Properties of X- Ray Rich Gamma- Ray Bursts and X -Ray Flashes

1
Properties of X- Ray Rich Gamma- Ray Burstsand
X -Ray Flashes

• Valeria DAlessio Luigi Piro
• INAF section of Rome, Italy

XXXXth Moriond conference, Very High Energy
Phenomena in the Universe
March 12th-19th, 2005, la Thuile, Aosta, Italy
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Outline

• 1. Introduction to X-Ray Rich Gamma-ray
  Bursts (XRRs) and X-Ray Flashes (XRFs)
• 2. Description of our analysis of the
  following XRR/XRF properties
• - Distribution of spectral parameters
  of the prompt emission
• - Distribution of X and optical fluxes
  of the afterglow
• 3. Discussion of the results in the framework
  of high redshift scenario
• and inhomogeneous jet model off axis
  scenario
• 4. Conclusions

3
What are X Ray Flashes and X Ray Rich Gamma-Ray
Bursts?

• X Ray Flashes (XRFs) are a subclass of GRBs,
• 1/3 of them, discovered by BeppoSax
• in 2001 (Heise in' t Zand, 2001) (fig.1) as
• Events no detected by GRBM (40-700 keV)
• Events with high non thermic emission in X range
  2-10 keV

• X Ray Rich GRBs (XRRs) are a subclass of GRBs
  characterized by
• Very faint Gamma to X fluence in comparison with
  that of GRBs.

Fig1 Light curves of GRB980329 (left) and
XRF971019 (right) in range 2-28 keV and 40-700
keV (Heise 2003)
XRF and XRR spectrum, as GRBs, is described by
Band law. Kippen et al. (2001) and Sakamoto et
al. (2004) found

• spectral slope a and ß are marginally consistent
  with those of GRB
• Ep is significantly lower than GRB one

• Lamb et al. (2003) defined a criterious of
  classification for different events,
  according the value of spectral hardness ratio
  HS(2,30)/S(30,400)
• GRBs events with log H lt -0.5
• XRRs events with -0.5 lt log H lt 0
• XRFs events with log H gt 0

Fig 2 Comparison of spectrum for a classical
XRF, XRR, GRB (Lamb et al. 2003)
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Origin of XRFs

• GRBs at high redshift (z gt5) (Heise 2003) Ep
  would be shifted by a factor (1z)
• GRBs seen off-axis
• -uniform jet model e cons off-beam
  (Yamazaki, Ioka Nakamura 2003)
• -universal structured jet model e ? -2
  (Zhang Meszaros 2002, Rossi et al.
  2002)
• -quasi-universal gaussian-like
  structured jet model e e-?²/2?²o (Zhang et al.
  2004, Lloyd-Ronning et al. 2003)
• Unified jet model (Lamb et al.2003) XRFs have a
  wide opening angle jet, while classical GRBs have
  an high collimated jet.
• Dirty fireball (Dermer et al. 1999) in the
  external Shock model , fireball with high baryon
  load would lead to a smaller Lorentz factor and
  consequently smaller Ep
• Clean fireball (Mochkovitch et al. 2003) in the
  internal Shock model , fireball with the bulk
  Lorentz factor gtgt 300 and the contrast between
  the bulk Lorentz factors of colliding shells
  small could produce smaller Ep
• Off axis cannonballs (Dar De Rujula 2003)
• Photosphere-domianted fireballs (Dermer et
  al.1999 Huang, Dai Lu 2002)
• Peripheral emission from collapsar jets (Zhang,
  Woosley Heger 2003)

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The analyzed sample of XRRs and XRFs

• We compiled a sample of all the events observed
  until 31 December 2003 and classified in
  litterature as XRRs/XRFs.
• They are 54 events, 17 observed by BeppoSax and
  37 observed by HETE-2.
• We classified them according Lamb et al. (2003)
  criterious established for events observed by
  HETE-2.

• We confirm that the 37 events observed by HETE-2
  have log H gt -0.5
• We find that also the 17 events observed by
  Beppo Sax have log H gt -0.5

All the 54 events have log H gt -0.5 and so they
are XRRs/XRFs in particular we find 26 XRFs and
28 XRRs, but we consider them as an unique class.
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Distributions of a, ß and Ep
We built up the distributions of spectral
parameters a, ß and Ep of Band law for the
bursts of sample which have these parameters well
constrained and we compared results of their mean
value with that of 31 classical GRBs, 21 observed
by BeppoSax and 10 by HETE-2 (tab 1.,fig 3, 4)
CLASS ltagt ltbgt ltEpgt
XRRsXRFs -(1.200.05) 37 -(2.830.22) 19 475 51
GRBs -(0.980.07) 31 -(2.860.23) 25 19428 30
Tab1.Mean value of a, ß and Ep for the class of
XRRs/XRFs and GRBs.
XRRs/XRFs
GRBs
Fig. 3 Distribution of spectral slope ??(left)
and???(right) for XRRs/XRFs (red line) and GRBs
(black line).
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XRRs/XRFs
GRBs
Fig. 4 Distribution of Peak energy (right) for
XRRs/XRFs(red line) and GRBs (black line).

• We find that
• ltagt XRRs/XRFs is compatible with that of GRBs
  within 3s
• ltßgt XRRs/XRFs is compatible with that of GRBs
  within 1s
• ltEpgt XRRs/XRFs is lower by a factor 4 compared
  to that of GRBs

We confirm results of Kippen and Sakamoto
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What we expect for X and Optical afterglow
properties

• In the high redshift scenario
• no detection of Optical afterglow for XRRs/XRFs
• X-Ray flux of the afterglow of XRRs/XRFs fainter
  than GRB one.
• In fact, the observed flux depends on redshift as
  (Lamb Reichart 2000)
• F(?,t) L?(?,t) / 4pD²(z)(1z)1-ad
• where a is the spectral index, d is the temporal
  decaying index and D(z) is the comoving distance
• In particular
• if XRR/XRF are at z 5
  the ratio between their X
• and GRB at z1
  afterglow is 7
• 
  

9

• In the off-axis scenario
• The afterglow of XRRs/XRFs is fainter than GRB
  one, more and more as observing angle increases,
  but only at early time from burst trigger (fig. 5)

• In particular in the universal stractured jet
  model Eiso 4?e??-²
• with Ep?Eiso1/2 ? Ep??-1
• Ep(GRB)/Ep(XRR/XRF)?XRR/XRF/?GRB
• From our results of Ep ?XRR/XR/?GRB4.130.67
• Assuming
• XRRs/XRFs and GRB at z1
• Afterglow observed 11.1 hr from burst trigger

Fig 5.light curves of an inhomogeneous jet
obsereved from different angle. From the top
??0.5, 1, 2, 4, 8, 16 (Rossi et al. 2002).
The ratio between observed afterglow flux of
GRBs and XRRs/XRFs, considering time dilatation,
is 6 if ?XRR-XRF4 and ?GRB1 24 if
?XRR-XRF8 and ?GRB2 74 if ?XRR-XRF16 and
?GRB4
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What we find for X and optical afterglow
The results of mean value for the logarithm of
the optical and X fluxes of XRRs/XRFs with
observations within 1.5 d, compared with 27 GRBs
( De Pasquale et al. 2004) are
CLASS Log(fo) s² Log(fx) s²
XRRsXRFs -(31.020.11) 0.38 -(12.320.21) 0.35
GRBs -(30.850.12) 0.39 -(12.24 0.12) 0.26
The fluxes of the X and optical afterglow of
the XRRs/XRFs are consistent with that of GRBs!!

• Ro fo GRB/ fo XRR/XRF 1.480.55
  
• Rx fx GRB/ fx XRR/XRF 1.200.64

Neither the inhomogeneous jet off axis scenario
nor the high redshift scenario are consistent
with the properties of the total XRR/XRF sample
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Fig 8 Distribution of logarithm of Optical flux
at 11.1 hr from burst trigger in unit??Jansky
?for 11 GRBs (black line) and 10 XRFs/XRFs (red
line) with optical afterglow with early
observations
Fig 9 Distribution of logarithm of X flux at 11.1
hr from burst trigger in unit erg cm -2 s-1 for
27 GRBs (black line) and 15 XRFs/XRFs (red
line) HGXRRs/XRFs with host galaxy,
OTXRRs/XRFs with optical transient,
X XRRs/XRFs with neither HG nor OT.
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What we find for optical afterglow observations
and redshift

• For the 54 events there are 40 events with
  Optical Afterglow observation
• 24/40 events have no detected candidate optical
  afterglow
• 9/54 events have an estime of spectroscopic
  reshift 3/9 from host
  galaxy and 6/9 from optical afterglow. The mean
  value is ltzgt1.410.39

• 3/54 events have redshift constraints redshift
  is always zlt 3.5

The high redshift scenario is not consistent with
the global class of XRRs/XRFs
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Conclusions

• We confirm spectral properties of XRFs and XRRs
  are similiar to those of GRBs, except the lower
  value of Peak Energy which is lower.
• We find that the X and Optical fluxes of the
  afterglow of XRRs/XRFs are compatible with that
  of GRBs and that the the mean value of redshift
  for 9 XRRs/XRFs is a low value.
• Nor high redshift scenario neither inhomogeneous
  jet model scenario can explain the properties of
  all the XRR/XRF class.
  
  
  
  

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Analysis

1. We analysed in particular X and Optical Flux of
   the afterglow at 11.1 hr from burst trigger and
   we compared distribution and mean value of logFo
   and logFx for XRRs/XRFs and GRBs.
   
2. When the value of Fo and Fx at 11.1 hr
   was not available in literature, we extracted
   them at 11.1 hr from the observations of the
   afterglow at different time with the best
   temporal slope between prompt and afterglow
   observations. We used only events with
   observations until 1.5 day from burst trigger.
3. 
   
   

• We find
• 15/54 X candidate afterglow , 9 of them with
  observations at early time (4 XRF and 5 XRR)
• 40/54 optical observations with16 Optical
  candidate afterglow and 24 DARK events, but 9
  of them have not early time observations ?31
  events, 21 DARK (11 XRR and 10 XRF) and 10 OT (7
  XRR and 3 XRF)

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Criterious of definition

• Lamb et al. (2003) defined a criterious of
  classification for different events, according
  the value of spectral hardness ratio H
  S(2,30)/S(30,400)
• GRBs events with log H lt -0.5
• XRRs events with -0.5 lt log H lt 0
• XRFs events with log H gt 0

Histogram of hardness ratio for GRBs ( blue),
XRRs (green) and XRFs (red) observed by HETE-2
(Lamb et al. 2003)
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Spectral properties of prompt emission of XRRs
and XRFs

• XRFs and XRRs, as GRBs, have a
• spectrum described by Band law (fig. 3)
• E? exp(-E/Eo) E (a-ß)Eo
• N(E)
• E? E
  (a-ß)Eo
• With Ep(2a) Eo
• Kippen et al. (2001) analysed 9 XRFs observed by
  BeppoSaX and by BATSE off line data. They found
• spectal slope a and ß are marginally
  consistent with those of GRBs
• Ep is significantly lower than GRB one, which
  is 300 keV.
• Sakamoto et al. (2004) analysed a sample of 16
  XRFs and 19 XRRs observed by HETE-2. He
  confirmed the results of Kippen et al. (2001).
  

Fig 3 Comparison of spectrum for a classical
XRF, XRR, GRB (Lamb et al. 2003)
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Implication on inhomogeneous jet model
(II)Isotropic Energy Distribution
Since Eiso 4??????????-²
Riso EisoGRB/ EisoXRR/XRF (?GRB/
?XRR/XRF)-2 With previous assumption and results
we obtain Riso48

• We calculated isotropic energy of the 14
  XRRs/XRFs with estimated redshift and we compared
  them with values of 17 GRBs (Bloom et al, 2001).
• We obtained mean value for this parameter and
  distribution (fig.9)
  
• ltEisogt (4618)1051 erg for XRRs/XRFs
• ltEisogt (330100) 1051 erg for GRBs

Eiso(GRB)/ Eiso(XRR/XRF) 7.17 3.55
Fig. 9 Distribution of logarithm of isotropic
Energy for XRRs/XRFs (red line) andRBs (black
line). G
This results is consistent within 1?, BUT there
are three events, XRR000615, XRR011030 and
XRF020903, whose Eiso is lower by 3 or 4 order
of magnitude compared to GRB one.
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What are X Ray Flashes and X Ray Rich Gamma-Ray
Bursts?

• Events no detected by GRBM (40-700 keV)
• Events with high non thermic emission in X range
  2-10 keV

X Ray Flashes (XRFs) are a subclass of GRBs,
1/3 of them, discovered by BeppoSax in 2001
(Heise in' t Zand, 2001), as

• X Ray Rich GRBs (XRRs) are observed as
• Events detected in Gamma range by GRBM
• Events with very faint Gamma to X fluence

Fig1 Light curves of GRB980329 (left) and
XRF971019 (right) in range 2-28 keV and 40-700
keV (Heise 2003)
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Implication on inhomogeneous jet model
observing angle

• Appling this result of Rx fx GRB/ fx XRR/XRF
  we extracted the value of the observing angle of
  XRRs/XRFs, ?XRR/XRF , assuming ?GRB1.

??????XRR/XRF (2-22 )
The inhomogeneous jet scenario is not consistent
with global class of XRRs/XRFs
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