Transcript Spectral Line VLBI

Spectral Line VLBI
Alison Peck
SAO/SMA Project
Alison Peck, Synthesis Imaging Summer
School, 20 June 2002
Spectral Line Observations
Galactic and Extragalactic Line Absorption – HI, OH
Extragalactic Megamasers – H2O, OH
Galactic Masers – SiO, H 2O, OH, methanol
Alison Peck, Synthesis Imaging Summer
School, 20 June 2002
Similarities to Continuum Observations
Scheduling an experiment, you will need:
At least one “fringe finder”
One amplitude calibrator
One bandpass calibrator
And possibly:
One phase reference calibrator (very nearby)
(if using phased VLA) one VLA phase calibrator
Alison Peck, Synthesis Imaging Summer
School, 20 June 2002
Similarities to Continuum Observations
Amplitude calibration: use measured Tsys recorded in telescope
logs, and a priori measurements of antenna gain (APCAL)
(Can also use auto-correlation spectra)
Estimate initial delay and rate errors using bright calibrator
(FRING, KRING), (but can’t record pulse cal signal)
Solve for residual delays and rates using target source (if bright
enough) or phase reference calibrator (“global” FRING)
Alison Peck, Synthesis Imaging Summer
School, 20 June 2002
Spectral Line Observations
Correlators are intrinsically spectral line, so
can do imaging and spectroscopy simultaneously
Spectral resolution is a function of bandwidth
and number of lags
Maser components are frequently very
narrow (0.5 km/s) while absorption features
can be very broad (>300 km/s) so need
wide range of spectral resolution and bandwidths
Hanning smoothing might be required to prevent
“ringing” on sharp features
Peck et al 2002
Alison Peck, Synthesis Imaging Summer
School, 20 June 2002
Main Differences
Bandpass calibration more important
When looking for weak lines toward strong continuum source
Doppler tracking
Need to remove frequency offset introduced at each
station by rotation and revolution of Earth
Continuum Subtraction
Only necessary if strong continuum present
Fringe fitting
Different techniques if no strong continuum source present
Giant data sets
Need to keep short integration times to avoid time smearing
Alison Peck, Synthesis Imaging Summer
School, 20 June 2002
Galactic HI Absorption
3C138
Can determine small
scale structure of HI
in our Galaxy
Several lines of sight through
gas on AU scales
Can estimate density based
on optical depth
Faison et al 1998
Alison Peck, Synthesis Imaging Summer
School, 20 June 2002
Extragalactic
HI Absorption
Circumnuclear torus seen in HST image
also detected in absorption on one
EVN baseline
Absorption appears only toward
counterjet
First scientific observation processed
at the new JIVE correlator
image courtesy Ylva Pihlstroem
Alison Peck, Synthesis Imaging Summer
School, 20 June 2002
Extragalactic HI Absorption
Peck and Taylor 2001
Alison Peck, Synthesis Imaging Summer
School, 20 June 2002
OH Emission and Absorption
18 cm observations toward IIIZw35
Masers lie in a ring of radius r~22pc
Enclosed mass m~7x106 M solar
Velocity field confirms rotation
Absorption lies outside maser ring
(Pihlstroem et al 2001)
image courtesy Ylva Pihlstroem
Alison Peck, Synthesis Imaging Summer
School, 20 June 2002
Bandpass Calibration
Linear fit across bandwidth on strong calibrator (BPASS)
Use cross-correlation spectra to correct for phase as well as amplitude,
(do fringe-fitting first to remove residual errors)
Need strong continuum source at same frequency as target source
Better to use unresolved calibrator, but can use model of CLEAN components
if calibrator is resolved
Can also do polynomial fit, but risky… (CPASS)
Could also use auto-correlation spectra, but doesn’t correct phase
Watch out for RFI!
Alison Peck, Synthesis Imaging Summer
School, 20 June 2002
Doppler Tracking
±30 km/s
±0.5 km/s
plot courtesy Mark Reid
Alison Peck, Synthesis Imaging Summer
School, 20 June 2002
Continuum Subtraction
Can be done in u,v plane using line-free channels on both sides
of line features (UVLSF, UVLIN)
Can also be done in image plane using model of clean components
made from averaged line-free channels
(might be preferable if source structure complicated)
Not necessary for many maser sources
Alison Peck, Synthesis Imaging Summer
School, 20 June 2002
VLBI Maser Studies
Extragalactic megamasers in accretion disks and a
few jet-cloud interactions
Galactic masers in star forming regions and
photospheres of AGB stars
Lines can be very narrow but cover large velocity
range
Masers can be spread over large field
Frequently very little continuum emission in
target sources
Alison Peck, Synthesis Imaging Summer
School, 20 June 2002
Fringe fitting
Removing residual delays and rates (FRING)
plots courtesy Mark Reid
Can use phase referencing to phase reference calibrator
or strong, narrow spectral feature
Alison Peck, Synthesis Imaging Summer
School, 20 June 2002
Fringe rate mapping
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qy (arcsec)
Large fields with many spectral
channels can be very CPU intensive
to image
Fringe rate mapping permits
determination of the location of the
emission without having to image
the whole field (FRMAP)
Fringe fitting in one spectral channel
at an instant on one baseline
constrains the location of the
dominant emission feature to a “line”
on the sky
Can use one scan on several baselines
or several scans on one baseline, so
also useful for pinpointing origin of
emission in single baseline observations
0
-1
-2
-2
-1
0
qx (arcsec)
Alison Peck, Synthesis Imaging Summer
School, 20 June 2002
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Molecular Accretion Disks
Considered best evidence of
a supermassive black hole
Can estimate central mass
Can estimate distance to
host galaxy
image courtesy Lincoln Greenhill
(see Miyoshi et al 1995
Herrnstein et al 1999)
Alison Peck, Synthesis Imaging Summer
School, 20 June 2002
Calibration using Auto-correlation
Spectra
Use template from most sensitive telescope (ACFIT)
Pros:
Relative amplitude calibration good to
within 1%
corrects for pointing errors on individual
telescopes
Cons:
Absolute calibration depends on accuracy
of flux density scale for template
plot courtesy Mark Reid
Need a strong line source for good SNR
Requires some “off” time, but can use
scans on continuum calibrators
RFI can be a big problem
Alison Peck, Synthesis Imaging Summer
School, 20 June 2002
Motions of Water Masers near IRAS
05413-0104
Young stellar object at a distance
of 450 pc
Observations made in four epochs
over 10 weeks
Claussen, Marvel, Wooten & Wilking 1998
image courtesy of Kevin Marvel
Alison Peck, Synthesis Imaging Summer
School, 20 June 2002
AGB Stars
Maser emission provides a
means to detect these shells
when star is obscured by dust
Can probe dynamics of the gas
in shells
Can also determine Galactic dynamics
using very accurate positions and
velocities (i.e. VERA project)
image courtesy Lorant Sjouwerman
Alison Peck, Synthesis Imaging Summer
School, 20 June 2002
TX Cam Movie
44 VLBI observations
over nearly 2 years
(Diamond and Kemball 2002)
movie courtesy Athol Kemball
and Phil Diamond
Alison Peck, Synthesis Imaging Summer
School, 20 June 2002
6.7-GHz methanol in G339.88-1.26
image courtesy Chris Phillips (Phillips et al 2002)
Methanol masers observed using Australian VLBI
network
Masers probably tracing shock rather than disk
Alison Peck, Synthesis Imaging Summer
School, 20 June 2002
Spectropolarimetry
Information about magnetic fields
image courtesy Crystal Brogan
Alison Peck, Synthesis Imaging Summer
School, 20 June 2002