iarise Zooming in on Black Holes

Transcription

1 iarise Zooming in on Black Holes An International, Two-Spacecraft, Microarcsecond Imaging Mission This document was prepared by E. Fomalont (NRAO), L. Greenhill (CfA), D. Murphy (JPL), R. Preston (JPL), M. Reid (CfA), J. Romney (NRAO), J. Smith (JPL) and J. Ulvestad (NRAO). It includes material provided by members of the ARISE Science Advisory Group and astronomers from the U.S., Japan, and Europe. 30 January 2002

2 iarise Whitepaper 1 The international ARISE mission (iarise) is a proposed NASA Roadmap mission for the microarcsecond imaging of Supermassive Black Holes (SMBHs) and stellar objects, using the technique of Very Long Baseline Interferometry (VLBI). This dual-spacecraft mission will have considerably more scientific capability than ARISE (Advanced Radio Interferometry between Space and Earth), at considerably lower cost to the U.S. than was envisioned in the ARISE mission. 1 Introduction The ARISE mission, as originally envisioned, involved the use of a 25m (possibly inflatable) aperture in an elliptical Earth orbit having an apogee height of 40,000 km. The original estimated cost of this mission was $400 million. The key science goals of ARISE were the imaging of accretion disks and jet formation near the event horizons of supermassive black holes (SMBHs), as well as using extragalactic H 2 O megamasers to study the physics of molecular tori in active galactic nuclei (AGNs) and to derive direct distance measurements to the galaxy hosts. The importance of the ARISE and iarise science goals has been recognized by various review committees over the last several years. Specifically, ARISE appears on the list of missions for the next decade that was recommended by the NRC Astronomy and Astrophysics Survey Committee (the decade committee ) in their report Astronomy and Astrophysics in the New Millennium. It also appears in the long-term roadmap of the Structure and Evolution of the Universe theme. Finally, the concept of using a Space VLBI mission such as ARISE is discussed in the NASA Space Science Strategic Plan as a mission for possible implementation after 2007, addressing the objective to explore the ultimate limits of gravity and energy in the universe. iarise will be a more sensitive, higher resolution and more robust imager than ARISE, at lower cost to NASA. It also will be an international mission, as denoted by the addition of i in the mission name. The USA participants are the Jet Propulsion Laboratory, the National Radio Astronomy Observatory, and the Harvard-Smithsonian Center for Astrophysics. The major foreign participant will be the Japanese Institute of Space and Astronautical Science (ISAS), which led the first dedicated space VLBI mission VSOP. Significant contributions are expected to be provided by the Canadian Space Agency and the European Space Agency. Like VSOP, iarise will be an open mission and available to all astronomers though a peer review system. The strategic plan of the Space Science Enterprise (SSE) states that the prize is to directly image a black hole, whose existence heretofore has been based on indirect evidence. If imaging a black hole is indeed the prize, then iarise is a premier mission. Using techniques that have been thoroughly demonstrated and are technically achievable today, iarise combines 10 µarcsecond (µas) resolution and sensitivity that are sufficient to image material near the event horizon of a SMBH. The SSE strategic plan identifies a prospective mission as a microarcsecond X-ray imaging mission pathfinder that would achieve 100 µas resolution as a step towards the 0:1µas resolution MAXIM X-ray mission. While iarise and the X-ray missions will independently resolve the structure of material at the event horizons of SMBHs, iarise, in contrast, could be flown in less than a decade, with minimal development. The long-term vision for Space VLBI is a multi-spacecraft mission, the Black Hole Imager (BHI). This mission will follow on from the technical and scientific developments of iarise, as well as possible short baseline space interferometers at millimeter and sub-millimeter wavelengths. BHI would achieve 1-microarcsecond resolution by operating at high frequencies in the range of 200 GHz to 500 GHz, processing the interferometric baselines among five or more spacecraft. The maximum baseline lengths will be on the order of 200,000 km, which provides 1-microarcsecond resolution at 300 GHz. The primary scientific goal of BHI will be to observe active galaxy cores on the scale of the event horizons of their central black holes, and to image these event horizons by their effects on nuclear radio sources (cf. Falcke, Melia, and Agol, 2000, ApJ, 528, L13). Many nearby elliptical galaxies have black hole masses ranging from 10 8 M to 10 9 M (cf. Table 1 of Milosavljevic and Merritt, astro-ph/ ). For a 10 9 M black hole at a distance of 40 Mpc (or M at the Virgo cluster distance of 16 Mpc), the event horizon diameter is 1 microarcsec, exactly the scale that BHI will image. This scale cannot be probed by

3 iarise Whitepaper 2 longer baselines at lower frequencies, due to the effects of interstellar scattering and to the hot gas very near the event horizons. The iarise mission will complement two NASA Roadmap missions: Constellation-X (Con-X) and the Gamma Ray Large Area Space Telescope (GLAST). Con-X will perform high-resolution spectroscopy of the accretion disks surrounding SMBHs and other material immediately surrounding SMBHs and will conduct time monitoring. This hot gas will also generate radio emission from thermal Bremsstrahlung. GLAST will target Gamma-ray emission caused by relativistic shocks within the portions of jets closest to SMBHs. Flares are by and large caused by inverse Compton scattering of radio photons caused by synchrotron processes in the jets. Con-X and GLAST will conduct critical spectroscopic investigations of SMBHs however, their respective angular resolution are relatively poor in absolute terms (> or >1 kpc at a distance of 20 Mpc) and in relative terms (10 6 and 10 8 coarser than that of iarise, respectively). The goals of these missions will be detailed models of time variable physical processes near SMBHs. Coordinated multi-wavelength investigations (radio, X-ray, Gamma-ray) that leverage the imaging and spectroscopic capabilities of all three mission, iarise, Con-X, and GLAST, would enrich the science output of each significantly, and enable substantially more robust, better constrained models of SMBHs and their environs. The prospect of a third, independent avenue of attack (i.e., high angular resolution imaging) is a major driver in the iarise concept and design: to decrease cost and development so that a launch around 2010 is possible and to increase specific technical capabilities so that the regions of X-ray and Gamma-ray production may be imaged flexibly. The iarise mission design reflects two major improvements over the original ARISE concept. First, iarise will comprise two space radio telescopes placed in complementary orbits, whereby the imaging capabilities compared to any single spacecraft mission are greatly enhanced. A two craft mission permits high angular resolution, high fidelity imaging of any celestial source at any time with a circular point source response, which is of critical value to monitoring programs and pursuit of targets of opportunity (neither of which, for instance, have been tackled seriously by VSOP). Second, iarise will achieve better detection thresholds for weak sources because it will be designed to support of VLBI phase referencing techniques, now used routinely with ground VLBI arrays. In part, this is possible because the 15-m iarise telescopes are individually smaller than the single 25-m ARISE telescope, and easier to position-switch on the sky. Although the iarise collecting area is reduced, the mission targets a larger list of potential sources than ARISE. The smaller iarise apertures also improve high-frequency performance at reduced expense and risk. With the experience gained from successful operation of the VSOP program in conjunction with the VLBA of the NRAO, and in light of recent design studies for a modest Japanese successor VSOP-2 mission, conducted in collaboration with Japanese and Canadian engineers, we are confident that the iarise mission goals can be met in the stated time and for the stated cost. iarise will be NASA s first microarcsecond imaging mission and NASA s first observatory-class radio astronomy mission. International cooperation, mainly with Japan, would make possible a major science mission for the cost to NASA of a MIDEX mission (note that the MIDEX program does not support highly international missions like iarise). The technical risks and development requirements of iarise are minimal, but the mission depends on an immediate and narrow window of opportunity, whereby NASA can leverage a present-day Japanese initiative to launch a second-generation radio telescope, of relatively limited capability, before 2010 (VSOP-2). If this opportunity is missed, there will not be another. 2 iarise Science 2.1 Overview of iarise Science Goals The main goal of the iarise mission is to broaden substantially our understanding of supermassive black holes and the active galactic nuclei (AGN) in which they lie. iarise will operate in tandem with arrays of ground radio telescopes to achieve an angular resolution as fine as 10 µas (about 10 times what can be achieved with ground installations alone). It will be the only astronomical tool that can image the hearts of AGN and track their evolution. An overlapping of the iarise mission with two other NASA Roadmap missions, Constellation-

4 iarise Whitepaper 3 X and GLAST, would significantly enhance the scientific dividends from all of these missions. Three iarise Key-Science Projects, described below, will attack the phenomenon of SMBHs from different perspectives: 1. The elliptical galaxy M87 contains the nearest AGN known to contain a multi-billion solar mass black hole, and it will be studied in detail. Observing with an array of ground-based radio telescopes, iarise would directly image (1) the innermost portions of the accretion disk, (2) the region in which material leaves the disk and spirals toward the SMBH event horizon, and (3) the acceleration and collimation of ultra-hot plasma by processes that are not understood. 2. H 2 O megamasers that lie in accretion disks orbiting SMBHs are unique tracers of the dynamics, stability, warping, and clumpiness of disk material. iarise observations will permit direct imaging of disk structure, mapping of disk dynamics, including complete resolution of rotation curves, and the determination of accurate SMBH masses. Over a period of several years, observations will detect the proper motion and centripetal acceleration of megamasers in several AGN and thereby the rotation of the disks. From these data, the distances to galaxies will be measured geometrically, forming a robust basis for calibration of extragalactic distances, beyond that which is now possible. 3. For a sample of hundreds of distant, luminous, and variable AGN, iarise will image the shocks and the superluminal ejecta near the base of jets, and observers will determine magnetic field configurations and densities in both entrained and confining material. These luminous AGN emit enormous X-ray and gamma-ray fluxes, in part from the radio-emitting regions, which makes coordination with GLAST and Constellation-X so important. A short review of other scientific investigations is also given. 2.2 M 87: A SMBH Rosetta Stone Chandra and VLA images show X-ray jets and complex extended regions in AGN and quasars on all scales from 10 to less than 0.5 arcsec. On smaller scales, HST and VLBI imaging, X-ray spectra, and theory all suggest fascinating structures will be found down to the event horizon. Indeed, X-rays from the Fe Kα fluorescence line are proving a valuable diagnostic of material within a few Schwarzschild radii (R Sch ) in some AGN. However, direct imaging of emission from regions close to the event horizon may ultimately revolutionize our understanding of the physical processes that operate in the extreme environments of black holes. This can be accomplished with iarise. The archetypal AGN jet is found in the giant elliptical galaxy M87. HST observations strongly suggest that this galaxy harbors a truly impressive SMBH containing M. A VSOP space-vlbi image of M87 at low radio frequencies (see Fig. 1) clearly shows that its jet is quasi-cylindrical, extremely limb-brightened, and spatially oscillatory, suggesting that relativistic magneto-hydrodynamic processes mold the jet at angular scales above 1 mas. Currently the telescope with the highest resolution is the VLBA, which when operated at its highest possible frequency of 86 GHz has an angular resolution of 100 microarcseconds (µas). For M87, the smallest VLBA beam is comparable to the region within about 25 R Sch of the central black hole. Hence, VLBA imaging can provide only incomplete, though tantalizing, views of the region jet collimation might be taking place. But, what happens in the region inside of 10 R Sch? An artist s conception of this region is shown in Fig. 1. At centimeter and millimeter wavelengths, there is strong emission unresolved by current observations. For example, the VLBA 43 GHz image (Junor, Biretta, & Livio, 1999, Nature, 401, 891) contains 0.2 Jy in an unresolved core, implying a very high brightness (> K). This is bright enough for detailed study at higher angular resolution with iarise. The currently unresolved core emission likely originates among three synchrotron emitting structures: 1. the inner edge of accretion disk or torus, which could support thermal synchrotron emission if the electron temperature approaches the virial temperature

5 iarise Whitepaper 4 iarise fringe at λ7mm for the M87 black hole Figure 1: The Black Hole in M87. (Left): The VSOP radio image of M87 at 1.6 GHz with 1.0 mas resolution. (Right): A cartoon of the black hole, accretion disk and theoretical magnetic collimation of the inner jet. The iarise resolution at 43 GHz of 15 µas is shown under the cartoon.

6 iarise Whitepaper 5 2. the magnetic field generated in the inner disk/torus and threading toward the SMBH which are likely sources of electron acceleration and non-thermal emission 3. the jet, which is probably formed, accelerated, and/or partially collimated within 10 R Sch For M87, iarise should be able to image DIRECTLY material at the event horizon of the M SMBH, the inner accretion disk, and the jet where it is collimated. None of the exotic physical processes active in this small but important region is well understood. For M87 (at a distance of 15 Mpc) a resolution of 10 to 20 µas (for 86 and 43 GHz, respectively) corresponds to the region within1 to 2 R Sch. Synchrotron-emitting material close to the SMBH is expected to exhibit an equivalent Blackbody temperature on the order 10 9 K or more, which would be detectable using VLBI techniques. Since the M87 jet is inclined at about 25 to the line of sight it will be possible to examine the inner accretion disk almost face-on with relatively little obscuration from either the outer parts of the disk or from the jet. iarise images of M87 could be extraordinarily important, providing key information needed to address the following fundamental questions: Is the innermost accretion disk thin or thick or does it resemble a torus? Do the radio jets show signs that they tap accretion energy and/or BH spin energy? Do some magnetic-field lines from the disk contact the SMBH? Are some magnetic-field lines from the disk spun-up to form a sheath surrounding the jets? Is the jet formed and collimated directly by a torus or through magnetic fields? 2.3 Accretion Disks, Black Holes, and Megamasers Accretion disks in AGNs are the key physical links between SMBHs and their host galaxies. Proximity to the black holes makes the disks the best available probes of the deep gravitational wells and black hole environs. For some AGNs, such as the well-known example of NGC4258 at a distance of about 7 Mpc, H 2 O megamaser emission (at 22 GHz) delineates warm, dense gas at radii < 1 pc in differentially rotating accretion disks. The maser emission, which is beamed, is visible when the disks are viewed nearly edge-on and the Doppler velocity of the masing material can be determined to an accuracy < 1 km s 1. At about the systemic velocities of host galaxies, maser emission from the near sides of disks is observed. At velocities symmetrically red and blueshifted from systemic, masers emission is observed from the regions corresponding to the maximally approaching and receding disk material (see Figure 2). Interferometric measurements of H 2 O maser positions, line-of-sight velocities, and proper motions provide unique and direct, well resolved maps of underlying accretion disks. These maps cover the molecular portions of the disks and complement images of synchrotron-emitting jets and the innermost portions of the disks where material spirals in toward event horizons (e.g., iarise observations of M87). Of equal importance, imaging studies of maser disks provide extremely detailed information on host AGNs that may be generalized and applied to the interpretation of traditional optical and X-ray data for the broad sample of objects that do not support masers. Twenty-two H 2 O masers are known to lie in AGNs. They are detected in roughly 5% of AGNs within 100 Mpc that have been surveyed. The use of newly deployed wide-band spectrometers in the next few years will lengthen the source list of masers since it will be easier to detect high-velocity emission (from rapidly rotating disks). The recent, surprising detection of H 2 O maser emission in an optically normal edge-on galaxy suggests that observable maser emission may also be detected toward a class of galaxy that has not been targeted in past surveys. If the alignment of parsec-scale accretion disks and kiloparsec-scale stellar disks is more common than not, and if black holes are commonplace in galactic nuclei, then tens or hundreds of new masers may be discovered in edge-on systems. Ground-based VLBI observations enable detailed study of only the nearest H 2 O megamasers. iarise observations will provide a needed order of magnitude jump in angular resolution over ground-based observations. Ground-based VLBI observations could resolve the structure and rotation curve of NGC4258 were

7 iarise Whitepaper 6 Figure 2: Accretion disk and SMBH of NGC4258. Top panel- An artist s conception of the thin, warped accretion disk, below which is a triply-peaked H 2 O maser spectrum characteristic of emission from an edge-on rotating disk. On the disk surface, maser emission is depicted as white glints. Middle panel- The mapped distribution of H 2 O maser emission in the accretion disk (dots), and the fitted dynamical model (mesh). Symbol color indicates Doppler velocity. An image of the innermost jet emission (color contours) is registered precisely and coincides with the dynamical center of the disk (black spot). Bottom panel, left- The fitted rotation curve of the disk, which obeys v r 0:5 at a level of < 1%. Bottom panel, right- The radio jet observed on kpc-scales. The jet extends along the disk axis in close proximity to the SMBH (see middle panel), but is deflected by material about 1 kpc from galactic center.

8 iarise Whitepaper 7 it at no more than 25 Mpc. In contrast, iarise should be capable of mapping accretion disks at distances of 100 to 200 Mpc. The measurement of proper motions is more difficult than mapping disks. Of the known megamasers, ground-based observations can tackle only NGC4258. In contrast, iarise should be capable of measuring motions in 4 to 6 AGN that are known today, at distances as large as 60 Mpc. The mission will address four key-science questions that are beyond the reach of ground-based study. What is the census of SMBH masses in a large sample of AGN, and how compelling is evidence for their existence, over a range of masses? Known maser-disks are associated with SMBHs of 10 6 to M outside the Local Group. While massive, at these distances, such light black holes are difficult to detect and weigh by other means. Evidence from maser studies is especially convincing because the enclosed masses are typically confined to 1 pc, which implies mean densities of 10 7 to M pc 3. How thick are accretion disks, what processes supply vertical support against gravity, and how clumpy are they? The thickness and substructure of accretion disks is a critical parameter in estimation of mass accretion rate, accretion mode (e.g., viscous, advective), stability, and physical conditions (e.g., temperature). No accretion disk thickness has been measured directly < 1 pc from a SMBH, but maser disks are excellent candidates because they are perforce nearly edge-on. Thicknesses of 10 to 100 µas (up to three iarise beams) are anticipated for several known disks within 15 Mpc, assuming hydrostatic support. Do warped accretion disks in many galaxies influence the perceived gross characteristics of AGN that we observe along lines of sight from Earth? AGN unification theory may depend on both warped accretion disks as well as the often posited dusty tori. H 2 O maser emission is a beacon of dense molecular material (on the order of 10 8 to cm 3 ). A warped maser-disk that crosses a line of sight toward a SMBH readily accounts for observed optical and X-ray absorption columns. Can the calibration of the extragalactic distance scale (EDS) be built on geometric distance measurements, rather than Standard Candles? The EDS is now underpinned by the distance of the LMC alone, which is based on Cepheid light curves. However, differences in the evaluation of systematic errors for different measurement techniques has hindered convergence on a single distance. The calibration of standard candles and the EDS should instead rest on measured geometric distances to anchor galaxies. In the case of NGC4258, proper motion and centripetal acceleration data have been used to estimate an initial distance with an uncertainty of 7%. With iarise, ultimately, uncertainties of perhaps a few percent could be achieved for NGC4258 and other galaxies. 2.4 The Physical Processes within the Inner Jets of AGN: Ground VLBI and VSOP images of the strongest and most distant AGN show radio emission with similar properties to that of M87. However, these luminous AGNs are extremely variable in intensity and structure and usually have X-ray and gamma-ray luminosities far in excess of that in the radio (see Figure 3). Although iarise will not have sufficient resolution to reach the event horizons and accretion disks in these AGNs, it can image the complex changing structures and superluminal motions occurring in the innermost regions of the jets relatively close to the driving SMBHs, especially at 43 and 86 GHz, for which the radio emission is semi transparent (optically thin). Radio polarimetry can also derive information about the jet magnetic field configuration and the plasma density within and surrounding the jets. The astrophysical topics addressed with these observations are: The observed motion, evolution and magnetic field configuration of the inner-jet components can distinguish between superluminal ejecta or shocks, both moving and stationary. Observations every few weeks will be necessary to follow the individual components. Many AGNs produce copious quantities of variable X-rays and gamma-rays, often from the same populations of electrons which produce the radio emission. With iarise resolution at 43 GHz and 86 GHz, the variable radio phenomena in the base of the jets should show clear correlations with X-ray and gamma-ray

9 iarise Whitepaper 8 The Heart of an AGN Radio Waves Supermassive Black Hole Accretion Disk/Torus X-rays & Gamma Rays Broad-line Clouds (optical emission) 0.1 pc IR (from dust) Gamma Rays 1 pc Narrow-Line Clouds (optical emission) Radio Jet 10 pc 100 pc Figure 3: The Heart of an AGN: schematic view of the central 100 parsecs of high luminosity AGN of the type that are also strong hard X-ray and Gamma-ray sources. Note the logarithmic scale. variability with time delays of less than one week, rather than the weak correlations now observed over several months. By observing a radio source at several frequencies, the magnetic field configuration within the jet can be determined. Whether the magnetic field configuration is radial, transverse or helical will have implications in the modeling of jet collimation and stability. The observed large and rapidly changing Faraday rotation of the jet components may be the result of the passage of the radio emission through the narrow-line region (NLR) associated with AGN (see Figure 4 for an example). Such observations, along with studies with optical telescopes, will provide estimates of the magnetic field strength (not determined by any other means), gradients in velocity and electron density, and filling factor of the NLR. Intra-day variable sources (IDV) confirm that some radio sources contain components smaller than about 20 µas, even if all the variations are caused by refraction and diffraction by dense gas in our galaxy. These high brightness temperatures greater than K are are very difficult to explain unless coherent emission processes exist that mimic synchrotron radiation. iarise resolution, at the high frequencies where the interstellar diffraction is small, is needed to measure (rather than place limits on) the brightness temperatures and to determine their locations relative to the accretion disk and any superluminal components. A major question is whether IDV radio sources differ intrinsically from non-idv sources or if a continuous range of high brightness temperature (coherent?) emission is exhibited. 2.5 Other Scientific Topics Binary Black Holes in Early-type Galaxies: It is widely believed that many elliptical and S0 galaxies result from mergers of disk galaxies. If each of the disk galaxies harbored a SMBH, recent theoretical models suggest that not all SMBH pairs would coalesce during the merger. Of all observatories in the near future, only iarise could

10 iarise Whitepaper Figure 4: The Rotation Measure Across 3C273. The contour levels show the 22 GHz total intensity distribution The colors indicate the rotation measure in rad m 2, with values shown by the colored wedge at the bottom. The insets display the RM fits at the points indicated. The bright, elongated radio component in the upper left is about 10 pc in size and is composed of several components whose rotation measures change significantly over six months. iarise will be able to track these changes at a 22 GHz resolution of 35 µas a factor of 20 more than these observations (Zavala, R. T. & Taylor, G. B., 2001, ApJ, 550, L147).

11 iarise Whitepaper 10 resolve the emission from multiple SMBHs that have not coalesced. One parsec at a distance of 100 Mpc subtends 100 iarise beams at 43 GHz. Advection-Dominated Accretion Flow (ADAF): The relatively low luminosity of some AGN and the Galactic Center (i.e., SgrA*) may be a consequence of ADAFs, where most of the energy of infalling gas passes through the event horizon before it can be dissipated in detectable radiation. When an ADAF is present, a thermal radio source on the order of 10 9 K should be observable, in addition to any jets. The order of magnitude improvement of iarise angular resolution over ground-based observations is required if we are to test this prediction. The Structure of Star-Forming Disks and the Impact of Protostars on their Environs: The processes of star formation above a few to 10 M are poorly understood because of high extinction by dust and gas, confusion and the formation of stars in clusters, and the rapid evolution of massive protostars before they emerge from their stellar birth clouds. These factors make it difficult to study massive star formation with traditional millimeter wave and infrared observing techniques. However, the star-forming accretion disks and outflows host H 2 O (22 GHz) and SiO (43 GHz) maser emissions, which pepper shocked dense gas and make it possible to measure bulk mass motions (via Doppler shifts and proper motions), from a few AU to 10 3 AU from the protostars. Study with iarise will resolve the surface layers of protostellar accretion disks, where they are disrupted by overlying outflows, which are responsible for halting accretion, which makes possible evolution of protostars to the Main Sequence. iarise study will also track small-scale interactions that dictate how outflow energy is translated into the turbulent and bulk mechanical energies that disrupt protostars natal molecular clouds. Furthermore, there is now a growing recognition that the interaction between the inner regions of the disk and the protostellar magnetic field may be important in generating outflowing knots in protostellar jets. Over the last decade, there have been several detections of nonthermal emission from magnetic structures on scales of 10 stellar radii in young stellar objects (e.g., Andre et al. 1992, ApJ 401, 667, and references therein). iarise will have the capability of resolving structures (at 8 GHz) on the scale of the stellar radius at the distance of the nearest star-forming regions, and could thus greatly advance our understanding of the disk-magnetosphere interaction and the production of stellar jets. The Evolution of X-ray Binaries (XRBs)/Microquasars: Ground VLBI images suggest XRBs appear remarkably similar to AGN: both display bright and variable radio cores, (usually) one-sided radio jets, relativistic motions in the jets between 0.2c and 0.95c, and radio lobes thus the designation microquasars. Since the temporal scaling of observed XRB phenomenona observed over ten minutes corresponds to about 100 yrs in AGN, long-term evolutionary changes in AGN may be understood by analogy with XRB observations. One example is the possible correlation between the ejection of components along radio jets and changes in the X-ray state of accretion disks. With a typical XRB distance of 4 kpc, over ten minutes, light travels 300 µas. With two orbiting telescopes, iarise can provides high quality snapshot imaging for the first time in space VLBI, which will be sufficient to follow motions in microquasar systems in real time. 3 iarise Mission Design Several preliminary studies of the iarise mission have already been undertaken and the key mission drivers have been identified. Table 1 compares the iarise mission both with the projected capability of the ground-based VLBA at the time of the iarise launch and the original ARISE mission as presented to the decade committee. The capabilities of the instruments stated here are those thought likely to be available in This table illustrates not only the enhanced capabilities of iarise over the VLBA but also the improvement of the iarise mission compared to the ARISE mission. Critical to the success of the iarise mission is the choice of complementary orbits for both iarise spacecraft. Work in this area has identified optimum orbit choices and a comparison of ARISE and iarise UV-coverages is shown in Figure 5.

12 iarise Whitepaper 11 Instrument VLBA ARISE iarise Number of Spacecraft Data Rate (Gbps) Observing bands (GHz) 8/22/43/86 8/22/43/86 8/22/43/86 Detection Limits (mjy) /0.1/0.2/45 1/3/10/ /0.1/0.2/80 Maximum Baseline (km) 8,000 50,000 90, GHz Resoultion (µas) Sky Coverage Moderate Low High Image Quality High Low Moderate 1. Detection limit calculations assume that phase-referencing can be undertaken at 43 GHz and below and that the observation length is 28 hours. Table 1: Comparison of the properties of the VLBA, ARISE, and iarise missions. Figure 5: Comparison of simulated northern hemisphere UV-coverages (synthesized aperture distributions) for the single spacecraft ARISE mission (top) and two-spacecraft iarise mission (bottom) observing with a ground array such as NRAO s VLBA. In the simulation all spacecraft are in 30 inclination orbits and have 1,000 km perigee heights. The two iarise spacecraft have 80,000 km apogee heights, orbits nodes that are 180 apart and arguments of perigee that are 60 apart. The ARISE spacecraft has a 40,000 km apogee height.(southern hemisphere coverages are similar to this plot.)

13 iarise Whitepaper 12 4 iarise Technology The technology of iarise is mature, but some areas will require some preflight engineering development or demonstration. Technologies that require little or no development include: cryo-coolers, microwave low-noise amplifiers, GPS receivers, and on-board accelerometers. Technologies that require some development include: the on-board observing antenna, a nod-and-nutate pattern of antenna pointing, and the high data rate system. All of these engineering developments are considered straightforward low-risk efforts that could be accomplished in a short time at low cost (few $M). All of these technology thrusts are aimed toward achieving the best possible system sensitivity. The on-board antenna will provide as large an observing aperture as affordable (in terms of stowage, mass, efficiency, pointing, and cost). The data system from spacecraft to ground will provide a high data rate to allow as wide an observing band as affordable (in terms of frequency allocation, mass, power, and ground equipment and operations costs). The nod-and-nutate system will provide phase referencing for longer integration times. The GPS receivers and onboard accelerometers will provide a reconstructed orbit determination of 1 cm. This makes possible the imaging of spectral-line sources (e.g., megamasers) and the use of phase referencing techniques, which are necessary for iarise to reach sensitivity limits that are better than those forecast for ARISE, despite the smaller aperture of iarise. The cryo-coolers and microwave low-noise amplifiers will provide an extremely low system temperature at each of the observing bands. iarise relies on the Astro-Aerospace antennas of TRW. 12-m versions of these deployable antennas are currently in orbit on the satellite Thuraya (see Figure 6) with a surface tolerance sufficient for 10 GHz radio astronomy operation with good efficiency under various orbital thermal conditions. A straightforward engineering effort is required to reduce the tolerances by a factor of 4 to 8 and to extend the diameter from 12-m to 15-m. The reduced tolerances will be achieved by use of tighter manufacturing and assembly tolerances, smaller antenna gores (flat triangular facets of mesh stretched over a pseudo-geodesic truss of composite filaments), and finer reflector mesh with more openings per inch. The larger size is the maximum that can be built within existing facilities. Figure 6: 12.3m TRW Astro Aerospace Thuraya AstroMesh antenna. This antenna concept could relatively easily be developed into a 15m antenna working at 86 GHz for the iarise mission. iarise relies on phase referencing at observing bands up to 43 GHz to lengthen the integration time. This requires a novel combination of nodding (moving the main reflector in a smooth sinusoidal pattern between source and calibration target) and nutating (moving the subreflector when the beam is within range of a few beam widths so as to remain on target as the reflector nods). Currently it is thought that it will not be possible to undertake phasereferencing observations at 86 GHz due to the short coherence time, lack of strong calibrator sources, and relative high system noise temperature compared to the lower frequency bands. Additionally, Water Vapor Radiometers

14 iarise Whitepaper 13 will be installed at selected ground telescopes to help calibrate the troposphere at 22 GHz and 43 GHz and so extend the phase referencing cycle time. iarise relies on the JPL Black Jack GPS receiver (from SRTM, Champ, and Sac-C missions) and 0.1 nanometer-per-second-squared accelerometers from the GRACE mission (with early versions on Champ). iarise relies on JPL/TRW for cryo-coolers and low-noise amplifiers; these are flight-qualified products with known operational parameters and little uncertainty. iarise relies on 1 to 4 Gbps data rates transmitted from spacecraft to ground in the 37 to 38 GHz Space Research band and corresponding wideband recording at both ground tracking stations and co-observing ground radio telescopes. Links at 1, 2, and 3 Gbps are well within the state-of-the art, and rely on a well-established Gbps receiver-chip technology program jointly managed at GSFC and JPL; the 4 Gbps link requires development of a suitable on-board modulator and station demodulator. The limiting factor in the data rate is not expected to be the link from spacecraft to ground, but rather, the costs of implementing and operating the recorders or disks and the co-observing telescope back-ends. Gbps recorders now available in Japan are expensive; lower-cost versions are under development at York University in Canada. The York group have developed a jukebox approach that will require tape changes once per 60 hours at 1 Gbps. Disk-based data acquisition systems are being developed both in Europe (EVN) and the U.S. (Haystack Observatory) as an alternative to recorders. Furthermore optical fibre technology and infrastructure are advancing at such a rapid pace that by the time of the iarise mission fibre links may allow iarise data to be correlated in real-time obviating the need for any telescope-based recording systems. 5 iarise Costs The costs of iarise have been determined by the same cost methodology used for ARISE in the Decade report, modified to accommodate mission changes, including two spacecraft rather than one. The on-board costs per spacecraft are dramatically reduced by selection of a standard feed, rather than an array feed, and by the choice of a TRW antenna, rather than an inflatable 25-m antenna, which would present substantial technical risks, as well as development and implementation costs. The data flow costs are dramatically reduced (1) by the reduction in data rate from 8 Gbps to the more manageable range of 1 to 4 Gbps, (2) by the design of new low-cost tracking stations, and (3) by cost-sharing of ground development costs by the European Space Agency (at least one European tracking station), National Science Foundation (use of the planned new correlator at the Expanded Very Large Array, EVLA), and Canadian Space Agency (donated recorders and further expansion of the EVLA correlator). The complete iarise mission is estimated to cost $370M. We expect the primary funding agencies to be NASA and the Japanese ISAS (and perhaps NASDA with which ISAS is merging). We have also been working with the Japanese on their development of the single craft VSOP2 mission, which though much less capable than iarise, provides parallels for development of the individual iarise craft. The estimated total cost of the VSOP2 mission is $200M. We believe that the clear superiority of iarise over VSOP2 will convince the Japanese to pool their resources with NASA for iarise. The net cost, then to NASA would be of order $150M after the contributions from the Canadian Space Agency and the European Space Agency. The iarise costs can be kept low by separating the mission into two well-defined parts. We suggest preliminarily that ISAS be responsible for the 2 spacecraft buses and launch, while NASA be responsible for the identical payloads associated with each radio telescope. The agencies would be equal partners - two partners, two telescope, working in tandem.

15 iarise Whitepaper 14 6 Conclusions iarise is an outstanding candidate for inclusion in the SEU roadmap. This international project would be NASA s first microarcsecond imaging mission and NASA s first observatory-class radio astronomy mission. iarise promises much higher scientific return in the study of supermassive black holes than the single-craft ARISE mission, which is part of the current roadmap. Substitution of iarise for ARISE accomplishes better science, decreased time to launch (before 2010), decreased risk, and decreased cost (only $150M approximately). All this is possible because the iarise mission is shouldered by international partners, chiefly the US and Japan, innovative in its design, and able to take advantage of improved but mature technologies. The Japanese space VLBI effort will likely proceed to a launch toward the end of this decade. If there is no investment by NASA, the Japanese effort will conclude with a limited VSOP2 mission, but if NASA collaborates, the efforts would lead to iarise. There is right now a narrow window of opportunity, which will close when the Japanese commit to development of VSOP2. If iarise is launched, it will join GLAST and Con-X in pushing back the frontier of research into SMBHs. These missions would have complementary but unique capabilities in the exploration of the ultimate limits of gravity and energy in the universe.