Black holes in the Milky Way and nearby galaxies

1. Black holesin the Milky Way and nearby galaxies • Jifeng Liu (SAO/NAOC/Eureka)

2. black hole is deep in the culture. to some, event horizon means infinite space, infinite terror; to some, black hole means a journey that begins where everything ends. to all, black hole is a hole not even curiosity can escape. Today I will talk about black hole from an astronomer’s perspective

3. outline • black holes: from theoretical curiosity to astrophysical reality • a brief history of the black hole concept • making black holes in astrophysical settings • detecting black holes on different scales • testing GR with black holes • two issues about astrophysical black holes • revealing “hidden” stellar mass black holes in the Milky Way • ultraluminous X-ray sources as intermediate mass black hole candidates • summary the talk will be organized as follows. First, I will give the background from theoretical curiosity to astrophysical reality. I will give a brief history of the black hole concept, I will describe how black holes can be made in astrophysical settings, I will then describe how the invisible black holes can be detected, and I will give examples how to use black holes to test GR. Black hole studies are not yet perfect, and I will describe two issues that we have been working on. First is on stellar mass black holes in the Milky Way and nearby galaxies, and them I will talk about ultraluminous X-tray sources as possible candidates for intermediate mass black holes.

4. “dark star” at Newton’s age • escape velocity • John Michell (1783) and Pierre-Simon Laplace (1796) • what if an object is massive but compact enough so that the escape velocity exceeds the speed of light? • it will be invisible from outside! a dark star! • but didn’t enjoy much popularity because it was not clear how gravity, the force between masses in Newton’s theory, could influence the massless light wave. the black hole concept can date back to late eighteenth century, when Newton ruled the world of physics. His theory stated that an object cannot escape the gravity of another object unless its speed exceeds the escape velocity, which is related to the mass and size of the object. John Michell in 1783 and Laplace in 1796 asked the question: what if a star is massive yet compact enough so that the escape velocity actually exceeds the speed of light? well, it will be invisible from the outside world, it will be a DARK star. How odd! yet this interesting dark star didn’t enjoy much popularity back then because it was not well understood how the gravity can influence the massless light wave in Newton’s theory

5. “black hole” in Einstein’s GR • Einstein (1915): mass distorts space-time as described by his field equation; the space-time distortion influences everything nearby including light • Karl Schwarzschild (1916) found a solution to the gravitational field equation of a point mass with a singularity at a radius later called the Schwarzschild radius • David Finkelstein (1958) identified the Schwarzschild surface as an event horizon, “a perfect unidirectional membrane: casual influences can cross it in only one direction”, or “a one-way exit to the outside universe” • any light impinged onto the event horizon won’t be reflected, but sucked in, so John Wheeler (1967) termed it as a “black hole” then came Einstein in 1915, who stated that mass distorts space-time as described by his field equation. the space-time distortion influences everything nearby, including light. Soon after, Karl Schwarzschild found a solution to Einstein’s field equation for a point source, with a singularity at a radius now called Schwarzschild radius. it took four decades of continuous efforts by many smart brains to figure out what this singularity really means. and in 1958, David Finkelstein identified Schwarzschild surface as an event horizon, “a perfect unidirectional membrane: casual influences can cross it in only one direction”, or “a one-way exit to the outside universe”. Because any light impinged onto the event horizon won’t be reflected, but sucked in, John Wheeler in 1967 termed it as a black hole.

6. fascinating facts about black holes there are many fascinating facts about black holes, and here I list some of them. First, no-hair theorem: a black hole can be completely described by only three parameters: mass, angular momentum and electrical charge. In comparison, a neutron star will require many more, like the equation of state, like density distribution, magnetic field etc. second, frame-dragging. A rotating black hole drags along the space-time surrounding it, making any object around it to rotate in the same direction. If an object wants to stand still relative to a distant observer, then it needs to move faster than light in the opposite direction. Also, a spinning black hole may be connecting another universe through a wormhole, although you need to prepare for its instability. There are many more, but I won’t list them all here. Now the question is, are black holes just theoretical, can they be made? and can they be detected? • no-hair theorem: a black hole can be completely described by only three parameters: mass, angular momentum, and electrical charge • frame-dragging: a rotating black hole “drags” along the space-time surrounding it, making any object around it to rotate in the same direction -- the object has to move faster than light in the opposite direction to stand still relative to a distant observer • a spinning black hole may be connecting another universe through a wormhole, although you need to prepare for its instability • and many more ... • Questions are: are black holes just a mathematical game? can they be made? do they really exist?

7. Making black holes in astrophysical settings (1) from death of massive stars progenitor mass < 8-10Ms The astronomer’s answer is clear “YES, we can make black holes, a variety of them, through different routes”. First, black holes can be made from the death of massive stars as this picture illustrates. The secret of a stellar life is a constant fight against the pull of self-gravity. At the normal stage of a star, you have nuclear burning that generates a lot of energy, most of it get radiated away in neutrinos, the rest transforms to thermal energy, and the thermal pressure gradient actually balances the gravity to keep the star stable. when the star exhausts all its nuclear fuel, it collapses with thermal pressure to balance the gravity. If the progenitor star is less massive than, say, 8 solar masses, then the electron degeneracy pressure can balance the gravity, and we have a whte dwarf. If the progenitor is more massive, but less massive than, say, 20 solar masses, then the neutron degeneracy can balance the gravity, and you have a neutron star. but if the progenitor is more massive than 25 solar masses, then the collapse cannot be stopped, you will have a black hole in the end. <20-25 Ms >25 Ms a stellar life is a fight against the pull of self-gravity

8. Making black holes in astrophysical settings (2) merging of stellar mass black holes • stars form in clusters, so are the remnant black holes, in a deep gravitational potential well • black holes sink to the center of clusters and merge into bigger black holes, possibly of a few thousand solar masses • possibly in the dense center of globular clusters today. It takes tens of millions of years to form an intermediate mass black hole there (Miller & Hamilton 2002) black holes can merge to form more massive black holes, and that is irreversible, according to Steven Hawking. In astrophysical reality, stars form in clusters of tens or hundreds of thousands of stars that form a deep gravitational well. The black holes will sink to the center of clusters. With increased space number density of black holes, they can merge into bigger black holes. possibly of a few hundreds or thousands of solar masses. This can really happen today in the dense center of globular clusters. It usually takes tens of millions of years to form an intermediate mass black hole there.

9. Making black holes in astrophysical settings (3) merging of stellar mass black holes • the merging of stellar mass black holes can also occur in star clusters in the early stage of galaxy formation. it is estimated there can be 10,000 intermediate mass black holes formed per proto-galaxy. • the merging does not stop there, because these intermediate mass black holes are in a deeper potential well of the whole proto-galaxy, so they will sink to the center of the proto-galaxy and merge further into an even more massive black hole • This even more massive black hole will accrete mass from surrounding gas, and eventually grow into the supermassive black hole of millions of solar masses at the center of the co-evolved galaxy we see today(Madau & Rees 2001) The merging of stellar mass black holes into intermediate mass black holes can also happen in the early universe when galaxies were forming. it is estimated that tens of thousands of intermediate mass black holes will form in a proto-galaxy. The merging doesn’t just stop at intermediate mass black holes, because they are in an even deeper potential well of the whole galaxy, so they will sink to the center of the proto-galaxy, and form an even more massive black hole then. This central black hole will accrete mass rom surrounding gas, and eventually grow into the supermassive black hole of millions of solar masses at the center of the co-evolved galaxy we see today.

10. detecting “invisible” black holes via matter around them that is “visible” • consider a black hole with a companion • the black hole and the companion rotate around the binary mass center. we can measure the motion of the companion, via doppler shift of spectral lines , and infer the mass of the black hole only one set of spectral lines will be seen for the “visible” companion radial velocity curve shows periodic change, can be used to get orbital period note: radial velocity = rotational speed * cos(i) inclination i=0 for edge-on so black holes of different masses can form. How can we detect them then? because black holes themselves are apparently invisible to us outsider observers. well, we can observe the motion of the visible matter around the black hole to infer the presence of the black hole itself. Here I show an animation expected for a binary, which rotate around the common mass center. The motion of the stars can be derived from the Doppler shift of the spectral lines from these stars. for a black hole binary, of course, we can only see one set of the lines from the visible companion. if we monitor the system over time, we can construct the radial velocity curve, which will show periodic changes. one thing to note is that the radial velocity we detect differs from the rotational velocity by a factor related to the inclination of the binary plane - so that when we see the system face-on, the radial velocity will be zero; when we see the system edge-on, the radial velocity will equal the rotational velocity.

11. detecting “invisible” black holes via matter around them that is “visible” • the companion as shaped by the black hole • the companion will be tidally distorted to pearl-like shape, the Roche lobe • gravity is normal to the surface, and different across the Roche lobe surface • this gravity difference means difference in temperature to maintain hydrostatic equilibrium - the so called gravitational darkening effects • temperature difference across surface makes it possible to derive inclination we consider the case when the companion is close to the black hole. in this case, the companion will be tidally distorted into a pearl-like shape as shown in the picture. this is called the Roche lobe. Note that the surface gravity is always normal to the surface, so that in an isolated, spherical star, the surface gravity all points to the stellar center. Here, however, not all surface gravity point to the center, and their amplitudes at different too. For example, at the point of L1, a test particle feels the pull from both objects and the centrifugal force, the total force is zero because they cancel each other. so the effective gravity is zero there. that means matter can stream freely through L1 point to the black hole, which will form an accretion disk around the black hole. the gravity is also reduced at the antipodal point of L1. When gravity is reduced, the temperature must drop to maintain the hydrostatic equilibrium, so the temperatures across the surface will differ from place to place. The L1 point will be the coolest, the antipodal point will be the second coolest, the two sides will be equally hotter, while the top and bottom will be equally hottest. • matter stream through L1 point to form an accretion disk around BH

12. detecting “invisible” black holes via the matter around them that are “visible” because of the temperature difference across the surface, the total flux from summing up the surface radiation will be quite different if we look from different viewing angle at different phase. so now it is in principle to derive the inclination angle if we have the flux variation over a period and compare it to a model. below i show the model we use for black hole binaries. This shows the binary geometry as seen at close. usually we cannot actually resolve the components, but we can collect the flux with photometric observations and construct the light curves in different bands. like shown on the right. The light curves here are called ellipsoidal light curves, which has a deeper minima when we see L1 point, a shallower minima when wee see the antipodal point, and two equal maxima when we see the two side. Ellipsoidal light curves means we are seeing tidally distorted stars, usually means presence of black hole as the close companion. These ellipsoidal light curve, if fitted to the model, will let us derive the inclination angle, the black hole mass, and the properties of the secondary etc. to summarize the procedures we use to discover the black holes: we monitor the companion spectroscopically to obtain the radial velocity curve , and photometrically to obtain the light curves, we then fit these curves to the binary model, and we will dynamically determine binary properties including black hole mass, inclination, and companion mass, radius, temperatures, and other parameters. • at different binary phase, we are seeing different parts of the secondary, with different temperatures, leading to different fluxes • measure the light from the binary with certain filters to only admit certain wavelength ranges • ellipsoidal light curves: a deeper minimum when seeing L1, a shallower minimum when seeing antipodal point, and two equal maxima when seeing the two sides Procedures for discovery: monitor the companion to obtain radial velocity curve + light curves ==fit to the binary model==> dynamically determine binary properties including companion mass, BH/NS mass, inclination etc credit: Liu, Orosz & Bregman (2012)

13. Schematic diagram for 20 dynamically confirmed black hole binaries here I show about two dozens of stellar mass black holes with dynamically determined masses. The sizes of the companions, the accretion disks are to the scale as shown here. We also show the inclination of each system. The color scale represents temperature of the star. There are three black hole binaries with high mass companions, and 17 with low-mass companions. the color scale represents temperature of the star missing here are M 33 X-3 IC 10 X-1 NGC 300 X-1 credit: Jerome Orosz

14. supermassive black hole at galactic centers • Milky Way: monitoring the stellar motion • a black hole of a few million solar masses 0.1”@8kpc=800AU Credit: Andrea Ghez

15. supermassive black hole at galactic centers • far away galaxies: no stars can be resolved! • use doppler shift of spectral lines to indicate motions • need to resolve the inner arc-second - only Hubble can do that! consistent w/ a ~1e9 solar mass black hole credit Bower et al (1998) Studies show there is a supermassive black hole at the center of every galaxy!

16. GR effect as revealed by matter around black holes now let’s see how black holes can be used to test GR. here I show around the black hole its accretion disk. When matter accretes onto the black hole, they don’t fall directly into black hole because of the angular momentum. instead, they spiral in, and form an accretion disk. • matter accreted onto a black hole forms a disk, with increasingly higher temperatures inward toward the black hole -- eventually so hot that the radiation peaks at X-ray • this intense X-ray (and UV) distinguishes black holes from normal stars! indeed, all known stellar mass black holes are X-ray binaries • the accretion disk truncates at the innermost stable circular orbit - inside which the materials undergo free-all toward the black hole X-ray optical inner edge = ISCO ultraviolet

17. GR predicts the space-time distortion around a black hole due to its spin, light bending, and that the innermost stable circular orbit shrinks for higher black hole spin • for higher spin, the accretion disk can extend closer to the black hole, reaching higher temperature and higher luminosity, resulting in X-ray continuum spectrum peaking at higher energy • model of accretion disk with full GR considerations, including spin, space-time distortion, light bending, ISCO (Li et al. 2005), can be compared to and tested by X-ray observations

18. The case of the black hole M33 X-7, among several others: model fits the X-ray continuum well, with the same spin for different observations over several years (well, we do not expect it to change suddenly) • this confirms the distortion of space-time around the black hole, and other GR effects the model adopts credit: Liu, McClintock, Narayan et al. (2008)

19. summary so far (1) • black holes are the inevitable prediction of General Relativity • black holes of various mass scales can be generated easily in astrophysical settings, and are confirmed by observations • two dozens of stellar mass black holes are discovered in the Milky Way and nearby galaxies • studies show there is a supermassive black hole at the center of every galaxy • we begin to use black holes to test GR • isn’t this a perfect black hole picture from theory to observation? • not yet ... still unsolved issues ... two issues I have been working on • where are most stellar mass black holes hidden in the Milky Way? • where are the intermediate mass black holes?

20. stellar mass black holes: theory vs observation • expected mass distribution for compact remnants from supernova explosions, for different theoretical considerations • define a black hole as above 3 solar masses, then neutron stars dominate black holes by a fact of 10 • always a continuous black hole mass distribution, with a cutoff at 10-15 Ms • about ~1e7 black holes and ten times more neutron stars expected for the Milky Way (or one black hole out of 10,000 stars given a total of 1e11 stars) credit: Fryer & Kalogera (2001)

21. confirmed stellar mass black holes there is a significant absence of black holes below 5-7 solar masses (Bailyn et al. 1998; Farr et al. 2010): either our picture for supernova explosion is seriously wrong, or we are limited by small numbers we need a lot more than two dozens of black holes to test the possibilities data taken from McClintock & Remillard (2006)

22. before we discuss how to get more, let’s see why so few. why so few so far? • the conventional method starts from an X-ray source in the sky, which usually corresponds to many counterparts in the optical: wait for X-ray outburst suggestive of NS/BH binaries-> the companion brightens up and is identified in the optical-> back to low hard state -> monitor the companion in the optical for the light curve and radial velocity curve -> fit to model to get binary properties and BH mass -> is it really a black hole? • so it really relies on X-ray outburst -- but we know most black hole binaries are very quiet without X-ray outbursts in our life span, so it will miss most black holes -- no wonder so few even after fours decades of work by hundreds of researchers (who just waited in idle most of the time)!

23. revealing the missed “quiet” black holes • The new approach: select black hole binary candidates in the optical/UV with the UV signature expected for the accretion disk, as manifested by the spectral energy distribution of known black hole binaries

24. spectroscopic verification to distinguish b/w we expect black hole binaries to have bright UV radiation, to be exact, it should have a late-type companion with spectral peak in the optical or near infrared, plus excessive UV radiation from the accretion disk that is far more than expected from the late-type companion. however, not all UV bright sources are black hole binaries. In fact, quasars can be pretty bright in UV, but quasars have distinctive broad emission lines, and it is easy to distinguish them with a medium resolution spectrum. WDs can be UV-bright sources too, and they can be distinguished by their signature broad absorption lines. also, early-type stars such as O/B/A stars can be pretty bright in UV, and they again can be distinguished by their unique sets of spectral features. These are quite different from our black hole binaries, and can be easily picked up from the optical spectral features. Less obvious are single late-type stars that are brighter than regular in UV because they are magnetically active or with giant flares. These can still be distinguished by presence of Ca II H&K around 3800A, and Ca II triplet around 8500A for active stars. Studies of GALEX sources with repeated observations also find that the FUV variations due to flares are generally less than 0.6mag, and certainly less than 6 mag. To summarize, UV bright sources can have different natures, and need spectroscopic verification to distinguish b/w different possibilities • possible UV bright sources: • late-type cool stars + NS/BH with excess UV radiation from accretion disk --- the UV-excess sources we are looking for, all the following are contaminants • quasars (with strong emission lines) • hot white dwarfs (with broad absorption lines) • early-type hot stars • late-type cool stars with giant flares, or magnetically active (w/ signature spectral lines Ca II H&K and Ca II triplet @ ~8500A)

25. such an approach possible only now • three components for the approach • all-sky UV survey with GALEX (a space mission in UV) and all-sky optical survey with Sloan to select UV bright sources • all-sky spectroscopic survey such as LAMOST, China’s large sky area multi-object fiber spectroscopy telescope, to weed out contaminants • LAMOST survey began last October, and will observe ~15K UV-bright sources over 5 years

28. 4000 fiber positioning units Guiding CCDs S-H sensor for MA

30. initial results for Dec,2011 • about 800 observed in Nov-Dec, 2011 (~1/20 of the total sample) • 345 targets with good spectra, of which • BH/NS binary candidates (21%) • late-type F/G/K/M stars with excessive UV too high for flares (6 mag brighter than the star itself) and w/ Ca II H&K or Ca II triplet for active stars (71: 21%) • contaminants (79%) • QSOs w/ broad emission lines (67) + GALAXY (19) (25%) • WDs w/ broad absorption lines, some w/ late-type companions(70: 20%) • early-type hot O/B/A stars (75:22%) • late-type F/G/K/M stars with slight or moderate UV-excess (<6mag) possibly from flares or magnetic activities (40: 12%) some w/ Ca II H&K 3937/3967 and Ca II triplet around 8500 • we expect ~1400 NS/BHs from the total sample, including 100-200 BHs

31. example 1: White dwarf

32. example 2: M1+UV-excess

33. confirming the initial results w/ HST • are excessive UV really from X-ray irradiated accretion disk, or from extremely rare, giant flares from late-type stars, or from some totally unexpected objects? • use HST UV spectrum to distinguish • X-ray irradiated disk has high-ionization emission lines such as N V 1240, C IV 1550, He II 1686 etc • nothing for late-type stars not in flaring state, or low-temperature emission lines from flares • we are proposing this cycle

34. confirming NS/BH binaries in X-ray • expectations for NS/BH binaries in low state: hard X-ray emission w/ Lx~1e32-34 erg/s, while stars have Lx <1e31 erg/s • test the initial results • only 3 out of 71 candidates were detected by ROSAT all-sky survey, not surprisingly given its low sensitivity to hard X-ray due to its small collecting area and soft X-ray band pass • they should be detectable with short Chandra exposures given the much higher sensitivity due to larger collecting area and harder X-ray band pass • we are proposing to observe them with Chandra this March • in the future, the candidates can be tested with NuStar (Nuclear Spectroscopic Telescope Array) with much higher sensitivity in hard X-ray (to be launched next year)

35. future plans: dynamical mass measurements • expect to finish LAMOST survey in five years • a total of 1400 NS/BH binaries expected, including 100-200 black holes • some targets will be observed multiple times, and may reveal radial motions • will follow up NS/BH candidates to measure the dynamical mass like for the known stellar mass black holes to determine whether a NS/BH binary candidate is a black hole or a neutron star • the addition of hundreds of black holes will change our view of black hole mass distribution, their formation and evolution • but huge amount of resources are needed!

36. resources available or unavailable • resources available for imaging photometry to obtain light curves • the 2.4m telescope in Lijian, China • MDM-2.4m telescope through Michigan • telescopes through National Optical Astronomical Observatory (NOAO) • we currently have a long-term project with CTIO-1.3m (10 nights in 2012) • American Association of Variable Star Observers (AAVSO): expect to have most candidates done by AAVSO • resources available for spectroscopy to obtain radial velocity curves • Palomar/P200 Hale telescope ( two nights in late March and late May) • MMT-6.5m telescope through Harvard, • Magellan-6.5m telescope through Harvard/Michigan • enough for a few black holes per year, but more telescopes are needed to obtain the total sample ( through NOAO or collaborations)

37. summary so far (2) • the mass distribution of known stellar mass black holes contradicts the current theory, which may completely invalidate our picture of supernova explosion if true, or just small-number statistics • we are searching for the “missed” black holes by looking at the UV signature of the accretion disk, a new approach made possible only by modern components, i.e., GALEX UV survey, Sloan optical survey, and the LAMOST spectroscopic survey • initial results show that we can expect 100-200 stellar mass black holes from our efforts, and ten times more neutron stars, which will certainly alter our view of stellar mass black holes upon completion, in five years. • the resources needed to measure the black hole mass dynamically are largely available, but we may be limited by our spectroscopic monitoring powers. • This project is pretty mature, we know what we are doing. It will also provide many research opportunities for students, to learn take the observations, collect, reduce and analyze the data, fit data to model, to work on individual sources, and to study the statistics

38. II: search for intermediate mass black holes • we haven’t found any intermediate mass black holes although they are expected today • we expect intermediate mass black holes to be detectable if they capture a companion (to accrete mass from and light up in X-ray) • X-ray emission is bounded by Eddington limit: the upper limit of the X-ray luminosity from the accrete disk, linearly proportional to the black hole mass • if the X-ray luminosity exceed the limit, radiation pressure will exceed the gravity pull on accreted matter and blow it away, and the X-ray luminosity will drop below the limit. • X-ray luminosity expectations based on black hole masses • stellar mass black holes => usually < 1e39 erg/s • intermediate mass black holes =?=> b/w 1e39 and 1e42 erg/s? • supermassive black holes if active => usually > 1e42 erg/s • ultraluminous X-ray sources: off-nuclear X-ray point sources in nearby galaxies with X-ray luminosities b/w 1e39 erg/s and 1e42 erg/s

39. 39 compact objects easily revealed with X-ray, including UltraLuminous X-ray sources (ULXs) The interacting pair of NGC4038/4039 (Antennae) has 14 ULXs: their luminosities are b/w 1e39-1e42 erg/s when placed at the distance of the host galaxy Ground-based optical HST Optical ULXs are a mixed class, and can include foreground stars and background quasars/active galactic nuclei. Chandra Fabbiano et al. (2003) do all galaxies have so many ULXs?

40. statistical properties of ULXs • do ULXs prefer star-forming galaxies? need to study a large sample of galaxies to address this question • we have administered two ULX surveys with huge datasets from archive (and tremendous skills, tricks, sweating work) • a ROSAT High Resolution Imager survey of 313 galaxies with 467 observations resulting 110 ULXs (Liu & Bregman 2005) • a Chandra Advance CCD Imaging Spectrometer survey of 383 nearby galaxies with 626 observations leading to 479 ULXs (Liu 2012) • average rate: 0.52 +/- 0.04 ULX per survey galaxy • late-type galaxy: 0.70+/-0.06 • early-type galaxy: 0.23+/-0.05 • confirms a connection between ULXs and young stellar populations

41. identifying the optical counterparts • need Hubble to resolve the dense stellar field (0.05 arcsecond versus one arcsecond from the ground) • need Chandra X-ray Observatory’s sub-arcsecond positional accuracy to match to the optical objects first optical counterpart identified: M81 ULX-1 as an O9V star: ULXs are very young systems! credit: Liu, Bregman & Seitzer (2002)

42. ultimate goal: mass determination via dynamical measurements • while ULXs are a mixed class, and some may not need intermediate mass black holes to interpret the observations, we need to measure the mass directly via dynamical methods, i.e., fit binary models to the light curves and the radial velocity curves to obtain the black hole mass • difficult because the optical counterparts usually • reside in dense stellar fields that cannot be resolved from ground, requiring Hubble to obtain the light curves • are really faint (V>23mag) and only the largest telescopes may obtain good spectra to derive the radial velocity curves • we have administered programs with Hubble, Keck, and Gemini for this purpose

43. best result so far NGC1313 ULX-2: Lx < 2 X 1040 erg/s V=23mag

44. light curve as revealed by HST Χ2=17.17 for 17 dof P=6.12±0.16 days A=0.102±0.016 mag Φ=143o±17o 6σ detection! (Liu+2009)

45. binary model w/ X-ray heating

46. disk motion as revealed by He II 4686A emission line from accretion disk Pakull & Grise (2006) 380+/-30 km/s Roberts et al (2010) 182+/-20 km/s

47. applying model to NGC1313 X-2... • it took sweating work to find the best model for the available data • modifying the model code and writing a solution finding algorithm suitable for the supercluster Odyssey at Harvard • it took a total of 10 million model evaluations on Odyssey (~3e5 cpu hours, or 34 cpu years), and months of my time • model works well: extinction value is highly constrained, and consistent with values derived from nebular lines • new astrophysics: the companion only fills ~50% of its Roche lobe (not filling 100% of its Roche lobe as everyone had assumed) • black hole mass can be a few tens of solar masses, or a few thousand solar masses if the true radial velocity is <40km/s credit: Liu, Orosz & Bregman (2012)

48. better constraints on black hole • new observations to determine mass • new HST UV spectroscopic observations of C IV, Si IV lines deep into the disk, less prone to dramatic change compared to He II from the upper atmosphere, and more representative of the black hole motion • new HST photometry in other bands • we are going to propose this cycle, with published results and detailed simulations to better our chances

49. Summary • black hole is not only a theoretical curiosity, but an astrophysical reality • black holes can work as a test bench for GR, and observations have revealed two dozens of stellar mass black holes in the Milky Way, and supermassive black holes at galactic centers • we are adopting a new approach to discover stellar mass black holes by combining the power of three all-sky surveys, i.e., the GALEX UV survey, the Sloan optical survey, and the LAMOST spectroscopic survey • initial results show that we can expect to discover 100-200 new stellar mass black holes, boosting current sample by ten times. This will change our view on stellar mass black holes. • ultraluminous X-ray sources could be the long-sought intermediate mass black holes -- the missing link b/w stellar mass black holes and supermassive black holes, and we are using the best facilities across wavelength to study them, including Chandra X-ray Observatory, Hubble Space Telescope, Keck-10m, Gemini-8m, Magellan-6.5m etc. • dynamical mass measurement of one ULX leaves room for IMBH, but further HST photometry and UV spectroscopy are needed to confirm

50. LAMOST spectra • pilot survey began 2011 Nov • observed a dozen of my targets in the first week • six are white dwarfs with broad absorption lines • one quasar with strong emission lines • others not recognized yet - are they NS/BH?