Highlights in Astronomical Polarimetry

1. 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

2. 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.

3. 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). From simple physical curiosity to a tool for constructing optical devices Polarization In particular, Fresnel succeeds in producing a device capable of transforming linear into circular polrization and viceversa (the so-called Fresnel rhomb).

4. 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.

5. 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.

6. 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).

7. 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.

8. 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)

9. 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.

10. 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

11. 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).

12. 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

13. Modern observations Nowadays things are evolved, though to have large Zeeman splittings it is necessary to observe in the IR.

14. 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.

15. 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

16. 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

17. 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 (Least-Squares 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)

18. 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%).

19. 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).

20. 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.

21. 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-dihydrogen-phospate). 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. .

22. 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. .

23. Weak solar magnetic fields III A modern magnetogram Obviously, technologies have evolved... .

24. 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

25. 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. .

26. 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 .

27. 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

28. 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. Véronique Bommier Wilhelm Hanle

29. 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 .

30. 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. .

31. 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.

32. 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. The polarization was found to be about 19% with a position angle ≈ 150% M.C. Weisskopf It was 1972!!!!

33. 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.

34. UV Spectropolarimetry I The Chromospheric Lyman-alpha Spectropolarimeter (CLASP)

35. UV Spectropolarimetry II CLASP launch, September 3, 2015

36. UV Spectropolarimetry III Slit-jaw image during the CLASP 5-minutes observations

37. 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

38. 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....

39. 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.

40. 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

41. Historical Introduction on Polarization III In the reflection and refraction of a beam of radiation, the polarization properties are deeply modified.

42. Historical Introduction on Polarization VI Left image: without polarizing filter Right image: with polarizing filter