Time lag in Sagittarius A* lightcurves from JVLA and ALMA Christiaan Brinkerink Radboud University Nijmegen The Strong Gravity Regime of Black Holes and Neutron Stars, March 31st 2014 Heino Falcke, Denis Barkats, Geoffrey Bower, Casey Law, Andreas Brunthaler, Charles Gammie, Violette Impellizzeri, Sera Markoff, Karl Menten, Monika Moscibrodzka, Alison Peck, Anthony Rushton, Reinhold Schaaf, Melvyn Wright
The objective To study the variability and spectral properties of radio emission in multiple bands from Sagittarius A*, the radio source associated with the black hole at the galactic center. To answer the question: what is Sagittarius A*, really? Accretion flow or jet? Brightness distribution @ 3.6cm, 8.3 GHz (NRAO/AUI)
The radio spectrum of Sgr A* Submm-Bump Zylka, Mezger, Lesch (1992) Falcke, Goss, Matsuo et al. (1998) flat spectrum core? ~3 Rs source Spectrum: Melia & Falcke (2001, ARA&A)
A jet model for Sgr A* Partially self-absorbed jet (Blandford & Konigl 1979, Falcke- Biermann 1995), with τ = 1 surface location dependent on wavelength Wavelength Distance
A jet model for Sgr A* Partially self-absorbed jet (Blandford & Konigl 1979, Falcke- Biermann 1995), with τ = 1 surface location dependent on wavelength Wavelength Distance
Size-frequency relation Observed in Sagittarius A* using VLBI wavelength size/λ 2 Bower et al. (2004, 2008) 50 µas GR ray tracing simulation wavelength (cm) angular size
Measured time lags Evidence for relativistic outflow using VLA ~20 minutes lag from 43-22 GHz Yusef-Zadeh et al. (2008) distance: 30 light-minutes (from size-freq)
The setup ALMA Cycle 0, eight-hour track (PI: H. Falcke): 100 GHz, 250 GHz, 340 GHz (bands 3,6,7), simultaneous. 19 antennas, 30-second band switching. Simultaneous track using JVLA at 18, 25, 27, 37, 39 and 48 GHz. Part of Chandra XVP Sagittarius A* monitoring program (PI: M. Nowak).
Sgr A* lightcurves Asymmetric bump in lightcurves can be fit with FRED (Fast Rise Exponential Decay) curves to determine time of maximum Error on fit checked with random decimation of data points: fits for 25 GHz and 48 GHz are not robust
Fitted maxima hint at lag vs frequency relation
Finding the lag Time before max @ λ=1.1 cm model from Falcke, Markoff, Bower (2009) Model from Falcke et al. 2009
Spectral shape Flat spectrum transitions into submm bump at higher frequencies Variability increases with frequency: indication for optically thin emission Absolute flux calibration for JVLA lightcurves is off ( but relative flux calibration looks fine)
ALMA lightcurves 100 GHz 250 GHz 340 GHz
Conclusions Time lags derived from measurements fit well with earlier results Data corroborates a picture of funneled, mildly relativistic outflow: an unresolved jet? To determine best-fit model inclination, we need more measurements!
Exciting times for Sgr A* and the galactic center! More lines of inquiry opening up: Intrinsic size measurements at 3mm BlackHoleCam / EHT project is under way
Galactic center results from our collaboration Early 2013: Magnetar discovered in galactic center region from outburst seen in NuSTAR data Confirmed with Effelsberg and other radio telescopes Angular separation only 3 arcsec from Sgr A*! Provides separate probe for interstellar scattering screen towards galactic center Eatough, HF, et al. (2013), Nature
Measured Major & Minor Axis Sizes (mas) Pulse shows temporal broadening according to λ 4 relation Angular broadening of source is in agreement with Sgr A* measurements: same scattering screen likely 100 10 Sgr A* Pulsar J1745 29 1 1 10 Bower et al. 2014 Wavelength (cm) Normalized Flux 8.34 GHz 4.85 GHz 3.22 GHz 2.56 GHz 1.63 GHz 1.55 GHz 1.42 GHz 1.34 GHz 1.27 GHz 1.19 GHz 0 0.5 1 1.5 2 2.5 3 3.5 t (s) Spitler et al. 2013
Temporal broadening of pulses is due to scattered rays having a longer path. ~ t ~½ t Sgr A* scattered light path direct light path apparent size of scattered image observer Local screen model remote screen model t (remote) t (local) For the same angular broadening, a local screen has much more temporal broadening! Temporal broadening measurements point to scattering screen distance of 3 kpc from us