Prof. Pawan Kumar Relativistic jets in tidal disruption events

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. 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. 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. 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. 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. 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. 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. Gamma-ray bursts (GRBs)
Another class of explosions:

11. Announcement of accidental discovery of γ-ray bursts by Vela:

12. 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!

13. 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:

14. 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.

15. AstroSat might be able to answer the
long unsolved question of MeV
radiation mechanism (via polarization
measurement). Sept 27, 2015

16. Another class of spectacular transient events:
Stars falling into black holes

17. 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.

18. 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.

19. 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

20. 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

21. 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

22. 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

23. 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)

24. 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)

25. Dispersion Measure:
Propagation of radio waves through ISM/IGM
Pulse broadening due to scattering
•
•
Unit: pc cm-3

26. 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

27. FRB 110220
Thornton et al. (2013)

28. Burst duration (milli-second)

29. (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)

30. FRB 121102 Statistics
Energy distribution function (EDF)
Wang (2016)
(VLBI localization – 3 milli-arcsec; z=0.19 – Chatterjee et al. 2017)

31. 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)

32. 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

33. 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)

34. 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

35. 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

36. 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

37. 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

38. Electric field generation in magnetic reconnection
in the neutron star magnetosphere
βin = Vin/c
Electric field

39. 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.

40. 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

41. 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.
•
•
•

42. 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

43. 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.

46. 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

47. 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
~

48. 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
•
•
•

49. 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

50. 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.
•
•

51. Millennium simulation – Springel et al.

52. 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

53. Properties to FRBs
Cordes+16

54. 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

56. Faraday rotation
FRB 110523: RM = -186 rad m-2
DM = 623.30 pc.cm-3
DMMW ~ 45 pc.cm-3
• not from the MW or IGM
• << typical B (few µG) in the MW

57. 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).

58. 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)