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

When Last We Met…

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

Cosmic X-ray Background

 Discovered by rocket flight in 1962  Equivalent to 10% of the CMB  Below 1017Hz, X-rays follow the plane of the Galaxy

 From hot gas in Galaxy

 Stellar coronae, Galactic halo

 T≈106 K

 Above 1017Hz, X-rays are isotropic ⇒ Unresolved point

sources  AGN accretion disks (doppler boosted)  Hot gas in galaxy clusters  Starburst galaxies

CRB CMB CIB COB CXB EGB Unresolved Active Galaxies Cutoff uncertain

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

CRB CMB CIB COB CXB EGB Universe runs out of Energy

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

60 Hz AC RFI 200 Hz RFI Second Harmonic

• f 200 Hz

Third Harmonic

• f 60 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

+ + Point Sources Galactic Diffuse Isotropic Background

Fermi-LAT data

What signal to look for?

 The signal you are looking for depends on the kind

• f science you are doing

 Point source  Extended source  Diffuse source  Spectral signature  Temporal signature  Anisotropy  Others? Plus combinations of some of these

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

00h00m00s (spring equinox) to 23h59m59.9s

 Declination

(analagous to Latitude)  Goes from -90°00m00s

(south celestial pole) to +90°00m00s (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 Coordinates (0 = North) Ecliptic Coordinates (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

• f 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