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Pawan Kumar Relativistic jets in tidal disruption events


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Prof. Pawan Kumar Relativistic jets in tidal disruption events

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Pawan Kumar Relativistic jets in tidal disruption events

  1. 1. • • Spectacular Explosive Events in the universe associated with black holes and neutron stars♪ Pawan Kumar Outline† TIFR, July 12, 2017 Recent progress Brief History of explosive events ♪Rodolfo Barniol Duran, Patrick Crumley, Wenbin Lu, Mukul Bhattacharya
  2. 2. One of the major accomplishment of the 20th century astronomy was to map the universe, and determine its composition and general properties. Ordinary matter (that we, and the stars and galaxies are made of) Dark Matter (does not emit light and does not interact much) Dark Energy (something mysterious that we know very little about) 4% 22% 74%
  3. 3. Most of the ordinary (baryonic) matter in the universe is in the form of gas, and a smaller fraction (~5%) is in stars. Most of the stars are similar to the sun and a typical astronomer finds these objects rather boring. However, a small fraction of stars (which are massive) leave behind black holes or neutron stars (compact objects) upon their death. Associated with these compact objects are spectacular events and explosions. This talk is about these events
  4. 4. We have known for at least 103 years that stars are not for ever Chinese court astrologers recorded a new “star” on July 4, 1054, that was visible even during the day (brighter than Venus) for about a month. Spectacular explosions and “compact objects” Optical Image The remnant of the “Chinese Guest Star” of 1054 – Crab nebula 33mspulsar Even after the death this “star” is ~105 times more luminous than the Sun! In about a million years though it will fade away. The Crab nebula was identified as the remnant of the stellar explosion of 1054 between 1921 and 1942 – at first speculatively by Hubble (1928) and beyond reasonable doubt by Jan Oort in 1942. A pulsar (spinning neutron star) was born in this explosion. Song dynasty emperor Renzong
  5. 5. A supernova in the Galaxy M96 Supernovae are spotted several times a day in modern astronomical surveys – a subclass of these explosions led to the discovery of dark energy in the universe! Black holes or neutron stars are born in core collapse spernova Galaxy M96
  6. 6. Recent developments (in the last ~ 7 years) in supernova research is the discovery of super- luminous events which are ~10—50 times brighter than a typical supernova; the discovery was made by a graduate student (Quimby) and my colleague (Wheeler) in Austin, Texas, using a small (45 cm) telescope. Another discovery is that of mildly relativistic SNe. These highly energetic supernovae are most likely powered by accretion onto a newly formed black hole or a rapidly spinning neutron star endowed with a super strong magnetic field (~1015) Gauss.
  7. 7. Life & Death of a Massive Star Gas & dust cloud Main sequence star (10 million years) Mass loss Supergiant Type II supernova Neutron star and supernova remnant Black hole & Accretion disk Schematic sketch of life cycle of stars
  8. 8. Gamma-ray bursts (GRBs) Another class of explosions:
  9. 9. Announcement of accidental discovery of γ-ray bursts by Vela:
  10. 10. Gamma-ray Bursts What are these? We see bursts of energy in gamma-rays from outer space, a few times a day, lasting for a few seconds. The energy involved is enormous!
  11. 11. Panaitescu&Kumar(2000) Energy release: ~ 1052 erg Relativistic jet Lorentz factor > 102 Angular size of the jet ~ 4o More energy comes out in these explosions in a few seconds than the Sun will produce in its 10 billion year lifetime! Almost certainly a black hole or a neutron star in born in these explosions Understanding the physical properties of these explosive events requires broad band data: 109 – 1024 Hz:
  12. 12. GRBs and the infant universe (similar to bursts at low z) GRB 090429B: z=9.4 (age of the universe 0.52 billion years) T = 5.5s, fluence=3.1x10-7 erg cm-2 Eiso=3.5x1052erg (Ep=49 keV) GRBs at high redshift are potentially powerful probes of the properties of the very first stars formed in the universe.
  13. 13. AstroSat might be able to answer the long unsolved question of MeV radiation mechanism (via polarization measurement). Sept 27, 2015
  14. 14. Another class of spectacular transient events: Stars falling into black holes
  15. 15. aT ~ GMBHR*/d3 Tidal acceleration: Star’s self-gravity: a* ~ GM*/(R*)2 d < RT = R* (MBH/M*)1/3Star is tidally torn apart if: aT > a*  If a star passes close to a black hole (BH), it is shredded by the tidal gravity. The star is partially accreted onto the BH, and relativistic jets are launched.
  16. 16. This event (and 2 similar events) – detected by the NASA satellite Swift – are excellent systems to address some long standing questions regarding relativistic jets:  How are X-ray photons produced?  What are jets made of? • • • A star wandered too close to a massive black hole on March 25, 2011, in a galaxy ~ 3.8 billion light years away (z=0.35), and the star was shredded by the tidal gravity of the black hole. An accretion disk was formed, and a relativistic jet was produced when roughly half of the star fell into the black hole. Powerful radiation from X-ray to mm bands was observed.
  17. 17. o X-ray data show a period of intense flaring lasting for ∼10 days. The variability time is ~102s. o Spectrum is a simple power-law function from 0.3 – 10 keV: fν α ν-0.8 Swift/XRT data Swift J1644+57: X-ray data Burrows et al. (2011)ν (keV) fν 10 μJy 101
  18. 18. t-5/3 decay Swift/XRT data Swift J1644+57: X-ray data Burrows et al. (2011) o X-ray data show a period of intense flaring lasting for ∼10 days. The variability time is ~102s. o Spectrum is a simple power-law function from 0.3 – 10 keV: fν α ν-0.8 ν (keV) fν 10 μJy 101
  19. 19. Jet turns off Swift J1644+57: X-ray data Swift/XRT data Burrows et al. (2011) o X-ray data show a period of intense flaring lasting for ∼10 days. The variability time is ~102s. o Spectrum is a simple power-law function from 0.3 – 10 keV: fν α ν-0.8 ν (keV) fν 10 μJy 101
  20. 20. What did we learn (Kumar et al. 2016)? A good fraction of star’s mass is converted to energy (E ~ m* c2/20) – in the form of a relativistic jet Piecing together the multi-wavelength data (mm to γ-rays) we are able to show that most of the jet energy is in magnetic fields, i.e. the blabk hole produced a Poynting jet. Magnetic field is dissipated at ~1015 cm from the BH and e- accelerated to ~TeV, and p+ to at least ~1018 eV; X-rays produced by the synchrotron process
  21. 21. These objects with relativistic jets are likely responsible for producing ultra-high energy cosmic rays and energetic neutrinos. ICECUBE result GZK cutoff Pierre Auger Observatory atmospheric ν astrophysical ν Aartsen et al. (2014)
  22. 22. A new class of bright transient events was discovered by radio astronomers at 1-3 GHz in 2007. These are called Fast Radio Bursts these events last for only about a milli-second (the remainder of the talk is about them)
  23. 23. Dispersion Measure: Propagation of radio waves through ISM/IGM Pulse broadening due to scattering • • Unit: pc cm-3
  24. 24. History Discovered in 2007 – Parkes 64m radio telescope (Australia) DM = 375 cm-3 pc δt(λ) λ4.4 (Consistent with pulse broadening due to ISM/IGM turbulence) Duration (δt) = 5ms Lorimer et al. (2007)Flux = 30 ± 10 Jy (3x10-22 erg s-1cm-2 Hz-1) (DM from the Galaxy 25 cm-3pc –– high galactic latitude) Estimated distance ~ 500 Mpc
  25. 25. FRB 110220 Thornton et al. (2013)
  26. 26. Burst duration (milli-second)
  27. 27. (so these events are not catastrophic; VLBI localization – 3 m-arcsec) Parkes ~ 14’; GBT ~ 6' Arecibo ~ 3'; VLA~0.1” Spitler et al. 2014, 16 Schol et al, 2016; Chatterjee et al. 2016. One object (121102) had multiple outbursts (>102 in 4 yrs) Z=0.19 (972 Mpc) Liso = 0.05-7x1042 erg s-1 0.1 arc-sec from a persistent radio source of 0.2 mJy. 2 bursts seen in VLA 2.5-3.5 GHz, but not in simultaneous obs. at Arecibo 1.1-1.7 GHz! DM = 558.1 ±3.3 pc cm-3 (same for all bursts) Wang & Yu (2016)
  28. 28. FRB 121102 Statistics Energy distribution function (EDF) Wang (2016) (VLBI localization – 3 milli-arcsec; z=0.19 – Chatterjee et al. 2017)
  29. 29. Properties of FRBs (summary) Duration: DM ~ 103 pc cm-3 Isotropic energy energy release: High galactic latitude, isotropic distribution • • • • One object can produce multiple outbursts• (so unlike supernovae, GRBs & TDEs, FRBs are not catastrophic events)
  30. 30. Models for FRBs Mergers (White Dwarfs, Neutron stars) Collapsing Neutron stars Magnetar flares Galactic flaring stars Giant pulses Asteroids colliding with Neutron Stars …… Extra-terrestrial intellegent life communication
  31. 31. Basic features of the model suggested by the data Duration:• This strongly suggests that the source is a neutron star (or a BH) Magnetic field should be very strong: >1014 Gauss (a typical pulsar has 1012 Gauss) And when the magnetic field undergoes reconfiguration, an intense pulse of radio waves is produced. Kumar, Lu and Bhattacharya (2017)
  32. 32. Radiation mechanisms Maser process: synchrotron, curvature radiation etc. – absorption coefficient can become negative under suitable conditions Collective Plasma wave emission: particle beam energy converted to plasma waves which are converted to EM radiation via non-linear process or mode coupling. These mechanisms produce radiation at a frequency that is often related to the plasma frequency νp Antenna mechanism – Coherent curvature radiation The brightness temperature is very high: Coherent radiation mechanism
  33. 33. Coherent curvature radiation Frequency of radiation:             : radius of curvature of field lines : Lorentz factor of particles Magnetic field produced by the current associated with particles streaming along the field lines This “induced” field is perpendicular to the original field B0
  34. 34. This suggests that we are dealing with a magnetar             The “induced” field will tilt the original magnetic field by different angles at different locations (because the “induced” field lines are closed loops in planes perpendicular to B0 ) This will cause the particle velocities to be no longer parallel and that will destroy coherent radiation, unless B0 > 1014 G B0 Lower limit on B0
  35. 35. The radiative cooling time of electrons is very short: To prevent this rapid loss of energy, we need an electric field that is parallel to B0 to keep the particles moving with Lorentz factor γ. This time is much smaller than the wave period (1 ns for 1 GHz radiation) The required electric field: Particle acceleration
  36. 36. Electric field generation in magnetic reconnection in the neutron star magnetosphere βin = Vin/c Electric field
  37. 37. Coherent radiation requires particle clumps of size ~λ Electrons & positrons moving in opposite directions due to the electric field suffer from 2-stream instability, which can generate particle clumps. The length scale for the fastest growing modes is of order 50 cm and the growth time is ~ a few μs.
  38. 38. Age of the magnetar It cannot be less than about 102 years Otherwise, the SNa remnant’s contribution to the DM changes in 3 years by an amount larger than we see for FRB121102. It cannot be older than ~104 years • • Crustal motion and flux emergence probably cease on this time scale Viganoetal.2013
  39. 39. Energetics The total energy release is modest: Whereas the total energy in the magnetic field is ~ 1045 erg So there is no problem powering a large number of bursts! The total number of electrons/positrons needed for producing a FRB radiation is ~ 1030. So about one kilogram of matter is producing the radiation we see at a redshift ~ 1. • • •
  40. 40. Predictions of the model We should see FRB like bursts at much higher frequencies (mm–optical) – if the model I have described is correct. The reason for this is that the peak frequency for curvature radiation depends strongly on γ: and The event rate
  41. 41. Summary The newest class of transient events, discovered in 2007, are Fast Radio Busts (FRBs). These are short, intense, radio pulses lasting for about 1 ms. We now know that they are most likely associated with young neutron stars with unusually strong magnetic field > 1014 G. The radiation is coherent curvature radiation; e± are accelerated in magnetic reconnection. Multiple outbursts from the same object is a natural expectation of this model. • • • In the last 30 years, and particularly in the last 10 years, a new frontier in astronomy has opened up, and that is the study of explosions (transient events). These events, in particular GRBs & TDEs, have taught us about the birth of black holes and neutron stars and interesting physics associated with these objects.
  42. 42. BH or Magnetar central engine Black-hole based central engine (Woosley 1994; Bucciantini et al., 2009) Usov (1992); Wheeler & Yi (1999) Zhang & Woosley (2006) Woosley (1993); Paczynski (1998)   Almost certainly a black hole or a neutron star in born in these explosio
  43. 43. General constraints on Maser & Plasma mechanisms These mechanisms are favored for radio pulsars, and so are obvious possibilities to consider for FRBs. Particle beam kinetic energy is converted to GHz radiation Lbeam > Lfrb ~ 1043 erg s-1 The outflow launch radius: R < 107 cm Lfrb,43 B14 -1/4 1/2 Lbeam < R B2 c/4π = 1049 erg s-1 B14/R6 22 4 ~
  44. 44. Kinetic energy luminosity of the outflow: We can eliminate ne in terms of ν (νp ne 1/2) Synchrotron maser (in plasma) and plasma waves produce radiation at a frequency ~ νp γa 0 < a < 1 γ: e- LF νp: plasma frequency Observed frequency: ν = Γ νp γa Γ: source Lorentz factor (ν ~ 1 GHz) Lbeam > Lfrb ~ 1043 erg s-1 The maser (or plasma-wave generation) is at R > RLC = 5x109P cm • • •
  45. 45. Ne is ~ 109 times larger than the total number of e± in the entire magnetosphre of a NS with Bns ~ 1014 G (assuming ne is of order Goldreich-Julian density)! (Curvature maser radiation suffers from the same problem) So the maser mechanism requires an outflow that is generated well inside the light-cylinder of the NS and the radiation is produced well outside the LC. This is a serious drawback for the Maser and plasma-wave mechanisms. where Since
  46. 46. Reconnection (σ ≈ 1013) The rate of energy flow into the current sheet: this turns out to be sufficient to power FRB emission • The total energy release in a burst is modest: The total energy in the magnetic field is ~ 1045 erg So there is no problem powering a large number of bursts! The total number of electrons/positrons needed for producing a FRB radiation is ~ 1030, i.e. ~ 1 kg of matter is producing the radiation we see at a redshift ~ 1. • •
  47. 47. Millennium simulation – Springel et al.
  48. 48. What can we say about the FRB source? Coherent radiation: electric fields of different particles must point in the same direction and have the same phase. The FRB luminosity is L ~1043 erg s-1 The strength of the electric field associated with the FRB radiation at the source is: Source size in the comoving frame cannot be much larger than λ’ = λ γ
  49. 49. Properties to FRBs Cordes+16
  50. 50. Electrons must stay in the ground state of the Landau levels, otherwise coherence will be destroyed.  0 1 2 3 4 n Energy for transition from 0 1 should be large, so that Coulomb collisions, two-stream instablity etc. cannot excite particles to higher Landau states & destroy coherence. Strong magnetic field helps to stabilize the system
  51. 51. Faraday rotation FRB 110523: RM = -186 rad m-2 DM = 623.30 DMMW ~ 45 • not from the MW or IGM • << typical B (few µG) in the MW
  52. 52. Could FRBs be analog of giant pulses (GP) from the Crab? Crab pulsar has ~MegaJy pulses of duration ~1 μs. The radio luminosity during the GP is of order 1% of the spin-down luminosity of the Crab, i.e. ~ 1036 erg/s. The GPs are seen once every hour and the rate declines with increasing flux as 1/f3. FRBs cannot be super GPs because the age of SNa remnant of FRBs is > 102 yrs (so that the DM does not change in a 4 year period), and the spin-down luminosity of >102 yr old pulsar is less than 1039 erg/s which is too small (assuming a Crab like efficiency for GP of 1%). In fact this argument rules out any rotation powered mechanism for FRBs – FRBs must be powered by magnetospheric activity. The Crab pulsar produces GP with flux 100--200 kJy once per hour (the brightest one is about an order of magnitude more luminous) --- see Cordes & Wasserman (2016).
  53. 53. How about magnetar flares producing FRBs? This too is rules out because the famous SGR 1806-20 (10 kpc from us) event with a luminosity of 1047 erg/s had a very stringent upper limit on radio luminosity of no more than 1041 erg/s. No radio from PSR J1119-6127 X-ray (radio efficiency < 10-8)