Transcript Highlights in astronomical polarimetry

Highlights in
Astronomical Polarimetry
Egidio Landi Degl’Innocenti
Department of Physics and Astronomy
University of Florence, Italy
COST Meeting “The Future of Polarimetry”
Bruxelles, September 21, 2015
Historical introduction on polarization I
Historically, the name “polarization” is due to Etienne-Louis
Malus (1809).
Polarization is an intrinsic property of light connected with
the transversality character of the “optical vibration”.
Malus shows how (linearly) polarized light
can be artificially produced by means of
reflections and refractions on the surfaces
of (what we now call) dielectric bodies
such as glass, water, etc.
Historical introduction on polarization II
Augustin Fresnel continues the
quantitative analysis of the polarization
properties of light. Fresnel publishes his
famous laws concerning the
polarization properties of light reflected
and transmitted at the surface of a
dielectric (circa 1820).
Polarization
From simple physical curiosity
to a tool for constructing optical
devices
In particular, Fresnel succeeds in producing a device
capable of transforming linear into circular polrization
and viceversa (the so-called Fresnel rhomb).
Historical introduction on polarization VI
Stokes introduces statistics in the
mathematical description of polarization.
By so doing he defines four quantities,
nowadays known as the Stokes
parameters, which have the advantage
of being operationally connected with
intensity measurments.
First astronomical applications I
The first polarimetric observations in
Astronomy were made by François
Arago in 1811. He directed his telescope
to the moon to see if the reflected
sunlight carried similar properties to
those seen by Malus in reflections by
glass surfaces.
Arago’s equipment was (obviously)
rather poor, but clever. It comprised a
quartz plate and a Wollaston prism.
At the time no photographic recording was yet available.
Polarization had to be detected by means of visual
measurements.
First astronomical applications II
Further astronomical applications of polarimetry went on
during the 19th century. They were however restricted to
luminous bodies, the moon being the first target. Arago
published his results on the moon polarization around 1850.
He also observed the polarization of comets. Other
astronomers who contributed to moon observations were
Father Angelo Secchi and Lord Rosse.
Unfortunately these first astronomical applications have
revealed to be rather difficult to interpret, the underlying
physics being quite complicated. In particular, it was found that
the polarization of the reflected solar radiation was strongly
varying with the moon phase and that it was larger on lunar
maria than on lunar highlands (Umov’s law).
First astronomical applications III
Umov’s law (also known as Umov effect) was established in
1905. It states that the polarization P of an astronomical
solid body, such an asteroid, is connected with its albedo α.
P ≈ 1/α
This effect has been very important in the
history of astronomy for establishing the
dimensions of asteroids from the
measurement of their luminosity and for
putting the bases of their taxonomy.
First astronomical applications IV
Modern results for the moon's polarization at various λ.
At small phase angles (phase < ≈ 25°) P is negative.
Coyne & Pellicori (1970) + Lyot (1929, dashed line, visual
observations)
First astronomical applications V
The problem is that on the moon surface (as well as on the
surfaces of planets and asteroids) one is faced with the
phenomenon of diffuse reflection instead of specular
reflection
This phenomenon is not restricted to astronomical bodies.
Many common materials behave this way, such as paper,
plaster, etc.
Modern results
The polarization
phenomena associated
with diffuse reflection are
nowadays widely used in
planetary sciences for
diagnosing the nature of
the surface of the minor
bodies of our solar
system.
From Bagnulo et. al
First spectropolarimetric observations I
At the middle of the XIX century, the phenomenon of
polarization is fairly well understood, but it is necessary to
wait for more than 50 years before the first astronomical
application combining polarimetry with spectroscopy. In
1908 Hale succeeds in observing the spectrum of a
sunspot in two opposite directions of circular polarization
and, from the observed shift of the spectral lines, deduces
the existence of a magnetic field in an
astronomical object, the sun.
Hale brings in Astronomy a real
revolution, comparable to those brought
by Galileo (telescope) and by
Fraunhofer (diffraction grating).
First spectropolarimetric observations II
By inserting in front of his spectrograph at Mount Wilson a
rotating Fresnel rhomb and a Nicol prism, Hale was
capable of observing the Zeeman effect in sunspots. He
published his results in the Astrophysical Journal, 28, 315.
spot
spot
λ5934.7, Fe I
redshift
blueshift
Modern observations
Nowadays things are evolved, though to have large
Zeeman splittings it is necessary to observe in the IR.
First spectropolarimetric observations III
With his apparatus Hale discovers his famous laws (1st
and 2nd Hale's laws) concerning the magnetic cycle of
the sun. As a result, the solar cycle changes its period.
Accounting for the polarity reversal of the magnetic field,
the cycle becomes of 22 years instead of 11.
Stellar spectropolarimetric observations I
The discovery of Hale opens the way
to the search of magnetic fields in
other astronomical objects, in
particular stars. After 36 years of
Hale's discovery, in 1946 Horace W.
Babcock, working at the Coudé
spectrograph of the Mount Wilson
Observatory, reports on the first
evidence of a magnetic field on the
star 78 Virginis, an A2 peculiar star.
Ap. J. 105, 105
Stellar spectropolarimetric observations II
The main difficulty of stellar observations in discovering the
presence of a magnetic field is due to the non-uniformity of
B over the surface. The first detections are restricted to
stars having a well organized dipole-type magnetic field
and have small rotational velocities. These are a sub-class
of the peculiar A stars for which anomalies in the elements
abundances had been previously discovered.
In 1958 Babcock publishes a catalogue of 89 magnetic
Ap stars which remains a reference for many years.
Ap. J. S. 3, 141
Modern research in stellar magnetic fields
Since the single-line signals coming from magnetic stars are
always very weak, it is nowadays used to collect together (by
means of optical fibers) the circular polarimetric signals
originating in a large number of spectral lines. This technique
has been used in a series of instruments (MuSicoS,
ESPaDOns, Narval) and is referred to as LSD (LeastSquares Deconvolution). The data, combined with the use of
sophisticated techniques of data reduction, like Doppler
imaging, allow the reconstruction of the magnetic field over
the surface of the star. This technique has been pioneered by
Donati, Semel, et al. (1997)
Resonance polarization I
In 1947 Chandrasekhar and Breen publish a theoretical
paper on the continuum polarization to be expected at the
limb of an electron-scattering atmosphere (Ap. J. 105, 435),
finding that the radiation is linearly polarized along the
parallel to the limb with a fractional value of 11.71% (today
corrected bu the use of modern computers to 11.53%).
Resonance polarization II
The paper by Chandrasekhar raises a strong interest
among astronomers who started looking for linear
polarization in eclipsing binaries. However, the net result of
this search was the totally serendipitous discovery of a
different phenomenon, interstellar polarization.
The prevision of Chandrasekhar was confirmed only in
1983 by J.C. Kemp and
collaborators who found
variable linear
polarization (of the order
of 0.01%) during the
eclipsing phase of Algol
(β Persei).
Interstellar polarization
The phenomenon of interstellar polarization was discovered
by W.A. Hiltner (Nature 163, 283) and J.S. Hall (Science 109,
166) in 1949. Now we know that it is due to the alignment of
anisotropic dust grains by the interstellar magnetic field.
The underlying theory is rather involved and there are
several competing models that try to explain the alignement
mechanism and the λ-dependence of polarization.
Weak solar magnetic fields I
A quantitative improvement of the sensitivity in measuring
solar magnetic fields is realized by H.W. Babcock and his
son, H.D. Bacock in 1953 with the introduction of a new
technique based on a difference amplifier and on variable
retarders made of ADP crystals (ammonium-dihydrogenphospate). The retardance is varied at the frequency of
120 Hz by applying a variable potential to the crystal.
The technique is illustrated by
this picture. It allows to
measure solar magnetic fields
of the order of few gauss.
.
Weak solar magnetic fields II
One of the first results produced by Babcock's instrument.
The instrument is called a (longitudinal) magnetograph and
the resulting image a magnetograph. The magnetic field is
not restricted to sunspots but is present almost evrywhere.
.
Weak solar magnetic fields III
A modern magnetogram
Obviously, technologies have evolved...
.
Magnetic white dwarfs
In 1970 came an another important highlight in astronomical
polarimetry, the discovery of circular polarization in the
featureless continuum radiation of the white dwarf
Grw+70°8247. The circular polarization was attributed to a
magnetic field of the order of 107 gauss acting through the
grey-body magneto-emissivity mechanism
.
John Landstreet
Quantum effects in Resonance polarization
at the solar limb
In 1980 Jan Stenflo discovered that the solar spectrum
observed close to the limb was presenting a peculiar, linear
polarization profile in a 100 Å interval around the Ca II H and
K lines. This effect was interpreted as due to quantum
interference berween the two possibilities offered to the
photon: scattering in the H-line or scattering in the K-line.
.
Quantum effects in Resonance polarization
at the solar limb II
The thoretical interpretation is already contained in the
Kramers-Heisenberg equation for Rayleigh scattering, but
such kind of phenomena are impossible to observe in
laboratory plasmas due to the very little absorption
coefficient in the far wings. Similar phenomena were later
found around the Na D lines
From Stenflo and Keller, 1997,
Landi Degl'Innocenti, 1998
.
Magnetic field measurements in prominences
In 1981, V. Bommier, J.L. Leroy and S. Sahal-Bréchot,
publish an important paper on magnetic field measurements
in solar prominences based on linear polarization
observations in the D3 line of helium I
A&A 100, 231
The Hanle effct
This physical phenomenon was discovered by the german
physicist Wilhelm Hanle around 1920, and now bears his
name. Stated in modern terms, the Hanle effect consists in
the relaxation of atomic coherence due to the presence of a
magnetic field. Véronique Bommier was the first to apply the
Hanle effect for the interpretation of astronomical
observations.
Wilhelm Hanle
Véronique Bommier
Polarization measurements help in
establishing the unified model of AGNs I
In 1982 R. Antonucci showed that by means of
spectropolarimetric observation it is possible to separate
the spectrum of hidden sources from any sources of direct
light. In Quasar 3CR234 the Broad Line Region is present,
but hidden from direct view. The scattering polarization
position angle helps in determining the geometrical
scenario.
These results were fundamental in the construction of the
unified model of AGNs
Antonucci R.R. et al., 1982
.
Polarization measurements help in
establishing the unified model of AGNs II
The polarized spectrum of the Narrow Line Region
.
shows a spectrum typical of the Broad Line Region.
Radio-waves polarimetry
Highlights in radioastronomical polarimetry:
In 1946 E.V. Aplleton and J-S. Hey discover strong signals
of circular polarization from sunspots areas at the frequency
of 85 MHz. The interpret them as due to cyclotron radiation.
In 1957 C.H. Mayer, T.P. McCullough and R.M. Sloanaker
detect linear polarization from the Crab Nebula. They find a
fractional polarization of the order of 6% with a position
angle (150°) differing by few degrees with the position
angle of the optical polarization already observed at optical
wavelengths. These observation confirm the interpretation of
synchrotron radiation for the radiation observed from the
Crab Nebula.
X-ray polarimetry I
Ap. J. 208 L125
The experiment was on board of a
sounding rocket. Two types of
polarimeters were used. One based on
the polarization dependence of
Thomson scattering and the other on
Bragg reflection.
M.C. Weisskopf
The polarization was found to be about
19% with a position angle ≈ 150%
It was 1972!!!!
X-ray polarimetry II
Since 1972 scientists have proposed multiple space
missions to explore other sources of these rays,
such as pulsars, black holes and supernova
remnants. Three spacecraft nearly flew, but space
agencies cancelled or passed over these polarimetry
missions. So far, the Crab Nebula, is the only source
of polarized X-rays that has been mapped.
Several scientists, including Martin Weisskopf,
Enrico Costa and many others are now struggling to
have a space mission flown.
Nowadays It looks that XIPE (X-Ray Imaging
Polarimeter Explorer) has good chances of success.
UV Spectropolarimetry I
The Chromospheric Lymanalpha Spectropolarimeter
(CLASP)
UV Spectropolarimetry II
CLASP launch,
September 3,
2015
UV Spectropolarimetry III
Slit-jaw image during
the CLASP 5-minutes
observations
Conclusions
I will like to conclude with a general comment concerning
spectropoalrimetry. For studying polarization, more than for
any other discipline of physics, the famous words of Galileo
still sound extremely approriate:
The Universe is written in
mathematical language, and its
characters are triangles,
circles, and other geometrical
figures, without which it is
humanly impossible to
understand a single word.
Thank tou for your kind attention
Theory V
Due to the Hanle effect, when a scattering process takes
place in a magnetic environment, the scattered polarization
results in being modified from its "magnetic-field-free" value.
This allows to use the Hanle effect as a diagnostic tool for
measuring magnetic fields.
If you want to know more....
Historical Introduction on Polarization IV
Fresnel succeeds in connecting what he was calling
“the amplitude of the optical vibration” (today we speak
about the electric and magnetic field components) along
the different unit vectors.
His equations show that in reflection the parallel
component of the optical vibration (the component
laying in the plane containing the incident and reflected
beams) always result in being smaller than the
perpendicular component. This was confirming Malus
experiments.
Historical Introduction on Polarization V
For instance, in the case of reflection on water the coefficients
connecting the reflected with the incident component are
shown in this graph. For a particular angle (the Brewster
angle) rpar = 0. The reflected beam is totally polarized
Historical Introduction on Polarization III
In the reflection and refraction of a beam of radiation,
the polarization properties are deeply modified.
Historical Introduction on Polarization VI
Left image: without polarizing filter
Right image: with polarizing filter