Transcript Diapositiva 1 - ego

Gamma-Ray Burst Searches with
Virgo: current analyses & future
prospects
Alessandra Corsi
 Formation of a GRB could begin either with
 If  not
Fireball
Model
the merger of two compact objects or
with the
uniform,

The fireball
expands
collapse
of a massive
star and
faster shells
collects ISM
collide with
54 ergs can
 As a result, an energy as high as E~10
slower
6

starts to in
be adecelarated
and
beItreleased
compact volume
of space
(~10
ones, and
an
external shock forms
cm)
internal
shocks form

The
inital
evolution
is
A
fireball
of e+/epairs, photons
and baryons is
inverted:
bulk
kineticconverting
energy is thermal
formed and
expands
energy
Part in
of
converted
into energy
internal carried
energy by the
bulk kinetic
the baryons
kinetic
across
the shock
front
energy
is
originally
present
in the explosion site (M
b)
converted

electrons
in internal
The
Whenaccelerated
the thermal
motion becomes
subradiate
prompt
relativistic, via
the bulksynchrotron
Lorentz
factor
saturates
to 
16 cm
13
~
10
~ 10
cm
emission
emission
emission
=E/Mbc2  afterglow
How to derive clues on the
nature of the progenitor?
The fireball model
n < 1 cm-3 and uniform medium
d
E.g. afterglow phase:
emission processes,
circum-burst
medium (density and
structure)
I will
consider the
link with the
GW domain
~ 1013 cm
~ 1016 cm
GW emission from GRB progenitors
Long GRB: no in-spiral phase, only
merger and ring-down
Short GRB: we expect the GW
signal to be emitted in 3 phases: inspiral, merger and ring-down
Long GRBs: the progenitor
Collapsar model (e.g. Woosley 1998):
 “GRB as the birth cry of a BH”: when the collapse of the iron core of a
rotating massive progenitor proceeds directly to a BH formation, the
stellar mantle falls into the newly formed BH and angular momentum
slows the collapse along the equator, ultimately forming an accretion disk
that, within a few seconds, launches particle jets along the rotation axis
powering a GRB”
 “The jets pass through the outer shells of the star and, combined with
the vigorous winds of newly forged radioactive metals blowing off the disk
inside, give rise to the supernova event”
 Collisions among shells of the jet moving at different velocities, far
from the explosion and moving close to light speed, create the GRB, which
can only be seen if the jet points toward us
Short GRBs: the progenitor
General picture:
Merger events of NS+NS or BH+NS systems widely favored:
 Seems unlikely that typical energies of short GRBs set free during the
dynamical merging; the following accretion phase in a postmerger system
consisting of a central BH and a surrounding torus is much more promising
 BH-torus system geometry: relatively baryon-poor regions along the
rotational axis  thermal energy release preferentially above the BH poles
via e.g.  anti- annihilation  can lead to collimated, highly relativistic jets
of baryonic matter if thermal energy deposition rate per unit solid angle
sufficiently large.
 -rays produced in internal shocks when blobs of ultra-relativistic matter
in the jet collide with each other. When the jet hits the ISM, the afterglow is
produced
GRB: progenitor models and time duration
Both progenitor types result in the formation of a few solar mass
BH, surrounded by a torus whose accretion can provide a sudden
release of energy, sufficient to power a burst. But different natural
timescales imply different burst durations:
 LONG: death of massive stars  free-fall time of the material
falling on the disk form outside,
tff ≈ 30s(M/10M⊙) −1/2 (R/1010cm)3/2
 SHORT: coalescing compact objects  duration set by the
viscous timescale of the gas accreting onto the newly-formed BH
(short due to the small scale of the system)
GRBs: EM signal and GW
GW emission
EM signal
Progenitor models
Source position,
(distance), trigger time:
on-source time window
Detecting GW
+
Total energy output:
hints on the disk mass
Nature of the progenitor
From the GW signal:
hints on the masses
involved in the process
Choice for an on-source time window: relevant timescales
The emitted GWs are in the frequency band accessible to
VIRGO & LIGO only for the last few minutes of in-spiral (e.g.
Cutler & Flanagan 1994);
Simulated launch + evolution of relativistic jets driven by
thermal energy deposition (e.g.  anti- annihilation) near BH
accretion torus systems: after 100 ms of constant energy supply (i.e.
tacc~100 ms), the fireball is at d ~3x109 cm. Standard fireball model:
GRB produced at d >1013 cm  another 400 ms after the central
engine supply shut off are needed to reach typical internal shock
distances (Aloy et al. 2004);
 But in long GRBs: delay between GWs and em signal dominated
by the time necessary for the jet to push through the stellar
envelope: can be of the order of 100s (Meszaros)
Connection between disk mass & energy output
Linking E,iso of a GRB to the energy output from the central engine and the mass
Macc accreted on the BH (during the phase when neutrino emission from the
accreted torus is sufficiently powerful to drive the jets): chain of efficiency
parameters corresponding to the different steps of physical processes between the
energy release near the BH and the -ray emission at distances of ~ 1013cm:
Disk mass:
Directly
depends on the
-1
2
E,iso = f1 f2 f3 f f4 Macc c mass ratio q 
measured: GRB
prompt emission
enters in the inspiral GW signal
Efficiency at which
(chirp mass)
accreted rest mass energy
Fraction of ultracan be converted to
f=2jet/4 =1-cosjet
relativistic jet energy
neutrino emission
Fraction of the sky
emitted in -rays via
covered by the two
dissipative shocks
Conversion efficiency of 
polar jets with semiwhen optically thin
anti- to e+e pairs
opening angles jet
conditions are
+
Fraction of e -photon fireball and solid angles jet:
reached: broad-band
energy which drives the ultra- afterglow light curve
afterglow modeling +
relativistic outflow with  >100
prompt emission
Torus formation in NS mergers
Oechslin & Janka 2006:
 q<1: less massive but slightly larger star
tidally disrupted and deformed into an
enlongated primary spiral arm which is
mostly accreted onto the more massive
companion. Its tail, however, contributes a
major fraction to the subsequently forming
thick disk/torus around a highly deformed
and oscillating central remnant.
 q~1 and ~ equally sized stars: both stars
tidally stretched but not disrupted and
directly plunge together into a deformed
merger remnant.
The disk mass is around 0.05 M for q=1
and 0.26 M for q=0.55.
Estimating the strains of GWs from GRB progenitor candidates
M=(m1, m2,)3/5 (m1, + m2,)-1/5 = (m1, ) q-2/5/(q+1)1/5
 In-spiral:
M=m1+m2
hc(f) ~ 1.4x10-21 (d /10 Mpc)-1 M 5/6 (f /100 Hz)-1/6
f  fi ~ 1000 [M/2.8M ]-1 Hz
=m1/(1+q)
Short GRBs,
optimal filtering
can be applied
 Merger: hc ~ 2.7x10-22 (d /10 Mpc)-1 (4 /M) F-1/2(a) (m /0.05)1/2
Kobayashi &
Meszaros
2003
fi  f  fq ~ 32 kHz F(a) (M/M)-1
Em= m (4/M)2 M c2
F(a)= 1- 0.63 (1-a)3/10
Short & Long GRBs,
order of magnitude
estimate of hc
 Ring-down: slowly damped mode, l=m=2, peaked at fq and width f:
f ~  fq/Q(a), where Q(a)=2(1-a)-9/20 Short & Long GRBs, but
frequency too high
hc(fq) ~ 2.0x10-21 (d /10 Mpc) -1 (/M ) [Q /14 F]1/2 (r /0.01)1/2
Typical values
a ~ 0.98 m ~ 0.05 r ~ 0.01
Kobayashi & Meszaros 2003
Detectability
Optimal filtering (in-spiral & ring down):
2opt=4  [hc(f) (f Sh(f))-0.5] 2 d(lnf)
  (d /10 Mpc)-1 M 5/6  5
Advanced LIGO
Advanced LIGO
Kobayashi
& Meszaros
2003
Fryer et al.
1999,
Belczynski et al.
2002
Advanced LIGO
GW signal amplitude and VIRGO sensitivity curve [f Sh(f)]1/2
ns-ns m1= m2 =1.4 M
- - - - in-spiral
-  -  -merger
bh-ns m1=12 M m2 =1.4 M
-  -merger
- - - in-spiral
Virgo
Virgo
Advanced Virgo
ring-down
Advanced Virgo
62 Mpc
53 Mpc
280 Mpc
220 Mpc
2300 Mpc
1100 Mpc
bh-he m1=3 M m2 =0.4 M
- - - - merger
Virgo
ring-down
Advanced Virgo
GRB 980425 d=40 Mpc
Collapsar
m1= m2 =1 M
- - - - merger
Virgo
Advanced Virgo
27 Mpc 23 Mpc
GRB 980425 d=40 Mpc
62 Mpc
95 Mpc
490 Mpc
110 Mpc
GRB analysis with Virgo
On behalf of Virgo collaboration
Virgo-Rome group: Fulvio Ricci
& EGO: Elena Cuoco
Vesf-EGO: fellowship support
Virgo C7 run & GRB 050915a
GRB 050915a: good position in one of the data stretch of the run, z unknown,
T90=53 s (15-350 keV), long GRB  we expect the progenitor to be a collapsar
and search for a burst type event in VIRGO data
Virgo sensitivity during C7 run
and nominal in black; LIGO
Hanford during S2 (coincidence
analysis with GRB 030329) in
red (2 km) and blue (4 km).
Definition of on-source and bkg region
http://gcn.gsfc.nasa.go
v/swift2005 grbs.html.
GPS 810818575 No events with
SNR>10
 Source region: a time window 180 s long, 2 min before the EM trigger and 1 min
after. Covers most astrophysical predictions, trigger uncertainty and accounts for
the favored ordering where GW precede the GRB
 bkg region: a single lock stretch of data, 16500s long around the source region,
used to estimate the statistical properties of the bkg and the pipeline efficiency
 On both the on-source and bkg region, we run the “Wavelet Detection Filter”
(WDF) by Elena Cuoco (VIR-NOT-EGO-1390-305 & VIR-NOT-EGO-1390-110),
selecting all events with SNR > 4.
 On-source and off-source distrib. are statistically compatible  UL analysis
Pipeline efficiency evaluation on bkg region:
e.g. sine-gaussian signals
Q=2sf0
f0=characteristic frequency EGW=(hSGrss)2 c3dL222f02/[5G(1 + z)]
If the GRB was optimally oriented, the UL would be a factor of ((F+)2+(Fx)2)-1 ~7 lower. At
nominal sensitivity, the noise strain @200 Hz is a factor of 15 lower. Under these
optimistic assumptions, our strain UL implies an energy UL of ~0.2 M at the distance of
GRB 980425 (40 Mpc). Kobayashi and Meszaros 2003: in the case of a merger of two blobs
of 1 M each, assuming that 5% of the mass energy goes in GWs, one expects EGW~0.1 M.
Prospects
WDF and population studies:
 Some changes to the wavelet bases used for the filter in order to apply the search also
to the in-spiral phase of short GRBs (Elena Cuoco)
 Need to set-up a Virgo procedure for population studies: while the GW signals from
individual GRBs may be too weak to be detected directly, the small correlations they
induce in the data near the GRB trigger time may still be detectable by statistical
comparison to data from times not associated with a GRB
Joint LIGO-Virgo searches within the External Trigger group:
 Virgo sensitivity during VSR1 improved nearly of a factor of 10 @ ~200 Hz with
respect to C7. LIGO-Virgo GRB searches for data collected during this period (S5-VSR1)
are planned (External Trigger group activities). ~50 GRB triggers received during the
joint LIGO-Virgo data-taking period (May – Oct. 2007). A coincident analysis will allow
us to quantify how much we gain by using more than one detector
Work in progress: definition of a common set of simulated signals to estimate
pipelines efficiency and compare LIGO-Vigo results (LIGO effort)
Currently, starting to test the kind of improvement we may have on Virgo ULs by
using coincidences with LIGO detectors (e.g. GRB 050520B and GRB 070729)
E.g.: GRB 070729
Starting to analyze 3 hrs of data around the GRB trigger time (~59 windows, 180s long).
A preliminary check for the maximum SNR of the Virgo triggers that survive in a 4
detector coincidence search (+/- 20 ms) gives a maximum SNR of ~3.5 (to compare with
SNR max = 9 during C7 run) in the 3 hrs of data.
SNR distribution of
triggers found in one of
the 59 windows (180s)
considered for bkg
studies in coincidence
with GRB 070729
… waiting for the plus and advanced configurations
Virgo /LIGO (red)
AdvVirgo
Virgo+/eLIGO
(green, 2008-09)
Virgo
Virgo semi-advanced
AdvVirgo/LIGO
(blue
-2013-14)
Virgo semi-advanced
"new mirror
model"
-20
10
-21
• BNS range: 121 Mpc
• BBH range: 856 Mpc
• 1 kHz sens.: 6 10-24
h(f) [1/sqrt(Hz)]
10
Advanced LIGO
Advanced Virgo without thermo-refractive
-22
10
10
-23
10
-24
10
10
100
1000
Frequency [Hz]
10000
The End