the material being accreted to very high temperatures (millions of degrees Kelvin), resulting in X-ray emissions.

With the Earth’s atmosphere blocking most X-ray emissions, space missions are essential to seeing the hot and energetic Universe. ATHENA is the future of X-ray astronomy, aiming to probe both known and unknown high-energy astrophysical processes. It will address two important questions in astronomy: how does baryonic matter clump together to form the large-scale structures of the Universe? And how do supermassive black holes grow and shape the cosmos?

ATHENA will explore the formation and evolution of galaxy groups and clusters. By mapping how hot gas clouds are distributed in these galactic structures at various redshifts, and determining the temperature and density of the gas, it will help astronomers understand how they formed and evolved. In addition to this, by measuring how abundant elements, such as nitrogen and oxygen, are at different redshifts, ATHENA can determine when and through which processes the baryonic gas in large-scale structures became enriched with these elements. Astronomers will use the mission’s instruments to identify the locations in galaxy clusters that are hotspots for the generation of metals, and to find out how these heavier elements are dispersed.

Looking into the cosmos at high redshifts (z>6), Athena will peer into a time in the history of the Universe when many supermassive black holes were just forming and starting to grow. The processes that are involved in the origin of this emergence and early evolution are not fully known. Athena will follow explosive events occurring at the end of their life, measure with unprecedented accuracy their metal content, and allowing for the development of a better understanding of if and how they could have formed black holes, and to study the mechanisms that dominated the early growth of these massive objects.

IXPE

In late 2020, NASA is set to launch the Imaging X-ray Polarimetry Explorer (IXPE) mission. IXPE will exploit the polarisation state of light from astrophysical sources to provide insight into the understanding of X-ray production in objects such as neutron stars, pulsar wind nebulae, and stellar/supermassive black holes. Objects such as black holes can heat surrounding gases to more than a million degrees. The high-energy X-ray radiation from this gas can be polarised – vibrating in a particular direction. The Imaging X-ray Polarimetry Explorer (IXPE) mission will fly three space telescopes with cameras capable of measuring the polarisation of these cosmic X-rays, allowing scientists to answer fundamental questions about these turbulent and extreme environments where gravitational, electric, and magnetic fields are at their limits.

The Imaging X-ray Polarimetry Explorer will be launched on or after 20 November 2020 into a 540-km circular orbit at 0° inclination. During IXPE’s two-year mission, targets such as active galactic nuclei (AGN), micro quasars, pulsars and pulsar wind nebulae, magnetars, accreting X-ray binaries, supernova remnants, and the Galactic centre will be studied. The cost of IXPE will be $188m (~€166m) which includes the cost of the launch vehicle and operations and data analysis after launch.

NASA’s Astrophysics Explorers Program requested proposals for new missions in September 2014. 14 proposals were submitted, and three mission concepts were selected for additional review by a panel of agency and external scientists. NASA determined the IXPE proposal provided the best science potential and most feasible development plan. NASA’s Explorers Program provides frequent, low-cost access to space using principal investigator-led space science investigations relevant to the agency’s astrophysics and heliophysics programs.

The program has launched more than 90 missions, including Explorer 1 in 1958, which discovered the Van Allen radiation belts around the Earth, and the Cosmic Background Explorer mission, which led to a Nobel Prize. NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the Explorers Program for the agency’s Science Mission Directorate.

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