HEA Missions: Pharos

The Pharos mission The amount of normal matter in the Universe is predicted precisely by the Big Bang model, and this matter is seen in the early universe as the "Lyman alpha forest" - absorption by diffuse cold gas seen in the spectra of distant, high redshift, quasars. But by the current epoch, about half of this matter has disappeard, creating the 'missing baryons' problem. Within the last decade models of the emergence of structure in the universe and of galaxy formation have become sophisticated enough to predict that this missing matter has become heated to a million degrees. Such hot gas no longer aborbs optical light and so disappears from normal inventories of matter. But this 'warm-hot intergalactic medium' (WHIM) will absorb low energy (0.1-1keV) X-rays - and tentative hints of this absorption have been seen with Chandra

"Pharos" is a satellite mission designed specifically to detect and study the WHIM, and thereby contribute to our understanding of the structure of the universe, galaxy formation and the 'ecology' of the universe - how galaxies and quasars re-cycle matter into the expanses of intergalactic space. For this purpose, Pharos will be more than 100 times more capable than Chandra, both because it uses newer technology and because it slews rapidly to catch the brightest high redshift objects in the sky - the Gamma-ray bursts (GRBs). As GRBs emit most of their X-ray light within a few minutes of their first eruption, it is vital to get to them within a minute.

Papers, Links:

Science Goals:

  • Primary Goal: detect the Warm-Hot Intergalactic Medium and study its structure, heating, growth, and enrichment.
  • Secondary Goal: study the host galaxies of gamma-ray bursts.
  • Open Science: respond to time critical events and guest observer requests.

Spacecraft Architecture:

  • short focal length X-ray mirror (0.1-1.5 keV)
  • rapid slew to GRBs (<1 minute)
  • high resolution (R>1500) grating spectrograph
  • 'all' sky monitor to provide GRB alert and rough (arcminute) position
  • zero order detector to provide accurate (arcsecond) GRB position.


The Pharos - lighthouse - of Alexandria, one of the Seven Wonders of the ancient world, was then the tallest building on Earth (120m). Its mirror, whose reflection could be seen more than 55km off-shore, fascinated scientists for centuries. It lasted from the time of Alexander the Great to the middle ages when it was felled by an earthquake.


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