Dual frequency interferometry and phase transfer at the Submillimeter Array

Todd R. Hunter, Jun-Hui Zhao (CfA) Sheng-Yuan Liu, Yu-Nung Su, Vivien Chen (ASIAA)

Summary of present SMA operationsAntennas: 8, diameter 6m, surface rms: 12 to 20 micronsBaselines: 28, from 16m to 500m (angular scales 0.3” to 15” @ 345 GHz)Receivers: DSB, 1-polarization, 3 bands: 180-245, 260-355, 620-695 GHz Correlator: 2 GHz BW per SB: 3072 channels x 2 sidebands x 2 receiversNext proposal deadline: March 2006 (http://sma1.sma.hawaii.edu)

Lack of Strong Gain Calibrators at High-Frequency

SMA sensitivity: Tsys ~ 100 K at 230 GHz (10 mJy in 5 min)(6-antennas) Tsys ~ 2000 K at 690 GHz (200 mJy in 5 min)

For good phase solutions, we need S/N > 10 per baseline

0.5 Jy at 230 GHz (70 quasars with F > 1Jy)8 Jy at 690 GHz (maybe 2 or 3 quasars)

This requires astrong source! {

Quasars are inadequate for the SMA at 690 GHz

Interferometry requires calibration of antenna-based phase & amplitude introduced by instrumental and atmospheric effects.

Typical flux density Typical230 GHz 658 GHz Diameter

Callisto 4.3 Jy 33 Jy 1.1”Ganymede 4.3 32 1.2”Titan 1.6 11 0.9”Ceres 1.4 10 0.5”Vesta 1.1 9 0.5”Pallas 0.6 5 0.3”

Other options in 690 GHz band: Minor planets

• These objects work adequately if one of them is available

synthesized beam in compact configuration: 1.1”

Other options in 690 GHz band: Water masers

658 GHz vibrationally-excited water line seen in oxygen-rich stars: (Menten & Young 1995)

(v2=1, J=110-101) 2328 K above the ground state

13 known sources (so far): VYCMa, RLeo, WHya, VXSgr, RCas, TXCam, RCrt, RTVir, RXBoo, SCrB, UHer, NMLCyg, NMLTau, RAql

Good candidates not yet searched: Mira, Betelgeuse, etc.

658 GHz H2O line in R Leo (1 hour, 15 baselines)

658 GHz H2O line in R Cas (2 hours, 15 baselines)

The same stars also show masers in 215 GHz SiO (5-4) v=1 line

Masers are detectable in both bands on short timescales (30 seconds) and make good targets for testing phase transfer

baseline = 59 meters

68 meters

45 meters

16 meters

25 meters25 meters

Phase transfer: hardware considerations

10 MHzCommon reference frequency

LO 1

ReceiverFeed 2

690 GHz

ReceiverFeed 1

230 GHz

LO 2

IF 1

IF 2 Correlator 2

Correlator 1

Co-aligned receiver feeds (< 6” or 1/10 beam @ 230)

Duplicate paths for simultaneous down-conversionand correlation

YIG,DDS

Antenna

Fundamental components of the SMA dual-band system:

Expected relationships for phase & amplitude

Effect on Fringe Phase The change in excess path length (L) is a function of the observing

frequency and leads to a ratio (RP) in the observed phase changes: RP = (L2/2) / (L1/) = (L2/L1) * (2/1) Using Scott Paine’s am model for Mauna Kea: = (0.9733) * (658.006 / 215.595) = 2.97

Consider a small change in atmospheric water vapor content above one antenna relative to another:

Effect on the Fringe Amplitude The change in opacity () is a function of the observing frequency and

elevation and leads to a ratio (RA) in the observed amplitude changes: RA = [exp(-2,wet) / exp(-1,wet) ] * exp(1,wet – 2,wet) for 0.4 mm water vapor and 658 vs. 215 GHz = about 3.5 (but is a function of airmass)

1. Strong, compact source (e.g. maser) to compare 215 GHz and 658 GHz phase and amplitude relationships

2. Science target 3. Quasar near the science target, bright enough at 2154. Strong continuum source to obtain bandpass information

in both bands (658: Callisto, Ganymede, lunar limb)

Proposed Observational strategy for “Phase transfer”

Observe four sources:

Antenna-based solutions on R Cas masers (658 vs 215 GHz phases)

Antenna 1(reference)

Antenna 2m = 2.6

Antenna 6m = 2.1

Antenna 3 m = 3.4

Antenna 5m = 4.7

Phase jump

Example: phase transfer on a calibrator658 GHz phase transfer image

(S/N=21, 0.3 beam offset)

Apply coefficientsfrom R Cas

to make690 gain

table

Quasar 3C454.3230 GHz self-cal image

658 GHz self-cal image (S/N=27) Self-cal recovers S/N=27

5 antennas,35 minuteson-source,

F ~ 6 Jy(beam =1.3”x1.1”)

658 GHz self-cal image of quasar 2232+117 (S/N = 16)

658 GHz phase transfer image (S/N = 11, 0.4 beam offset)

Self-cal recovers S/N = 16

Example: phase transfer on a “science target”

Take the 690 gain table derivedfor 3C454.3 (from R Cas) and apply it to the raw data for the science target, in this case another fairly bright quasar (2232+117)

5 antennas,30 minuteson-source,

F ~ 4 Jy

What limits this method of calibration?

1. Phase jumps and drift

Sometimes seen in one band only, sometimes both. Under investigation. Also, slow changes in phase offset with time between the two bands may require frequent measurement of phase transfer coefficients.

2. Bandpass determination in extended configs.No compact sources (< 0.4”) are bright enough ! Lunar & planetary limbs sometimes give signal, but are not ideal.Hardware noise source to measure bandpass phases (in progess) Autocorrelation on the ambient load for amplitudes?

Summary and future workWe have made a first attempt at phase transfer at submillimeter frequencies. Need to investigate some remaining instrumental problems. Then try the technique in more extended configurations.

New receiver band is coming! (320-420 GHz) Will allow more frequent dual-band observations (due to less stringent weather requirements at 230/345) and higher S/N testing of phase transfer.

Water vapor radiometry? Two ALMA prototypes being tested at SMA

Conclusion: The SMA is a path-finding instrument and we remain hopeful to realize its full potential.

5 second integration on a 123 meter baseline 226 meter baseline

658 GHz maser easily detectable on long baselines

22 GHz water masers in R Cas imaged by the VLA

0.2 arcsec

Ceres 690 Selfcal

Example #0: phase transfer using Ceres (on itself)

Ceres phase transfer imageCeres 690 uncalibrated data

Derivecoefficients

and 690 gain table

Apply690 gain

table

Ceres 230 Selfcal (rms = 50 mJy, S/N = 260)

(rms = 70 mJy, S/N = 193)Proof of software function

+

The phase transfer analyses in the previous slides were done in Miriad. Here is an example done in MIR / IDL (see poster 4.69 by Su & Liu).In this case, the frequency ratio (rather than the fit) was used in the scaling.

Phase transfer from quasar 1743-038

Example #2: phase transfer on IRAS 16293-2422

Direct 690GHz calibration (Ceres)

Linear fit of Ceres 690 phase vs 230 phase

Antenna Correlation Slope 1 0.97 3.0 2 0.85 2.3 3 0.95 2.4 4 0.88 2.3 5 0.94 3.2 6 reference antenna theory 1.00 3.0

Antenna Correlation Slope 1 0.94 3.1 2 0.72 2.0 3 0.85 2.0 4 0.83 2.2 5 0.85 3.3 6 reference antenna theory 1.00 3.1

USB data

LSBdata

Investigation of “Phase transfer” Part II. Search for phase relationships

1. Run phase-only selfcal on calibrator at 230 & 690

2. Examine correlations of 690 vs 230 phase solutions

3. Flag any phase jumps or unstable periods that degrade the correlation

4. Compute slope and offset relating 230 and 690 phases on each antenna

Eight nights with low opacity during the recent 690 GHz Campaign

Jan 28: W Hya / Ceres / CallistoFeb 14: VY CMa / Titan / 0739+016

Mars / Ceres / NRAO 530Feb 15: Orion-KL / Titan / 0607-157

Arp220 / Ceres / Callisto / MarsFeb 16: G240 / VY CMa / Titan / 0736+017

Sgr A* / Ganymede / 1924-292 / SgrB2NFeb 17: TW Hya / Callisto / 1037-295Feb 18: CRL618 / 3C111 / Titan / Callisto

IRAS16293 / Ceres / 1743-038 / Callisto Feb 19: Orion KL, Sgr A* (repeat) Mar 02: Arp220 / Ceres / Callisto / Mars

Appendix A: Phase noise measurements

• Antenna 230 GHz 690 GHz• 1 10.8 deg 33.6 deg• 2 9.3 27.2• 3 10.2 25.2• 4 11.8 32.0• 5 11.2 30.7• 6 11.1 30.0• 7 12.3 24.8

• Integration range: 100Hz to 10MHz from the 6-8GHz YIG carrier

January 28, 2005: First dual-IF fringes

SiO J=5-4, v=1 at 215 GHz H2O 1

1,0-1

0,1 v=1 at 658 GHz

Simultaneous maser lines from W Hydra

These screens show only 2% of the total correlator data product.

First Dual-IF Phase vs. Time solutions 215 GHz maser in LSB 658 GHz maser in USB

Ant 1

Ant 3

About 3x larger phase change and opposite sign (as expected)

Ant 1

Ant 3

2 hours

360o360o

Antenna 690 vs 230 Rx pointingNumber A1(arcsec) A2(arcsec) 1 -2.7 +7.0 2 -0.9 +1.6 3 +4.1 -0.8 4 +0.3 +1.0 5 +4.8 -3.4 6 +4.4 +3.4 7 -3.5 +6.8

By comparison, the SMA primary beam at 230 GHz is 56 arcsec. The worse case antenna is better than 1/7 beam.

Receiver “Feed offsets” measured by single-dish radio pointing with the chopping secondaries