Gamma-ray burst light curves show quite different patterns: from very simple
to extremely complex. We present a temporal and spectral study of the light
curves in three energy bands (2-5, 5-10, 10-26 keV) of ten GRBs detected by the
Wide Field Cameras on board BeppoSAX. For some events the time profiles are
characterized by peaks superposed on a slowly evolving pedestal, which in some
cases becomes less apparent at higher energies. We describe this behaviour with
the presence of two components (slow and fast) having different variability
time scales. We modelled the time evolution of slow components by means of an
analytical function able to describe asymmetric rising and decaying profiles.
The residual light curves, after the subtraction of the slow components,
generally show structures more similar to the original curves in the highest
energy band. Spectral study of these two components was performed evaluating
their hardness ratios, used also to derive photon indices. Slow components are
found generally softer than the fast ones suggesting that their origin is
likely different. Being typical photon indices lower than those of the
afterglows there is no evidence that the emission processes are similar.
Another interesting possibility is that slow components can be related to the
presence of a hot photosphere having a thermal spectrum with kT around a few
keV superposed to a rapid variable non-thermal emission of the fast component.Comment: 16 pages, 20 figures (18 color, 2 B&W), accepted for publication in
  Astronomy and Astrophysic

Costa, E.

Massaro, E.

Soffitta, P.

Ventura, G.

Vetere, L.

English

arXiv

Gamma-ray burst light curves show quite different patterns: from very simple to extremely complex. For some events the time profile in the 2–5 keV range is characterised by peaks superposed on a slowly evolving pedestal. We describe

this behaviour with the presence of two components (slow and fast) having different time scales. The slow component is usually less pronounced in the 5–10 keV and almost disappears at higher energies. We present a time and spectral study of the slow components of several GRBs detected by the Wide Field Cameras on board BeppoSAX. The origin of the slow components is likely different from that of the fast ones and can be related either to the presence of a hot photosphere or to the overlapping of the prompt emission with the initial phase of the afterglow

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2005-10060-2IL NUOVO CIMENTO Vol. 28 C, N. 3 Maggio-Giugno 2005Slow components in the X-ray light curves of Gamma-Ray Bursts(∗)L. Vetere(1)(2), E. Massaro(1), E. Costa(2) and P. Soffitta(2)(1) Dipartimento di Fisica, Universita` La Sapienza - Piazzale A. Moro 2, I-00185 Rome, Italy(2) INAF, IASF - Sezione di Roma - via del Fosso del Cavaliere, I-00113 Rome, Italy(ricevuto il 23 Maggio 2005; pubblicato online il 19 Settembre 2005)Summary. — Gamma-ray burst light curves show quite different patterns: fromvery simple to extremely complex. For some events the time profile in the 2–5 keVrange is characterised by peaks superposed on a slowly evolving pedestal. We de-scribe this behaviour with the presence of two components (slow and fast) havingdifferent time scales. The slow component is usually less pronounced in the 5–10 keVand almost disappears at higher energies. We present a time and spectral study ofthe slow components of several GRBs detected by the Wide Field Cameras on boardBeppoSAX. The origin of the slow components is likely different from that of thefast ones and can be related either to the presence of a hot photosphere or to theoverlapping of the prompt emission with the initial phase of the afterglow.PACS 95.85.Nv – X-ray.PACS 98.70.Rz – γ-ray sources; γ-ray bursts.PACS 01.30.Cc – Conference proceedings.1. – IntroductionThe shapes of GRBs’ light curves are very different: they can be very simple orextremely complex, in particular those showing several peaks and spikes are observed inlong duration events. This particular behaviour has been studied many times lookingfor possible relations between different parameters of the light curve. Using the entiredatabase of the Wide Field Cameras we realized that the light curves of several GRBsseem to be made of two components: a fast and structured emission superposed on aslower and smoother one. We found that 15 GRBs out of 56 (27%) show this pattern,although almost the 20% of all the events in the database are too faint to exclude thepresence of this component. In this contribution we present some results of the analysisperformed on 11 well structured events.(∗) Paper presented at the “4th Workshop on Gamma-Ray Burst in the Afterglow Era”, Rome,October 18-22, 2004.c© Societa` Italiana di Fisica 359360 L. VETERE, E. MASSARO, E. COSTA and P. SOFFITTATable I. – Values of the Hardness Ratios and the spectral indices of the slow component.GRB HR1 HR2 HR3 Γ1 Γ2GRB980515 1.27 1.38 1.76 0.98 0.92GRB980519 0.6 1.56GRB990123 1→1.09 1→0.79 1→0.863 0.97→0.91 0.97→1.14GRB990704 0.71 1.22GRB990705 1.4 1.21 1.7 1.01 1.12GRB990908 0.86 0.58 0.5 1.27 1.54GRB000528 2 → 0.46 → 0.91 0.65 → 1.69GRB001011 0.67 1.45GRB001109 0.75 1.21GRB010222 0.6 1.39GRB010412 0.83 1.122. – Time and spectral modelling of Slow ComponentsFor each burst we produced the light curves in three different energy bands (2–5,5–10, 10–26 keV) and derived the model for the time evolution of the Slow Component(hereafter SC) in each band. We used a simple analytical expression for the SC timeevolution able to represent adequately even asymetric rise and fall of the light curve.This law is combination of a power law with an exponential function and contains onlyfour free parameters:(1) Fs(t) = AtB exp[CtD].The parameters’ values were found by adapting Fs to the local minima of the lightcurves. The goodness of the result was verified a posteriori by comparing at differentenergies the light curves of the Fast Components (hereafter FC) obtained by subtractingthe SC from the original data. In the majority of cases SCs were found quite soft anddisappear at high energies, but for about one third of the events SCs grow from 2–5 keVto 5–10 keV and to 10–26 keV. Some examples are given in the next section.To obtain spectral information we computed the Hardness Ratios (HR) between thecounts in the three used energy bands both for the SC that for the FC comparing themwith the corresponding values of the original one. We then estimated the correspondingspectral indices Γ by convolving a power law input spectrum with the WFC responsematrix in the direction of the burst. We will indicate with HR1 the HR between 2–5 keVand 5–10 keV energy range, HR2 the HR between 5–10 keV and 10–26 keV, while HR3is computed between 2–5 keV and 10–26 keV. In analogy Γ1 is the spectral index for2–5 keV and 5–10 keV energy ranges and Γ2 for the 5–10 keV and 10–26 keV ranges. Intable I are presented the calculated values of the Hardness Ratios and the spectral indicesfor the SC.Having verified that the SC is not caused by merging of single pulses increasing theirwidth at lower energies, we then searched for a possible spectral relation between theSC and the initial phase of the afterglow. A more detailed analysis will be presented inanother paper (Vetere et al. 2005, submitted).SLOW COMPONENTS IN THE X-RAY LIGHT CURVES OF GAMMA-RAY BURSTS 361Fig. 1 Fig. 2Fig. 1. – X-ray light curves of GRB010222 in the three considered energy ranges. Left panelsshow the total counts and the SC model; right panels show the corresponding FCs after the SCsubtraction.Fig. 2. – First panel: evolution of HR1 and HR2 for GRB 010222. Second panel: evolution ofΓ1 and Γ2. The last panel shows the total light curve (FC+SC) in the 2–26 keV energy range3. – ExamplesAs stated above the GRB light curves can be divided in two main types characterisedby the presence or not of a detectable SC in the highest energy band (10–26 keV). Herewe present a case for each type.3.1. GRB010222 . – It is a very strong burst detected by the WFC1 with a redshift z =1.477 [1] and an estimated isotropic energy output Etot = (154.2±17)×1052 erg [2]. The2–5 keV light curve shows several peaks superposed on an evolving SC which decreasesat 5–10 keV and seems to disappear at 10–26 keV. SC parameters were estimated fromthe 2–5 keV light curve, the same function was used to fit the SC at 5–10 keV, leavingfree only the normalisation.The comparison of the light curve at 10–26 keV with the FCs in the other bands showsthat they really look alike indicating that the SC was well modelled (fig 1). Photon indiceswere estimated from the HRs for the total light curve and the SC (fig 2).3.2. GRB990123 . – It is the strongest bursts detected by the WFC with a redshift z =1.6 [3] and an isotropic energy output Etot = (278.3± 31.5)× 1052 erg [2]. Unfortunatelythe X-ray light curve is not complete because of earth occultation just before the endof the burst The LC at 2–5 keV is characterised by the presence of a SC, but here it isgrowing at higher energies. The model of eq. (1) was not able to describe well the SCand it was necessary to consider the sum of a couple of functions shifted in time.4. – Summary and conclusionsOur analysis provided evidence of a SC in the X-ray light curves of some GRBsindicating that their emission is due probably to two different mechanisms: one thatproduces the higly varying structure and another responsible of the SC, seen in the 27%362 L. VETERE, E. MASSARO, E. COSTA and P. SOFFITTAFig. 3 Fig. 4Fig. 3. – X-ray light curves of GRB990123 in the three considered energy ranges. Left panelsshow the total counts and the SC model; right panels show the corresponding FCs after the SCsubtraction.Fig. 4. – First panel: evolution of HR1 and HR2 The three ticks represent (left from right) theSC’s HR3, HR2 and HR1 values. Second panel: evolution of Γ1 and Γ2 for GRB 990123, thetwo ticks represent (left from right) the SC’s Γ2 and Γ1 values. The last panel shows the totallight curve (FC+SC) in the 2–26 keV energy range.of the events detected by the WFCs. We tried to investigate if this second mechanismcould be the initial phase of the afterglow and did not find evidence of a direct relationbetween the SC spectra and those the afterglows. The latters are usually characterisedby a photon index Γ2 while the Γ values of SCs are all smaller (see table I). Note thatthe Γ values of the all bursts (FC+SC), which are stable in time, seem to set around ∼ 2approaching the end of the bursts. This could be actually the beginning of the afterglow.Moreover, SCs seem to be generally softer than the FCs and likely the spectra are notgiven by a single power law, as indicated by the different values of Γ1 and Γ2 at variancewith the afterglow spectra.REFERENCES[1] Stanek K. Z., Garvanich P., Jha S. and Pahre M., IAU Circ., No 7586 (2001).[2] Amati L. et al., A&A, 390 (2002) 81.[3] Kulkarni S. R., Djorgovski S. G., Odewahn S. C. et al., Nature, 398 (1999) 389.

Slow components in the X-ray light curves of Gamma-Ray Bursts

Gamma-ray burst light curves show quite different
patterns: from very simple to extremely complex. 
We present a temporal and spectral study of the light curves in three
energy bands (2-5, 5-10, 10-26 keV)
of ten GRBs detected by the Wide Field Cameras on board BeppoSAX. 
For some events the time profiles are characterized by peaks
superposed on a slowly evolving pedestal, which in some cases
becomes less apparent at higher energies. 
We describe this behaviour with the presence of two components 
(slow and fast) having different variability time scales. 
We modelled the time evolution of slow components by means of an 
analytical function able to describe asymmetric rising and decaying
profiles. The residual light curves, after the subtraction of
the slow components, generally show structures more similar to
the original curves in the highest energy band.
Spectral study of these two components was performed 
evaluating their hardness ratios, used also to derive photon
indices.
Slow components are found generally softer than the fast ones
suggesting that their origin is likely different.
Being typical photon indices lower than those of the afterglows
there is no evidence that the emission processes are similar.
Another interesting possibility is that slow components can be
related to the presence of a hot photosphere having a thermal
spectrum with kT around a few keV superposed to a rapid variable
non-thermal emission of the fast component

L. Vetere

E. Massaro

E. Costa

P. Soffitta

G. Ventura

EDP Sciences OAI-PMH repository (1.2.0)

Slow and fast components in the X-ray light curves  of gamma-ray bursts

Gamma-ray burst light curves show quite different patterns: from very simple to extremely complex. For some events the time profile in the 2-5 keV range is characterised by peaks superposed on a slowly evolving pedestal. We describe this behaviour with the presence of two components (slow and fast) having different time scales. The slow component is usually less pronounced in the 5-10 keV and almost disappears at higher energies. We present a time and spectral study of the slow components of several GRBs detected by the Wide Field Cameras oil board BeppoSAX. The origin of the slow components is likely different from that of the fast ones and can be related either to the presence of a hot photosphere or to the overlapping of the prompt emission with the initial phase of the afterglow

Loredana Vetere

MASSARO, Enrico

Archivio della ricerca- Università di Roma La Sapienza

Gamma-ray burst light curves show quite different patterns: from very simple to extremely complex. We present a temporal and spectral study of the light Curves in three energy hands (2-5, 5-10, 10-26 keV) of ten GRBs detected by the Wide Field Cameras oil board BeppoSAX. For some events the time profiles are characterized by peaks superposed oil a slowly evolving pedestal, which in some cases becomes less apparent at higher energies. We describe this behaviour with the presence of two components (slow and fast) having different variability time scales. We modelled the time evolution of slow components by means of an analytical function able to describe asymmetric rising and decaying profiles. The residual light Curves, after the subtraction of the slow components, generally show structures more similar to the original curves in the highest energy band. Spectral study of these two components was performed evaluating their hardness ratios, used also to derive photon indices. Slow components are found generally softer than the fast ones Suggesting that their Origin is likely different. Being typical photon indices lower than those of the afterglows there is no evidence that the emission processes are similar. Another interesting possibility is that slow components can be related to the presence of a hot photosphere. having a thermal spectrum with kT around a few keV superposed to a rapid variable non-thermal emission of the fast component

Slow and fast components in the X-ray light curves of Gamma-Ray Bursts

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Archivio della ricerca- Università di Roma La Sapienza

Archivio della ricerca- Università di Roma La Sapienza