Intraday variability of Sgr A* at radio wavelengths:A Day in the Life of Sgr A*

Intraday variability of Sgr A* at radio wavelengths:A Day in the Life of Sgr A*

Doug RobertsNorthwestern University

Adler Planetarium & Astronomy Museum

Collaborators Collaborators

• Farhad Yusef-Zadeh – Northwestern University

• Geoff Bowers – University of California Berkeley

• Craig Heinke – Northwestern University

Motivation: CampaignMotivation: Campaign

• Started as part of multi-wavelength campaign in March 2004.

• Follow-up observations of Sgr A* and nearby X-ray/radio transient were done throughout 2004-05.

Motivation: Previous determinations of radio variability

Motivation: Previous determinations of radio variability

• Long term radio variability – 106 day variability 15 & 22 GHz (Zhao et al. 2001)– 2.5 yr observations at 15, 22 & 43 GHz suggest

bimodal flux densities and spectral indices (Herrnstein et al. 2004)

• Hourly variability at high frequencies:– 1.5 hour (x 2 flux) flare at 100 & 140 GHz (Miyazaki et

al. 2004)– 2.5 hour variability at 100 GHz (Mauerhan et al. 2005)

Radio observations used various telescopes (5-83 GHz)

Radio observations used various telescopes (5-83 GHz)

ATCA (17, 19 GHz)

VLA (5, 22, 43 GHz)

BIMA (83 GHz)

Observations designed to observe intraday variability Observations designed to

observe intraday variability

• All observations were done with frequent pointings between Sgr A* and a nearby (3º) calibrator (cycled between 90 seconds on source and 30 seconds on calibrator).

• Less frequent observations of a strong calibrator further away (10º).

• Pointing scans every hour.• Tipping scan to determine the atmospheric

opacity at the beginning of the run.

Lots of data! Lots of data! Date Telescope Freq Start time End time Duration Sampling Average Flux2004.03.28 BIMA 83 GHz 11.3 hr 15.8 hr 4.5 hr ?? min 1.5094 +/- 0.1812 Jy2004.03.29 BIMA 83 GHz 11.8 hr 15.8 hr 4.0 hr ?? min 0.8697 +/- 0.1568 Jy2004.03.30 BIMA 83 GHz 11.3 hr 14.8 hr 3.5 hr ?? min 0.4032 +/- 0.2000 Jy2004.03.31 BIMA 83 GHz 11.3 hr 15.8 hr 4.5 hr ?? min 0.6835 +/- 0.2080 Jy2004.04.01 BIMA 83 GHz 11.3 hr 15.8 hr 4.5 hr 15 min 1.1266 +/- 0.1240 Jy2004.03.28 VLA-CnB 43 GHz 12.2 hr 13.7 hr 1.5 hr 2 min 1.5797 +/- 0.0767 Jy2004.03.29 VLA-CnB 43 GHz 10.7 hr 15.1 hr 4.4 hr 2 min 1.8249 +/- 0.0317 Jy2004.03.30 VLA-CnB 43 GHz 12.1 hr 13.6 hr 1.5 hr 2 mn 1.6906 +/- 0.0604 Jy2004.03.31 VLA-CnB 43 GHz 10.5 hr 15.0 hr 4.5 hr 2 min 1.7098 +/- 0.0389 Jy2004.06.11 VLA 43 GHz 5.8 hr 9.3 hr 3.5 hr 2 min 1.5933 +/- 0.0354 Jy2004.06.12 VLA 43 GHz 5.8 hr 9.2 hr 3.4 hr 2 min 1.2938 +/- 0.0237 Jy2004.06.13 VLA 43 GHz 5.7 hr 9.2 hr 3.5 hr 2 min 1.4181 +/- 0.0336 Jy2004.07.06 VLA 43 GHz 4.7 hr 9.2 hr 4.5 hr 2 min 1.3658 +/- 0.0614 Jy2004.07.07 VLA 43 GHz 4.6 hr 9.0 hr 4.4 hr 2 min 1.7617 +/- 0.1082 Jy2004.07.08 VLA 43 GHz 4.7 hr 9.0 hr 4.3 hr 2 min 1.4978 +/- 0.0913 Jy2004.09.01 VLA 43 GHz 0.5 hr 3.9 hr 3.4 hr 2 min 1.4486 +/- 0.1612 Jy2004.09.02 VLA 43 GHz 0.4 hr 3.8 hr 3.4 hr 2 min 1.5058 +/- 0.0819 Jy2004.09.03 VLA 43 GHz 0.3 hr 3.8 hr 3.5 hr 2 min 1.5997 +/- 0.0366 Jy2004.09.04 VLA 43 GHz 0.3 hr 3.7 hr 3.4 hr 2 min 1.8571 +/- 0.2532 Jy2005.02.10 VLA 43 GHz 13.8 hr 17.6 hr 3.8 hr 2 min 1.6541 +/- 0.0543 Jy2005.02.11 VLA 43 GHz 14.0 hr 17.2 hr 3.2 hr 2 min 2.1744 +/- 0.0572 Jy2005.02.10 VLA 22 GHz 14.0 hr 17.7 hr 3.7 hr 2 min 1.1223 +/- 0.0217 Jy2005.02.11 VLA 22 GHz 13.8 hr 17.5 hr 3.7 hr 2 min 1.2586 +/- 0.0185 Jy2004.03.28 ATCA 19.5 GHz 13.6 hr 25.3 hr 11.7 hr ?? min 1.4683 +/- 0.0591 Jy2004.03.29 ATCA 19.5 GHz 13.2 hr 25.3 hr 12.1 hr ?? min 1.5452 +/- 0.0694 Jy2004.03.30 ATCA 19.5 GHz 13.2 hr 25.3 hr 12.1 hr ?? min 1.4795 +/- 0.0666 Jy2004.03.31 ATCA 19.5 GHz 13.2 hr 25.3 hr 12.1 hr ?? min 1.0761 +/- 0.0768 Jy2004.03.28 ATCA 17.6 GHz 13.6 hr 25.3 hr 11.7 hr ?? min 1.3284 +/- 0.0585 Jy2004.03.29 ATCA 17.6 GHz 13.2 hr 25.3 hr 12.1 hr ?? min 1.4000 +/- 0.0731 Jy2004.03.30 ATCA 17.6 GHz 13.2 hr 25.3 hr 12.1 hr ?? min 1.3091 +/- 0.0719 Jy2004.03.31 ATCA 17.6 GHz 13.2 hr 25.3 hr 12.1 hr ?? min 1.1537 +/- 0.0814 Jy2004.11.19 VLA 5 GHz 18.0 hr 24.0 hr 6.0 hr ?? min 0.9300 +/- 0.0118 Jy

• 32 Datasets• Each about 5 hr

in length• 20 GB of raw

data

Data analysis Data analysis

• Phase calibration on calibrators applied before ordinary calibration.

• Phase self-cal on Sgr A*.

• Flux of Sgr A* determined by fitting point source model in visibility plane, using only long spacing that filter out diffuse emission from Sgr A West.

First lookFirst look

• Radio variability of 5-10% at various timescales.

• High time resolution samples show few percent variation every sample (2 minutes).

• Significant day-to-day variability.

Example light curve – 1 day

Example light curve – 1 week

Calibration issuesCalibration issues

• Is variation real or artifact of observation or problems with calibration?

Calibration issuesCalibration issues

• Is variation real or artifact of observation or problems with calibration?– Variation within an observation is

pretty reliable.

– Day-to-day variation is harder to verify.

Analysis of light curveAnalysis of light curve

• Lomb-Scargle analysis of each light curve separately.

• Monte Carlo simulation of red noise – P(f) ~ f-1 – using the statistics of the relevant observation – following technique of Mauerhan et al.

Hourly variability: VLA (43 GHz)

Hourly variability: VLA (43 GHz)

Hourly variability: ATCA (17.6, 19.52 GHz)

Hourly variability: ATCA (17.6, 19.52 GHz)

Hourly variability: BIMA (83 GHz)

Hourly variability: BIMA (83 GHz)

Hourly variability: radio statistics

Hourly variability: radio statistics

Sgr A* variability distribution (15-83 GHz)

0

1

2

3

4

5

6

7

8

9

< 2 hr 2-3 hr 3-4 hr 4-5 hr 5-10 hr

Period

Nu

mb

er

83 GHz

43 GHz

22 GHz

19.5 GHz

17.6 GHz

Interpretation of variability distribution

Interpretation of variability distribution

• Lower limit to time scale for variability of 2.5 hr.

• Similar to peak in OVRO 83 GHz power spectrum.

• If the time scale for this variability is related to dynamics of emitting gas, this suggests a dynamical radius of 10 Rs.

Radio flare simultaneously detected at 22 & 43 GHz

Radio flare simultaneously detected at 22 & 43 GHz

Radio flare simultaneously detected at 22 & 43 GHz

Radio flare simultaneously detected at 22 & 43 GHz

• 43 GHz emission leads 22 GHz by about 20 minutes.

• Spectral index is steeper at higher frequencies and during flares, consistent with Herrnstein et al. (2004).

• Consistent with expansion of self-absorbed synchrotron source.

Scintillation vs. intrinsic changesScintillation vs. intrinsic changes

• Similar shape of light curve and delay between 22 & 43 GHz.

• Scintillation effects cannot explain this delay.

SummarySummary

• Typical variability within one day is 5-10%.• Variability on various timescales from 2 hr

to 5 hr (to 10 hr for long ATCA tracks).• Lower limit to dynamic length scale is

around 10 Rs.• Delay in 22 GHz and 43 GHz observations

of flare.• Model of variability suggests high optical

depth from expanding synchrotron source.