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X-ray Astronomy: An Overview

X-ray Astronomy: An Overview. Jimmy Irwin University of Alabama Walter P. Maksym (CfA), Yuanyuan Su (CfA)Ka-Wah Wong (Eureka Scientific), Lucas Johnson (University of Alabama), Dacheng Lin (University of New Hampshire). XXIII Ciclo de Cursos Especiais - August 14-17, 2018. Alabama.

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X-ray Astronomy: An Overview

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  1. X-ray Astronomy: An Overview Jimmy Irwin University of Alabama Walter P. Maksym (CfA), Yuanyuan Su (CfA)Ka-Wah Wong (Eureka Scientific), Lucas Johnson (University of Alabama), Dacheng Lin (University of New Hampshire) XXIII Ciclo de Cursos Especiais - August 14-17, 2018

  2. Alabama

  3. Course Schedule Day 1 – Introduction to X-ray astronomy Day 2 – X-rays from lone stars X-ray binaries/Ultraluminous X-ray sources Day 3 – AGN/Gas inside the Bondi radius/tidal disruptions Hot gas in galaxies Day 4 – Gas in groups and clusters of galaxies X-ray Spectral Fitting

  4. X-ray Astronomy Basics http://www.radioqrv.com/RadioQRV%20-%20Electromagneti %20Spectrum.html Units of measurement: keV (kilo-electron volt) 1 keV = 1.6 x 10-16 Joules = 1.6 x 10-9 ergs Temperatures given in units of kT, where 1 keV corresponds to a temperature of 1.16 x 107 Kelvin

  5. X-ray Astronomy Basics http://www.radioqrv.com/RadioQRV%20-%20Electromagneti %20Spectrum.html Typical “soft” X-ray energy/wavelength band is 0.1 – 10 keV or 124 – 1.24 Angstroms “Hard” X-ray is energy band 10 – 100 keV or 1.24 – 0.12 Angstroms

  6. Why Can’t We All Get Along?? Optical Radio X-ray Wavelength/Energy: Angstrom cm/mm/Hz keV or Angstrom Temperature: Kelvin Kelvin keV Energy Flux/ magnitude Janskys ergs s-1 cm-2 Density: Luminosity: abs. mag. or L Watts ergs s-1 Telescope hours hours kilosec Exposure:

  7. Typical X-ray Parameter Values Temperature: hot gas in supernovae remnants or elliptical galaxies – 0.3 – 1 keV hot gas in groups/clusters of galaxies – 2 –10 keV X-ray binary accretion disks – 1 – 2 keV stellar coronae – 0.1 keV X-ray Luminosity (typically in 0.1 – 10 keV energy band): Sun – 1027 ergs s-1 Eddington-limited neutron star – 1038 ergs s-1 Typical X-ray AGN - 1041 ergs s-1 Gas-rich elliptical galaxy – 1040-41 ergs s-1 Rich cluster of galaxies – 1044-45 ergs s-1 Brightest X-ray quasars – 1047 ergs s-1

  8. Optically thick gas at temperatures exceeding a few million Kelvin will emit in the X-ray regime X-ray Emission Mechanisms – Blackbody Radiation L  R2T4 Accretion disks around stellar-mass black holes are hot enough/dense enough to emit as blackbodies with characteristic temperatures of ~1 keV. In fact, a single accretion disk can contain gas with a cascade of different blackbody temperatures blended together.

  9. X-ray Emission Mechanisms – Bremsstrahlung electron • Optically thin gas is so hot that electrons are all or mostly all ioinzed • Hot X-ray gas cools and emits X-rays through a process called thermal bremsstrahlung, or “braking radiation” in an optically thin gas • Also called “free-free emission” because the electron remains free before and after the interaction proton Energy emitted is proportional to T1/2 * ne * ni, where T= temperature, ne = electron density, ni = ion density

  10. Hot electrons can upscatter lower energy photons to higher energies via inverse Compton scattering. For example, optical/UV photons from the disks of AGN can be upscattered to X-ray energies by energetic electrons in a corona around the black hole. Radiation Processes – Inverse Compton Scattering

  11. Relativistic electrons spiraling down strong magnetic field lines will emit radiation as they do so, generically a power law spectrum. This can lead directly to X-ray emission (pulsars), or radio photons that are then inverse Compton scattered up to X-ray energies (AGN). Radiation Processes – Synchrotron + Self Comptonization

  12. Radiation Processes – Metal Line Emission http://ixo.gsfc.nasa.gov/images/science/goals17Starburst.png OVII – oxygen missing 6 electrons Ne X - neon missing 9 electrons Mg XI – magnesium missing 10 electrons Note how highly ionized the metals are – nearly all electrons have been removed, because of the very high temperature of the gas (0.3 keV – 15 keV, depending on the object). In the hottest clusters, even iron (Fe) is almost completely ionized.

  13. Radiation Processes – Metal Line Emission http://ixo.gsfc.nasa.gov/images/science/goals17Starburst.png OVII – oxygen missing 6 electrons Ne X - neon missing 9 electrons Mg XI – magnesium missing 10 electrons In additional to thermal bremsstrahlung emission, there are also electron transition emission lines from metals within the gas: Metal lines from relevant elements (Fe, Ni, O, Mg, Ne, S, Si)

  14. X-rays are susceptible to absorption by the Milky Way, the ISM of external galaxies, and/or absorption intrinsic to the object itself (i.e., absorbing gas in the torus surrounding an AGN). Amount of absorption depends on whether the gas is optically thick or optically thin at a particular energy, as well as the elemental abundance of the absorbing gas. The optical depth, (E), describes the opacity of the gas as a function of energy, and is more significant at softer energies. X-ray Absorption

  15. Opacity and  •  is a measure of how many interactions on average a photon will encounter while traveling through a medium • Opacity = A measure of the number of absorbers a photon will run into over some length • Opacity is often measured in terms of  • = density of absorbers x cross section of each sphere x path length (L) Units: 1/volume x area x length = dimensionless number Imagine that dots represent the absorption cross-section of molecules in a gas

  16. Opacity and  For very small ,  is approximately the percentage of photons that get absorbed passing through the material. i.e., if  = 0.01, ~1% of the incident photons are absorbed by the material, with ~99% getting through. What if  is larger?

  17. Opacity and  Each narrow slab of material absorbs a small percentage of the incident photons. Over many layers, photon flux diminishes more and more. Differential (negative) change in flux, dI in going through any thin slab of optical depth d is proportional to the incident flux I: dI = - I d --> dI/I = -d

  18. Opacity and  dI/I = -d Solve differential equation: ∫ dI/I = ∫ -d ln I - ln I0 = -  ln (I/I0) = - I = I0 e- Incident flux I0 on left side of slab diminishes exponentially with increasing 

  19. (E) = σ(E) NH, where σ(0.07, 0.25, 1, 10 keV) = (600, 40, 2, 0.1) x 10-22 cm2 In highly obscured AGN, with (intrinsic) absorption exceeding 1023 cm-2, virtually all photons below 3 keV are absorbed  ROSAT missed highly obscured AGN! Galactic Hydrogen Column Density NH Galactic NH ranges from 7 x 1019to 2 x 1022 cm-2across the sky. Dickey & Lockman 1990

  20. Effects of absorption on a pure power law spectrum. Effect of NH on X-ray Spectra Courtesy Neil Brandt notes

  21. X-ray Telescopes Cosmic Perspective, Bennett et al. Earth’s atmosphere is opaque to X-rays  need to get above atmosphere 1960s/70s – balloon/sounding rocket experiments demonstrated that the sky was X-ray active

  22. X-ray Telescopes Cosmic Perspective, Bennett et al. 1970s/80s – non-imaging (UHURU), and early imaging (Einstein, EXOSAT, Ginga) X-ray satellites – limited spatial/spectral resolution 1990s – X-ray telescope technology begins to expand with first soft X-ray all-sky survey (ROSAT), first X-ray detector utilizing CCDs (ASCA), as well as joint hard/soft X-ray detectors (BeppoSax) and X-ray timing experiments (RXTE)

  23. Current X-ray Telescopes NASA’s Chandra X-ray Observatory Launch: July 23, 1999 Energy: 0.2 – 10 keV ACIS - Advanced CCD Imaging Spectrometer (10 CCDs) + high resolution gratings (LETG, HETG) HRC - High Resolution Camera (timing) BY FAR the best spatial resolution of any X-ray telescope Spatial resolution: 0.5”

  24. Instruments of Chandra ACIS-I/S have timing resolution of 3.2 seconds, and spectral resolution of E/ΔE = 10–20 HRC has timing resolution of 16 microseconds HETG/LETG have coverage of 0.1 – 10 keV and an energy resolution of E/ΔE = 100–1000

  25. X-ray (pre-Chandra) vs. Optical ROSAT PSPC Digital SkySurvey NGC4697

  26. ROSAT vs. Chandra 30’’ resolution 0.5’’ resolution NGC4697

  27. Current X-ray Telescopes ESA’s XMM-Newton X-ray Observatory Launch: December 10, 1999 Energy: 0.2 – 12 keV EPIC – European Photon Imaging Cameras (3 separate instruments with CCDs) RGS – Reflection Grating Spectrometers (E/ΔE = 300) Largest collecting area of any telescope in 0.2-10 keV range – currently 9x that of Chandra Spatial resolution: 6”

  28. Instruments of XMM-Newton EPIC: 2 MOS detectors s and a PN detector: CCDs with E/ΔE = 200–800 at 6.5 keV 2 Reflection Gratings Spectrometers (RGS): 0.4 – 2.5 keV coverage, E/ΔE = 200–800 Optical Monitor (OM) – 30-cm telescope with 6 filters covering 1800–6000 Angstroms. Limiting magnitude B ~ 24.

  29. Current X-ray Telescopes NASA’s Neil Gehrels Swift Observatory Launch: November 20, 2004 XRT – CCD sensitive in 0.2 – 10 keV band) UVOT – simultaneous ultraviolet/optical coverage Capable of fast slew to position of GRBs. Also suitable for short monitoring observations. Spatial resolution: 18”

  30. Current X-ray Telescopes NASA/Caltech’s NuSTAR Telescope Launch: June 13, 2012 Sensitive in 3 – 79 keV energy range First telescope capable of imaging at hard (>10 keV) X-ray imaging. Harrison et al. 2013 Spatial resolution: 58”

  31. X-ray Telescopes Comparison Harrison et al. 2013 2007 X-ray Astronomy School - K. Arnaud

  32. Future X-ray Telescopes JAXA/NASA’s X-ray Recovery Mission (XARM) Expected Launch: 2021 RESOLVE – soft X-ray calorimeter spectrometer (0.3 – 12 keV) – energy resolution of 5 – 7 eV (R = 1000 at 6 keV) XTEND – soft X-ray imager in the 0.4 – 13 keV energy range Hopefully the first X-ray microcalorimeter to survive more than a few days!

  33. Future X-ray Telescopes NASA’s Midex Mission Arcus Expected Launch: 2023 (if approved) Large collecting area gratings (0.25 – 1 keV) – energy resolution of R ~ 3000

  34. Future X-ray Telescopes ESA’s Athena Observatory Expected Launch: early 2030s 5” spatial resolution 3 eV spectral resolution 13x collecting area of XMM-Newton Effectively, super-XMM with a spectral calorimeter.

  35. Future X-ray Telescopes NASA’s Lynx Observatory Expected Launch: 203? 0.5” spatial resolution 3 eV spectral resolution 50x collecting area of Chandra Everything you could ever want in an X-ray telescope, but will it ever come to fruition?

  36. X-rays must be “skipped” in at shallow angles (“grazing incidence”) – very long focal lengths Single photon counters – position, energy, time of each photon is recorded (much unlike optical telescopes)  get spatial, spectral, timing information simultaneously. X-ray Telescope Optics

  37. X-ray Detection – Spatial Centaurus A Perseus Galaxy Cluster M31 Chandra Science Center images

  38. X-ray Detection – Spatial Supernova remnant Cas A blue – Ti-44 map Elliptical galaxy NGC4649 in 0.3 – 2.0 keV (upper) and 2.0 – 7.0 keV (lower) energy band. X-ray surface brightness profile of galaxy cluster Abell 3667

  39. X-ray images can be affected by: – CCD chip gaps – vignetting – worsening off-axis PSF X-ray Detection – Spatial

  40. X-ray Detection – Spectral X-ray spectra can be obtained for any source (or region of a spatially extended source) for which a sufficient number of counts can be obtained. Elliptical galaxy NGC 4374 AE Aquarii - Oruru & Meintjes (2012)

  41. X-ray Detection – Spectro-Spatial Temperature map of galaxy cluster Abell 1914 Fe abundance map of M87 – Owen et al. (2000)

  42. Since each photon arrival time is recorded, timing analysis can be done traditionally (# of photons in fixed time bins), a power density spectrum, or on a photon-by-photon basis: X-ray Detection – Timing http://science.psu.edu/alert/images/Circinus_graph300.jpg Gao et al. 2017 Irwin et al. 2016

  43. Spectra can be obtained as a function of X-ray intensity level by separating an observation into time intervals based on the X-ray count rate during that interval. X-ray Detection – Timing Enoto et al. 2014

  44. Method #1: Write a telescope observing proposal! Call for proposals happens usually once a year: – GO, or Guest Observer proposals – ToO – Target of Opportunity – TC – Time-constrained observations (or coordinated with other observatories) Most observatories have Director’s Discretionary Time (DDT) that can be requested throughout the year, for compelling or particularly time crucial observations. How Can I Acquire X-ray Data?

  45. Method #1: Write a telescope observing proposal! What makes a good X-ray observing proposal? A compelling, and to-the-point science case – don’t get bogged down in details, think ‘big picture’ 2) Technical feasibility is crucial – panel will not accept proposals that they think cannot be accomplished with the requested exposure time 3) Have good secondary science – can you accomplish more than one goal with the observation? How Can I Acquire X-ray Data?

  46. Method #1: Write a telescope observing proposal! What makes a good X-ray observing proposal? 4) Remember, reviewers are looking at a lot of proposals at once (up to 70 for Chandra proposals). Do not assume they will have time to appreciate very subtle points that are crucial to your argument. Be blunt. How Can I Acquire X-ray Data?

  47. Method #1: Write a telescope observing proposal! Usual Deadlines: Chandra: mid-March XMM-Newton: early October Swift: late September NuSTAR: late January How Can I Acquire X-ray Data? Most X-ray observatories have joint request avenues with the other X-ray observatories.

  48. Method #2: Download data from the data archive Unlike some ground-based observatories, X-ray data becomes publically available after a (usually) 12-month proprietary period. Photon-by-photon listing of photon position, energy, and arrival time stored in an event file. Other secondary files may be needed to analyze the data properly. How Can I Acquire X-ray Data?

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