Supernovae Leave Behind Cosmic X-ray Generators
The primary message of this article is that astronomer need to put
telescopes in orbit in order to study the wavelengths that don't pass
through the Earth's atmosphere. This article also reinforces the idea of
multiwavelength astronomy that was introduced in the 1955 Cosmic Times
article, "Radio 'Ear' on the Universe Being Built".
For thousands of years, the only form of astronomy observation was
optical observation – the light we could see with our eyes. All
telescopes, lenses and cameras were sensitive only to optical light.
The 20th century saw the advent of astronomy in other
wavelengths. Radio astronomy began in the 1930s (as we found in the
1955 Cosmic Times article titled "Radio 'Ear' on the Universe Being
Built"), with the first sky-survey in radio wavelengths conducted in the
1950s. The subject of the main 1965 article, "Murmur of a Bang"
describing the discovery by Penzias and Wilson of the cosmic microwave
background, was done with an antenna sensitive to microwave wavelengths.
There is a limit, though, to the portions of the electromagnetic
spectrum that can be studied from the Earth's surface. Earth's
atmosphere stops many wavelengths of light from reaching the Earth's
surface – lucky for us, because many of these wavelengths are
harmful to humans. However, this means that for the wavelengths stopped
by the atmosphere, observations must take place above the atmosphere.
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A sounding rocket
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X-ray wavelengths are among those stopped by the atmosphere, so must be
observed above the Earth's atmosphere. The 1960s saw the birth of X-ray
astronomy, with rockets, called "sounding rockets", to put instruments
above the bulk of the atmosphere. A "sounding rocket" is one that is
used to perform research, and is named after the nautical term to
sound, meaning, "to take a measurement". A sounding rocket has a
typical science observing time of 5-40 minutes (in other words, the
rocket remains above the bulk of the atmosphere for between 5 and 40
minutes, depending on the rocket and how high it flies).
When astronomers first thought about X-rays from astronomical sources,
the general opinion was that extra-solar sources would be too faint to
be detectable. The first X-ray detector flown on a sounding rocket was
set to look at the moon, under the idea that heavy particles from the
sun would cause X-ray fluorescence of the moon's surface. They did not
detect the moon. Luckily, it just so happened that an X-ray
source was near enough to the moon that the detector could "see" it.
This source is now called Scorpius X-1, because it was the first X-ray
source observed in the direction of the constellation Scorpius. It was
clear that Scorpius X-1 came from outside our solar system – disproving
the idea that extra-solar sources would be too faint to see!
This article discusses two specific X-ray sources in the sky, Taurus
XR-1 and Ophiuchus XR-1. Early X-ray observations were named after the
constellation in which they were found, and numbered in the order that
they were discovered. So Taurus XR-1 was the first X-ray source found
in the constellation Taurus. Similarly, Ophiuchus XR-1 was the first
X-ray source observed in the constellation Ophiuchus.
As previously mentioned, astronomers originally thought that cosmic
X-ray sources would be too weak to be observed, so the appearance of
numerous X-ray sources was a bit of a mystery. This article reports one
of the first attempts to unravel the mystery, pointing out the
similarity between the distributions of supernovae remnants in our
Galaxy and that of the observed X-ray sources. This view was bolstered
by the possibility of two X-ray sources coinciding with known supernova
remnants.
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X-ray image of the Crab nebula as observed
by Chandra. The
details seen in this image are only possible with modern instrumentation
– the original observations of the Crab would have been in the form
of a spectrum, rather than an image. More information on
this image available here.
Image credit: NASA/CXC/ASU/J.
Hester et al.
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Taurus XR-1 was conclusively tied to the Crab nebula – a remnant of
the supernova of 1054. The Crab nebula is currently one of the
most-studied objects in the sky, exhibiting emission at all wavelengths
from radio to gamma-ray. It is unclear, however, if Ophiuchus XR-1
turned out to be from the remnant of SN 1604, also known as Kepler's
Supernova. Ophiuchus XR-1 was only conclusively observed by one rocket
observation. However, later observations did find X-ray emission from
SN 1604. It is possible that the first observation was merely noise.
We now know that supernova remnants can leave behind X-ray sources
(neutron stars or black holes); however, supernova remnants are hardly
the only source of cosmic X-rays. X-ray astronomy has grown since the
1960s, with several dedicated satellites, including the well-known
Chandra observatory.
Other resources
The following web pages have more detailed information:
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