When Last We Met 1eV Hot gas in ISM or Halo Unresolved galaxy - PowerPoint PPT Presentation

When Last We Met…

1eV Hot gas in ISM or Halo Unresolved galaxy clusters & AGN Coronae CRB CMB CIB COB CXB EGB

Cosmic X-ray Background  Discovered by rocket flight in 1962  Equivalent to 10% of the CMB  Below 10 17 Hz, X-rays follow the plane of the Galaxy  From hot gas in Galaxy  Stellar coronae, Galactic halo  T ≈ 10 6 K  Above 10 17 Hz, X-rays are isotropic ⇒ Unresolved point sources  AGN accretion disks (doppler boosted)  Hot gas in galaxy clusters  Starburst galaxies

Unresolved Active Galaxies Cutoff uncertain CRB CMB CIB COB CXB EGB

Extragalactic Gamma-ray Background  Measurement in progress (unavailable before 2008)  Unresolved AGN jet emission + Local CR interaction with Galactic hydrogen  Compton spectrum  Record of galaxy formation and evolution  FSRQs, BL Lacs, at cosmological distances  High-energy cutoff due to intergalactic absorption/ downscattering

Universe runs out of Energy CRB CMB CIB COB CXB EGB

Astrophysical Sources and Backgrounds Astr288C Lecture 2

Data has Signal + Background + Noise  Any observation has multiple components  Signal = thing you are interested in  Background = anything astrophysical that makes it harder to detect the signal  Noise = anything instrumental that makes it harder to detect the signal  Step 1 = Remove the noise  Step 2 = Subtract the background  Step 3 = Measure the signal

Noise Sources  Noise is present for any attempt to make a measurement  Instrumental (e.g. CCD readout)  Environmental (e.g. Radio Frequency Interference)  Statistical Noise (Probablilistic)  To de-noise the data, you must  Determine the noise signature  Modify the data to minimize that signature  NOTE: The weaker your signal, the more you have to consider noise in your data analysis

200 Hz RFI Third 60 Hz AC RFI Harmonic of 60 Hz Second Harmonic of 200 Hz Arecibo Observatory RFI plot

Sources of Background  Background is anything astrophysical that gets in the way of making your measurement  Foreground absorption reducing signal  Background emission overwhelming source  Foreground sources biasing measurement  Continuum flux affecting spectral measurement  Other?  Remember: One person’s signal is invariably another person’s background  I.E. Data can be used for multiple purposes

Background Subtraction  You can remove background in a number of ways  Measure/fit the background and subtract  Model the background and subtract  Fit the background and source simultaneously

Fermi-LAT data + + Point Sources Galactic Diffuse Isotropic Background

What signal to look for?  The signal you are looking for depends on the kind of science you are doing  Point source  Extended source  Diffuse source Plus combinations of  Spectral signature  Temporal signature some of these  Anisotropy  Others?

Point Sources  Point-like sources have a number of characteristics measurable in a single waveband  Position (measured in RA/Dec)  Also proper motion, parallax  Time of measurement (measured in JD, MJD, seconds past epoch, etc.)  Flux (energy units)  Distance (light years, parsecs, redshift)  Spectral signature (flux per unit energy)  Also spectral shape, doppler shift  Temporal signature (per unit time)  Polarization

Point Source Classes  Galactic:  Planets, planetoids & planetessimals, stars & brown dwarfs, stellar remnants, x-ray binaries, microquasars, etc.  Extragalactic:  Distant galaxies, distant clusters, active galaxies, (super)novae, variable stars, intermediate-mass black holes, etc.  Cosmological:  High-redshift AGN, gamma-ray bursts, rapid radio transients

Classes of Extended Sources  Galactic:  Nebulae (star-forming, reflection, planetary, etc.), Supernova remnants, pulsar wind nebulae, molecular clouds, globular clusters, Galactic gas, Galactic dust, etc.  Extragalactic:  Nearby galaxies, radio galaxies, galaxy clusters, intercluster medium, AGN jets and lobes, superclusters, etc.  Cosmological:  Any?

Position Measurements  Need a reference frame  Earth is convenient  “Equatorial” or “Celestial” Coordinates  Gives measurements in Right Ascension and Declination

Equatorial system  Right Ascension (equivalent to longitude)  Goes eastward from 00 h 00 m 00 s (spring equinox) to 23 h 59 m 59.9 s  Declination (analagous to Latitude)  Goes from -90 ° 00 m 00 s (south celestial pole) to +90 ° 00 m 00 s (north celestial pole)

Conversions  Often, (RA, Dec) positions are given in decimal degrees  To convert, remember:  60 seconds in a minute  60 minutes in an hour  24 hours in a day  So, 24 hours in RA = 360°  Therefore 1 hr RA = 15°  Conversion Tool available online: http://heasarc.gsfc.nasa.gov/cgi-bin/Tools/convcoord/convcoord.pl

Other Systems Alt-Az Ecliptic Coordinates Coordinates (0 = North) (0 = Vernal Equinox) Galactic Coordinates (0 = Galactic Center)

Position Epochs  Positions are given with a specific epoch  Needed due to the precession of the pole (26,000yr period)  Current epoch is J2000 (position as if in year 2000)  Previous epoch was B1950  Positions can change with time  Parallax  Due to Earth’s orbit  Proper motion  Transverse motion of the source  Conversion tool available online http://heasarc.gsfc.nasa.gov/cgi-bin/Tools/convcoord/convcoord.pl

Time  Time of measurement is important  Variability studies  Periodicity signatures  Event direction (for some detectors)  Duration of observation is also important  Integration time is used to calculate flux  Need a reference time (time epoch)  Usually use Universal Time (UT , UTC, GMT , Z)  Also use Julian Day (JD) - # days since Noon, 1 Jan 4713 BC  Some instruments have non-standard epochs  Conversion Tool available online http://heasarc.gsfc.nasa.gov/cgi-bin/Tools/xTime/xTime.pl

Flux & Luminosity  Energy per unit area, per unit time  How much energy from the source crosses the plane of the detector integrated over a given amount of time?  If distance to source is known, flux ⇒ luminosity  Distance can be measured by:  Parallax, Absorption/dispersion measures, Standard candles, Redshift, etc …  Nice compilation available at: http://www.astro.ucla.edu/~wright/distance.htm

Broad-band Spectra  Different emission mechanisms produce radiation in different wavebands  Thermal (Stars, Dust, etc.) ⇒ depends on temp  Typically infrared, optical, UV , soft X-rays  Synchrotron (strong magnetic field) ⇒ depends on particle speed and radius of curvature  Typically radio and microwave  Compton upscattering (external seed photons) ⇒ depends on particle energy and photon energy  Hard X-rays and gamma rays  Particle decay ⇒ depends on the particle

Multi-messenger data sets

Variability  Flux changes with time are important probes of source structure and emission mechanisms  One-time/infrequent events  Gamma-rays bursts, (super)novae, lensing, occultations, etc.  Rapid reporting leads to follow-up observations  Multi-wavelength characterization  Persistent time-variable sources  Pulsars, Active Galactic Nuclei, Binary Stars, Eclipsing systems, etc.  Provides information about system geometry, masses, size and configuration of emission regions, particle propagation, and more

Which Observatory?  Depends on the waveband

Diffuse Sources  Some “sources” extend across the entire visible sky  Milky Way Galactic structure  Extragalactic Background  Cosmological Sources  Characterizing these sources requires observations from multiple hemispheres, or from space

Planck all-sky map