DETECTION OF HIGH-ENERGY GAMMA-RAY EMISSION DURING THE X-RAY FLARING ACTIVITY IN GRB 100728A

The Fermi Gamma-Ray Space Telescope, launched in 2008 June, has taken the study of gamma-ray bursts (GRBs) into an energy realm that so far has been poorly explored. Fermi/LAT (Large Area Telescope; Atwood et al. 2009) observations of GRBs allow for the first time a detailed study of the temporal and spectral behavior at high energies (>100 MeV). One of the most interesting features is the detection of a delayed and rapidly decaying high-energy (HE) emission, lasting hundreds to thousands of seconds longer than the observed sub-MeV γ-ray emission (Abdo et al. 2009a, 2009b, 2009c). Extended GeV emission, first hinted at in EGRET observations (Hurley et al. 1994), appears now as a common feature of Fermi/LAT bursts. The nature of such long-lived HE emission is far from being established. One possibility is that it is generated via synchrotron radiation of the external forward shock (Kumar & Barniol Duran 2009, 2010; Ghisellini et al. 2010). An alternative scenario is that it reflects the gradual turnoff of the central engine activity (Zhang et al. 2011). Such interpretations predict very different afterglow behaviors (Piron & Nakar 2010; Mimica et al. 2010) and therefore can be directly verified through broadband (from optical/X-ray to GeV energies) early-time observations. To date, only one burst (GRB 090510; De Pasquale et al. 2010) of the 20 LAT detected GRBs has been simultaneously detected by the Swift multi-wavelength observatory (Gehrels et al. 2004). In this case, an afterglow emission provides a likely explanation of the broadband data set (e.g., De Pasquale et al. 2010; Corsi et al. 2010).

In this Letter, we report on the Fermi/LAT detection of a temporally extended emission from GRB 100728A and the simultaneous Swift observations of an intense X-ray flaring activity. We further discuss the possibility that in the case of GRB 100728A the observed HE emission is related to X-ray flares and ultimately to the long-lasting activity of the inner engine. Observations and analysis are reported in Section 2; our results are discussed in Section 3; we draw our conclusions in Section 4. Unless otherwise stated, the quoted errors are at the 90% confidence level and times refer to the Fermi/Gamma-ray Burst Monitor (GBM) trigger T₀.

2.1. Swift Data

The bright GRB 100728A came into the Swift field of view during a slew to a pre-planned target, when the trigger system is disabled. After the spacecraft settled, the burst triggered the Burst Alert Telescope (BAT; Barthelmy et al. 2005) on board Swift at 02:18:24 UT on 2010 July 28. Swift slewed immediately to the burst. The two narrow field instruments, the X-ray Telescope (XRT; Burrows et al. 2005) and the Ultraviolet Optical Telescope (UVOT; Roming et al. 2005), began settled observations of the field ∼80 s after the BAT trigger. An X-ray afterglow was promptly localized at a position of R.A. = 05^h55^m201, decl. = −15°15'191 (J2000) with an uncertainty of 14 (Beardmore et al. 2010), while no counterpart was observed in the early UVOT unfiltered exposures down to a limiting magnitude wh > 20.5 (3σ confidence level; Oates & Cannizzo 2010).

Swift data were analyzed in a standard fashion; we refer the reader to Evans et al. (2007, 2010) for further details. As shown in Figure 1, the early X-ray afterglow (top panel) is characterized by a series of bright X-ray flares superimposed on a power-law decay (∝ t^−1.5). Each flare can be described by a Fast-Rise Exponential Decay (FRED) profile (solid line) with 0.04 < Δt/t < 0.2. In the same time interval, the BAT light curve (bottom panel) shows a long-lasting emission extending up to ∼800 s, with several peaks visible in coincidence with the X-ray flares (vertical dot-dashed lines).

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The post-flare X-ray afterglow decays as a power law with slope α₂ = 1.07 ± 0.05, which steepens to α₃ = 1.63 ± 0.07 at t ∼ 10 ks. No significant spectral evolution is observed. The time-averaged photon index is Γ = −2.07 ± 0.09.

By combining the simultaneous BAT and XRT observations, we performed a joint spectral analysis of the X-ray flares. We modeled the absorption with two different components: the former was fixed at the Galactic value of N_H = 10²¹  cm⁻² (Kalberla et al. 2005) and the latter, representing the absorption local to the burst, was fixed to the value of N_H = 2.6 × 10²¹  cm⁻² derived from the late-time (10⁴–10⁶ s) afterglow spectrum. This constraint prevents artificial N_H variations caused by the intrinsic spectral evolution, commonly observed in the brightest X-ray flares (Butler & Kocevski 2007). A strong spectral evolution is observed during the first 100 s, showing a peak energy that softens from 95 ± 15 keV during the first flare (from T₀ + 167 s to T₀ + 192 s) to less than 10 keV in the following flares. Excluding the first harder episode, the time-averaged spectrum, from T₀ + 254 s to T₀ + 854 s, is well described by a Band function (Band et al. 1993; χ² = 614 for 477 degrees of freedom, dof) with α = −1.06 ± 0.11, β = −2.24 ± 0.02, and a peak energy E_pk = 1.0^+0.8 _{− 0.4} keV.

2.2. Fermi Data

Below we summarize the results that are relevant to address the origin of the GeV emission.

• 1.  
  Significant GeV emission is found in the same interval where the X-ray flaring activity is enhanced. However, the backgrounds and limited statistics in the LAT data do not allow us to search for a one-to-one correlation between the GeV emission and the single flare episodes. There is marginal evidence of GeV emission after the end of the flaring period.
• 2.  
  The GeV flux is consistent with the extrapolation of the power law describing the flare spectrum above 1 keV. Assuming that an afterglow component is present below the flares and that it has the same spectrum observed at later times, it is found that the GeV flux is also consistent with the extrapolation of this putative component. The LAT data exhibit a harder spectrum than that observed in X-rays (see Figure 2), though marginally consistent (within 3σ) with the X-ray spectral slope.

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The last result suggests that the HE emission can simply represent the HE tail of the synchrotron component. The presence of an additional Inverse Compton (IC) component dominating over the synchrotron just above ∼1 GeV cannot be excluded, and it would be consistent with the observed flatter GeV spectrum. These deductions apply to whichever is the source of electron acceleration, internal or external shocks.

The discovery of HE emission in a time frame of vigorous flaring activity in X-rays lead us to consider first the association of the GeV emission with X-ray flares. We now discuss this scenario. Given the large number of flares, we can exclude that a delayed external shock is the dominant process originating the X-ray flares (Galli & Piro 2007). In fact, in this model only a single outstanding flare, corresponding to the onset of the afterglow from a long-duration central engine, is produced. We thus consider internal shocks from a long-lasting relativistic outflow as the source of flares (e.g., Zhang et al. 2006).

In order to allow the GeV emission to be observed we require two conditions. First, the source has to be optically thin for pair production. By computing the optical depth for e⁺e⁻ production (Lithwick & Sari 2001) by photons of energy E_GeV in GeV on the X-ray to GeV power-law component observed at about 300 s, we derive a lower limit on the Lorentz factor:

$$\begin{equation} \Gamma \ge \Gamma _{\gamma \gamma }\approx 30\,E_{\rm GeV}^{1/6}\,t_{v}^{-1/6} D_{28}^{1/3} \left(\frac{1+z}{2}\right)^{1/3}, \end{equation} \tag{ 1 }$$

where we specialized the equation for a photon index 2. The tightest constraints on Γ are derived from the shortest timescale for variability t_v that can be associated with the relativistic flow produced by the central source. In the scenario of a late internal shock, X-ray flares and the GRB prompt emission are both related to the central engine activity. We therefore consider that a variability timescale of millisecond typical of the prompt phase is a reasonable possibility. In this case, a single flare is produced by the superposition of several internal shock events from a relativistic wind active for the whole duration of the flare. By assuming t_v = 10⁻³ s, a timescale similar to that characterizing the prompt phase, one derives Γ_γγ ∼ 115 E^1/6_GeV for a typical redshift z = 1. On the contrary, if the flare is associated with a single internal shock event, i.e., the interaction of two shells, then t_v should be equal to the flare duration, i.e., about 100 s. In this case Γ_γγ ∼ 20 E^1/6_GeV.

The lower boundary on the Lorentz factor derived above for the flaring phase encompasses the range of values typical of the prompt phase (Lithwick & Sari 2001; Liang et al. 2010), consistent with the notion that a long-duration relativistic outflow with a Lorentz factor of the order of ≈100 is producing both the prompt emission and the flares. In principle, the parameters describing the relativistic shocks (_e, _B, L, Γ, t_v) can all be time dependent, i.e., be different during the prompt and late flaring phases. On the other hand, one wishes to reduce the number of variable parameters (Occam's razor). It goes beyond the scope of this Letter to find the best self-consistent internal shock model reproducing both the prompt and X-ray flaring phases. We just note the following. The model should be able to reproduce a peak energy that shifts from the ≈100 keV region during the prompt phase to the keV range observed during X-ray flares. Recalling that the peak of the synchrotron spectrum is given by (e.g., Zhang & Mészáros 2002)

$$\begin{equation} \nu _m\propto \epsilon _e^{3/2} \epsilon _B^{1/2} L^{1/2} \Gamma ^{-2} t_v^{-1}, \end{equation} \tag{ 2 }$$

it follows that the decrease of the luminosity L from the prompt to the flare phase already accounts for a decrease of the peak energy by a factor of 20, with all the other parameters remaining constant. The further reduction that is needed can be obtained, e.g., by the very reasonable assumption that the magnetic field weakens at the larger radii where flares are produced or by a smaller contrast in the Lorentz factor between colliding shells (Barraud et al. 2005).

The second condition is derived by requiring that the maximum energy at which the electrons are accelerated is large enough to produce photons of energy E via synchrotron radiation:

$$\begin{equation} \Gamma > 60 \left(\frac{1+z}{2} \right) (1+Y) E_{\rm GeV}, \end{equation} \tag{ 3 }$$

where Y is the Compton y parameter. This equation gives a condition on Γ comparable to that derived from Equation (1). In conclusion, we find that both the prompt emission and the later X-ray flares and HE emission can be explained by internal shocks produced by a long-duration central engine with a Lorentz factor of ≈100 and decreasing luminosity.

This simple internal shock model predicts an emission that is cospatial and simultaneous in the X-ray and GeV ranges. On the other hand, we find a marginal evidence of delayed HE emission. This is naturally predicted when the X-ray photons, produced by internal shocks at smaller radii, are upscattered to GeV energies via IC by the electron population of the forward shock (Wang et al. 2006; Fan et al. 2008).

Finally, given the quality of the present data set, we cannot exclude the possibility that the GeV emission is actually related to an afterglow underlying the X-ray flares. This requires that the afterglow onset takes place before 200 s. Such a condition is satisfied when the Lorentz factor of the relativistic flow at the beginning of the deceleration phase is

$$\begin{equation} \Gamma _{{\rm ext}} > \left\lbrace \begin{array}{lr}171 \left(\frac{1+z}{2}\right)^{3/8} \left(\frac{E_{54}}{n}\right)^{1/8} & \mbox{ISM}, \\ 74 \left(\frac{1+z}{2}\right)^{1/4} \left(\frac{E_{54}}{A^{*}}\right)^{1/4} & \mbox{Wind}, \end{array} \right. \end{equation} \tag{ 4 }$$

where A* is the density scaling factor in units of 5 × 10¹¹ g cm⁻¹. In other GRBs, the HE emission has been indeed associated with the forward shock synchrotron emission (Ghirlanda et al. 2010; Kumar & Barniol Duran 2010), though the shorter timescales observed require much larger values of Γ_ext than those derived here. We further explore the external shock scenario in GRB 100728A by analyzing the late-time X-ray behavior. The afterglow spectral and temporal laws are bound to obey specific relations (the so-called closure relations, e.g., Zhang & Mészáros 2004) that depend upon the density profile of the external medium, the jet opening angle, and the relative position of the typical frequencies of the synchrotron spectrum with respect to the observed range. Within the simple external shock model, the closest solution envisages a jet with a rather narrow opening angle of ≈1–2 deg expanding in a medium with a wind-like density profile, though the lack of multi-wavelength afterglow observations does not allow us to firmly characterize the circumburst environment. This scenario is consistent with the lack of spectral variations before and after the break at 10 ks, albeit in a wind-like medium a jet transition is expected to take place on much longer timescales (Kumar & Panaitescu 2000). In this scenario the cooling frequency falls below the X-ray band, and the early GeV emission (if afterglow) likely belongs to the same synchrotron regime. In this case, the LAT emission should display a similar decay slope of ∼1.07 and a photon index of ∼−2.07, softer than the observed value of −1.4 ± 0.2 (1σ) but still consistent within the large uncertainty.

GRB 100728A is the second case to date with simultaneous Swift and Fermi observations. HE gamma rays are detected by the Fermi/LAT until 850 s (TS = 42) and possibly continuing until 1600 s (TS ≈ 10). Very interestingly, the early X-ray afterglow exhibits intense and long-lasting flaring activity, visible both in BAT and XRT. Although an afterglow origin of the GeV emission cannot be excluded, the presence of bright X-ray flares unveiled by Swift observations opens the possibility that a prolonged central engine activity is powering the temporally extended HE emission observed in this burst.

Within the internal shock scenario a relativistic outflow with a Lorentz factor of ≈100 and decreasing luminosity can explain the prompt emission, the later X-ray flares, and HE emission. The presence of a delayed HE emission naturally arises from IC scattering of low-energy flare photons off the relativistic electrons at the external forward shock radius.

We thank the referee for a careful reading of the Letter and a rapid reply.

E.T. was supported by an appointment to the NASA Postdoctoral Program at the Goddard Space Flight Center, administered by Oak Ridge Associated Universities through a contract with NASA.

The Fermi LAT Collaboration acknowledges support from a number of agencies and institutes for both the development and the operation of the LAT as well as scientific data analysis. These include NASA and DOE in the United States, CEA/Irfu and IN2P3/CNRS in France, ASI and INFN in Italy, MEXT, KEK, and JAXA in Japan, and the K. A. Wallenberg Foundation, the Swedish Research Council, and the National Space Board in Sweden. Additional support from INAF in Italy and CNES in France for science analysis during the operations phase is also gratefully acknowledged.