PoS(10th EVN Symposium)052 http://pos.sissa.it/ e, as predicted by general rela- mitting region remained constant. equency VLBI places constraints VLBI and the new generation of radio iscriminating among models of jet illimetre VLBI array will result in producing transformative science. BI at 1.3 mm wavelength has ma- gular resolution better than 60 mi- nt with the 1.3 mm emission in Sgr embarked on a project of observing Sagittarius A*) and the giant ellipti- nsity of the compact emitting region f the black hole in Sgr A*. A series of ⊙ M ted models, our long-baseline detections e Commons Attribution-NonCommercial-ShareAlike Licence. † ∗ [email protected] Speaker. on behalf of the Event Horizon Telescope collaboration Due to recent technological andtured scientific to advancements, a VL point where observations can readily achieve an an croarcseconds. Using millimetre VLBI arrays, our group has the black holes at the centre of our Galaxy (the 4 million can be used to place constraints on theobservations orientation in and April spin 2009 o revealed an increasein in Sgr the A* flux on de a timescale of less than a day while the size of the e cal galaxy M87 on event horizon scales. Our data are consiste tivistic effects. Within the context of physically-motiva In M87, the size of the emission region obtained from high-fr A* coming from a very compact region offset from the black hol on the high-energy emission region and maygenesis be important near for a d black hole. Futurea true improvements Event to Horizon the Telescope (sub)m Horizon Telescope capable of ∗ † Copyright owned by the author(s) under the terms of the Creativ c

MIT Haystack Observatory, USA E-mail: Vincent Fish Observing event horizons with high-frequency VLBI 10th European VLBI Network Symposium andarrays EVN Users Meeting: September 20-24, 2010 Manchester, UK PoS(10th EVN Symposium)052 ) after of a few Sch Vincent Fish astrophysical om a face-on SMT) in Ari- ave Astronomy itational lensing r sizes at shorter as (3.7 R ed fringes on the k hole at the centre line (with a fringe l” observations are µ y consisting of the lack holes. A large parameters. igure 1). Physically, created by the black olutions as small as r the (sub)millimetre as at 345 GHz). For electromagnetic radi- ut across the intensity ent horizon as viewed out the possibility that µ tivity in a strong-gravity 900 km (SMT-CARMA) ge. Millimetre-wavelength ing side of the disk to dom- 3 mm) are therefore neces- . as, or 37 1 µ for a Schwarzschild black hole as. The black hole at the centre to interstellar scattering, which ≤ µ Sch λ plane, we considered two simple mod- ) v , u ( 3 mm and longer [1]. The intrinsic size of the 2 = λ as) conclusively demonstrate the existence of structure on spatial scales µ 60 ∼ as at 230 GHz on Hawaii-Chile and Europe-Chile baselines (and 20 Of all known black hole candidates, Sgr A* has the largest apparent ev In 2007, we observed Sgr A* at 230 GHz using a three-telescope arra Very high angular resolution is the key to understanding a variety of complex The circular Gaussian model of the emission yields a size of 43 Due to the relative sparseness of detections in the µ from the Earth, with a Schwarzschild radius of approximately 10 phenomena. Among these isvariety the of accretion theoretical and models outflow forpresently region unable the surrounding to emission b distinguish region whether exist, aemission in since disk the or “single-pixe Sagittarius an A*, outflow the is radioof source responsible the associated fo Galaxy. with the Sgr massive A* blac environment, may but be the an accretion and excellent outflow source astrophysics,ation, for which must testing dominate be general the understood rela first. 2. Observations of Sgr A* in 2007 sary to probe the inner accretion/outflow region of Sgr A*. James Clerk Maxwell Telescope (JCMT)zona, in and Hawaii, the one Submillimeter telescope(CARMA) Telescope in ( of California. The the lengths of Combinedto these baselines Array over range for from 4000 about Research kmSMT-CARMA in and (JCMT-CARMA Millimeter-w JCMT-SMT and baselines JCMT-SMT). [1].spacing We successfully The obtain fringes on the latter base of the nearby galaxy M87 has aVLBI Schwarzschild radius has that is the nearly as requisite lar 30 resolution to observe theseSgr A* scales, especially, with high-frequency observations angular aredominates res necessary over due the intrinsic source size at Observing event horizons with high-frequency VLBI 1. Introduction the effects of interstellar scattering arethe removed. 230 This GHz very emission small is sizewould optically rules cause thick such and emission centred to appear on to the have black a hole, diameter of since 5.2 grav R emitting region is itself wavelength-dependent duewavelength to (higher opacity frequency). effects, Short with millimetre smalle observations ( els of the emitting region ofa Sgr circular A*: Gaussian a might circular Gaussian correspond and approximatelyprofile a of to uniform an a inclined ring one-dimensional disk, (F c where Dopplerinate boosting the causes emission the approach [2].disk, Likewise, where a the uniform central ring portion mighthole of approximate [3]. the the Both disk emission models is could fr not fit visible the due 2007 to data the for physically “shadow” reasonable model Schwarzschild radii. PoS(10th EVN Symposium)052 Vincent Fish coverage of are excluded e structure to ) rly lower than v λ n prior physical , u s of analysis can ( -on [2]. GRMHD flows, etc.) parame- del visibility predic- comes from a uniform ckends) [9]. Sgr A* adiatively inefficient the long baselines to denotes the upper limit n 20 k at Sgr A* has an event λ spin vector of the black VLBI baseline and con- ting of the JCMT, SMT, ar 3 G ne shows the best Gaussian fit to the data, 3 as. The dotted line shows a possible fit assuming the emission µ Millimetre VLBI detections of Sgr A* in 2007 [1]. The arrow ne An alternate approach to model fitting is to generate a library of models based o In April 2009, we observed Sgr A* with a four-telescope array consis First, unlike in 2007, the flux density on the SMT-CARMA VLBI baseline is clea from nondetections on the JCMT-CARMA baseline.implying a The size solid of 43 li Observing event horizons with high-frequency VLBI Figure 1: to remove contributions from dust in the greater Galactic Centre region.) The ring instead. [1]. The very compact size ofhorizon the emitting [4]. region also strengthens the case th assumptions (e.g., jets, general relativisitic magnetohydrodynamicterized (GRMHD) by unknowns about the Sgrhole A* and system the (e.g., accretion rate the of three-dimensional thetions. system) and For then instance, compare if the the data emission againstaccretion mo around flow Sgr (RIAF), A* can it bemodels is approximated by reach likely a a that r similar the conclusionalso disk be about is useful the in not disk planning being future inclination viewed observations [5, [7, near 6]. 8]. face These type 3. Observations of Sgr A* in 2009 and two CARMA telescopes operatedwas independently detected (with on separate all VLBI ba baselinesHawaii. (Figure Three features 2), of including the numerous 2009 detections data point on toward new results. the “zero-spacing” flux measured by both the ultrashorttemporaneous CARMA-CARMA CARMA interferometer measurements. (Baselines shorter tha the VLBI detections is still sufficiently sparse to permit multiple models of the sourc PoS(10th EVN Symposium)052 as. The µ sion with a Vincent Fish ird night of nent and an 00 n disk models as [9]. Sgr A* MT) occur over s, since the total lly exceeds that µ litude plots. In a MT-CARMA flux stant. Treating the 43 igher flux density but the baseline tracks to ent [10], thus placing ependent data sets, the ce size [13, 14] but, due etion flow at several tens elativity predicts that the ly on the ratio of the mass of 3 and 7 mm are inconclusive hed line indicate the best estimate = λ tions on the long JCMT-CARMA and blue points show detections on the short best Gaussian fit to the data on each day. In 4 as. µ space. Future observations and analyses may be able to identify ) v , u ( Observations of Sgr A* in April 2009 [9]. The red point and das Second, the correlated flux density on the JCMT-CARMA baseline genera Third, the flux density of the compact component is seen to be larger on the th Detections of Sgr A* on the longest baselines (JCMT-CARMA and JCMT-S Observing event horizons with high-frequency VLBI fit the data. Onextended the component, assumption the latter that not thedensities, contributing total significantly the emission to lower the consists limit correlated of onflux S a the density compact in size the compo of extended the componentflux extended must density be component is slowly much is variable higher approximately on than long 3 in timescale 2007. appears to be in a newsimilar steady size state on as the compared third to night the of previous observations, nights. with Observations a at h of the longer JCMT-SMT baseline.ring model Whilst when it the inner is radiusapparent still size is possible of treated the to as inner a fit radius of freeSgr the an parameter, A* compact accretion general to disk r emis its should distance depend on a from us, strong which constraint is on known thewide to expected variety an location of accuracy of physically of a realistic a null familiesare few in of disfavoured perc correlated by accretion the visibility flow JCMT-CARMA models, amp detections low-inclinatio [2, 5, 11, 12]. Figure 2: observations in 2009 compared to2007 the data first and two nights, each but ofbest-fit its circular the size Gaussian three remains is days con always of consistent observations with in having 2009 a as FWHM four size ind of as to whether a significant correlationto exists the between increased flux opacity density of and the sour of accretion flow, Schwarzschild are radii only from sensitive the to black the hole. accr a time span of 3trace hours out on substantial one arcs night in of observing, which is long enough for of the average total fluxSMT-CARMA density baselines. of Sgr The A* magenta onJCMT-SMT and each baselines, cyan day. respectively. points The The show dark black detec line shows the each case, the size is consistent with 43 whether the source exhibits measurable deviations from axisymmetry [12]. PoS(10th EVN Symposium)052 ) v , LBI u from ( ◦ . Current Vincent Fish t 7 mm, the existence of an 30 t whose size ole candidate at , 16, 17]. High- nd telescopes of nefit from higher ompact jet launch I may be necessary 9. In the context of footprint, its collima- d by grants from the the spin vector of the ted into the observing hanges in Sgr A* [19, ed sensitivity and case that Sgr A* contains ting area from up to eight vations in the near future will coverage, enabling detailed studies of the ) v , u ( 5 , and wider-bandwidth observations in the near future 1 − 78. 455 Event-horizon-scale structure in the supermassive black h Nature , the Galactic Centre The nearby galaxy M87 is also detected on very long baselines at 1.3 mm. As a Several technological advancements will permit future observations to be Millimetre VLBI observations of Sgr A* in 2007 and 2009 have demonstrated the High-frequency VLBI work at MIT Haystack Observatory is supporte [1] S. S. Doeleman et al. 2008, Observing event horizons with high-frequency VLBI 4. M87 1.3 mm VLBI data detectsresolution both observations the such large-scale as jet those and providedto by a place millimetre-wavelength strong compact VLB constraints component on [15 parameterstion, of and the the jet, spin such of as the the size black of hole the in jet M87 [18]. 5. Future Directions sensitivity, which will be necessary20]. for A probing phased-array time-variable structural processortelescopes c has on Mauna been Kea developed (the tothe JCMT, sum Submillimeter the Array), the Caltech and collec Submillimeterobservations a have Observatory, been similar a recorded instrument at 4 will Gbit s be deployed at CARMA [21] coverage. 6. Summary will double or quadruple thearray, bit rate. producing New an telescopes Event will also Horizon be Telescope incorpora with significantly enhanc structure on scales of a fewa Schwarzschild radii, black furthering hole the scientific withdata, an but most event of horizon. the millimetrewas emission Multiple not appears seen to geometrical to originate models in changephysically-motivated a can in models compact the fit componen of aftermath the the of accretion millimetre ablack region, brightening hole V our is of inclined data the to source suggest the in that line 200 of sight with the accretion disk being viewed more th face-on. Long-baseline detections ofregion M87 around a will different enable class similarhave of studies significantly supermassive of black greater the holes. sensitivity c and Obser better quiescent and variable accretion flow in Sgr A*. Acknowledgments National Science Foundation. References PoS(10th EVN Symposium)052 , , et ApJ ApJ 1357. , , Detection 701 Vincent Fish 704. l Supermassive Variable ApJ L142 Radiative Models 304 , 703 Detecting Flaring Detecting Changing 1353. 960. Science ApJ , 706 , 012047 eung 2009, 706 , 131 ApJ , P -Ray Emission Region in the ApJ , in press, arXiv:1010.1287 γ 59. , 45. Estimating the Parameters of Using Millimeter VLBI to Constrain Evidence for Low Black Hole Spin and A&A . E. Rogers 2009, . E. Rogers 2009, 695 , I Observations of Sagittarius A* , itude Imaging . Goss, & D. C. Backer 2004, er Black Hole ncy VLBI iath, G. Witzel, A. Witzel, & J. A. Zensus 697 The Submillimeter Bump in Sgr A* from ApJ , ApJ , J. Phys. Conf. Ser. , 444. 6 The Event Horizon of Sagittarius A* 497. Testing the Accretion Flow with Plasma Wave Heating 325 706 1164. L14. , submitted, arXiv:1011.2472 Millimeter Flares and VLBI Visibilities from Relativistic 1044. 1092. High-Frequency VLBI Imaging of the Jet Base of M87 ApJ Viewing the Shadow of the Black Hole at the Galactic Center 697 692 , 689 717 ApJL Science , , Imaging the Black Hole Silhouette of M87: Implications for J ApJ ApJ , ApJ ApJ , , , Radio Imaging of the Very-High-Energy Measuring Distance and Properties of the Milky Way's Centra 1.3 mm Wavelength VLBI of Sagittarius A*: Detection of Time- A submillimeter VLBI array L13. ´ scibrodzka, C. F. Gammie, J. C. Dolence, H. Shiokawa, & P. K. L 528 200. Formation and Black Hole Spin Structures in Sagittarius A* with High-Frequency VLBI ApJ Polarization Structures in Sagittarius A* with High Freque of Sgr A* from GRMHD Simulations Central Engine of a Radio Galaxy Sagittarius A*'s Accretion Flow via Millimeter VLBI Simulations of Magnetized Accretion onto the Galactic Cent Emission on Event Horizon Scales Relativistic MHD Simulations Black Hole with Stellar Orbits RIAF Models of Sagittarius A* Physically Motivated Accretion Models from Millimeter VLB Mechanism for Sagittarius A* by the 1.3 mm VLBI Measurements submitted, arXiv:1011.2770 of the Intrinsic Size of Sagittarius A* Through Closure Ampl 2010, Multiwavelength VLBI observations of Sagittarius A* 660 [5] M.Mo [3] H. Falcke, F. Melia, & E. Agol 2000, [4] A. E. Broderick, A. Loeb, & R. Narayan 2009, [2] A. E. Broderick, V. L. Fish, S. S. Doeleman, & A. Loeb 2009, [6] J. Dexter, E. Agol, P. C. Fragile, & J. C. McKinney 2010, [7] V. L. Fish, A. E. Broderick, S. S. Doeleman, & A. Loeb 2009, [9] V. L. Fish et al. 2010, [8] L. Huang, R. Takahashi, & Z.-Q. Shen 2009, [19] S. S. Doeleman, V. L. Fish, A. E. Broderick, A. Loeb, & A. E [20] V. L. Fish, S. S. Doeleman, A. E. Broderick, A. Loeb, & A. E [17] S. S. Doeleman et al., in[18] preparation. A. E. Broderick & A. Loeb 2009, [21] J. Weintroub 2008, Observing event horizons with high-frequency VLBI [11] J. Dexter. E. Agol, & P. C. Fragile 2009, [10] A. M. Ghez et al. 2008, [12] A. E. Broderick, V. L. Fish, S. S. Doeleman, & A. Loeb 2010 [13] G. C. Bower, H. Falcke, R. M. Herrnstein, J.-H. Zhao, W. M [14] R.-S. Lu, T. P. Krichbaum, A. Eckart, S. König, D. Kunner [15] C. Ly, R. C. Walker, & W. Junor 2007, [16] V. A. Acciari et al. 2009,