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.
- 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.
- 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.