Infrared/optical - X-ray simultaneous observations of X-ray flares in GRB 071112C and GRB 080506
T. Uehara, M. Uemura, K. S. Kawabata, Y. Fukazawa, R. Yamazaki, A. Arai, M. Sasada, T. Ohsugi, T. Mizuno, H. Takahashi, H. Katagiri, T. Yamashita, M. Ohno, G. Sato, S. Sato, M. Kino
aa r X i v : . [ a s t r o - ph . C O ] J un Astronomy&Astrophysicsmanuscript no. uehara c (cid:13)
ESO 2018November 19, 2018
Infrared/optical — X-ray simultaneous observationsof X-ray flares in GRB 071112C and GRB 080506
T. Uehara , M. Uemura , K. S. Kawabata ,Y. Fukazawa , R. Yamazaki , A. Arai , M. Sasada , T. Ohsugi ,T. Mizuno , H. Takahashi , H. Katagiri , T. Yamashita ,M. Ohno , G. Sato , S. Sato , and M. Kino Department of Physical Science, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8526, Japane-mail: [email protected] Hiroshima Astrophysical Science Center, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8526, Japan Department of Physics and Mathematics, Aoyama Gakuin University, 5-10-1 Fuchinobe, Sagamihara 252-5258, Japan Faculty of Science, Kyoto Sangyo University, Motoyama, Kamigamo, Kita-Ku, Kyoto-City 603-8555, Japan National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan Institute of Space and Astronautical Science, JAXA, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan Department of Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, JapanReceived ; accepted
ABSTRACT
Aims.
We investigate the origin of short X-ray flares which are occasionally observed in early stages of afterglows of gamma-raybursts (GRBs).
Methods.
We observed two events, GRB 071112C and GRB 080506, before the start of X-ray flares in the optical and near-infrared(NIR) bands with the 1.5-m Kanata telescope. In conjunction with published X-ray and optical data, we analyzed densely sampledlight curves of the early afterglows and spectral energy distributions (SEDs) in the NIR–X-ray ranges.
Results.
We found that the SEDs had a break between the optical and X-ray bands in the normal decay phases of both GRBs regardlessof the model for the correction of the interstellar extinction in host galaxies of GRBs. In the X-ray flares, X-ray flux increased by 3and 15 times in the case of GRB 071112C and 080506, respectively, and the X-ray spectra became harder than those in the normaldecay phases. No significant variation in the optical—NIR range was detected together with the X-ray flares.
Conclusions.
These results suggest that the X-ray flares were associated with either late internal shocks or external shocks fromtwo-component jets.
Key words. gamma rays: bursts
1. Introduction
Gamma-ray bursts (GRBs) are transient gamma-ray sourceswhose durations are ∼ ∼ Swift satellite have discoveredshort flares in X-ray afterglows 10 − s after GRBs. These X-ray flares are unexpected in the framework of the standard ex-ternal shock model because it predicts a monotonous decayof the X-ray afterglow with a power-law form (Burrows et al., 2005). The X-ray flare is observed in a half of GRB after-glows (Falcone et al., 2007). The afterglow emission from theexternal shock could exhibit short-term modulations when ashell passes a high-density region of the interstellar medium(Wang & Loeb, 2000), or slow shells catch up with the mainshell (Rees & Meszaros, 1998). Such modulations of the emis-sion from the external shock are candidates for the origin of theX-ray flare. It is also possible that a late-time internal shockcauses the X-ray flare (e.g. Burrows et al. 2005; Zhang et al.2006a; Chincarini et al. 2007; Butler & Kocevski 2007). Thisscenario with the late internal shock requires a long activity ofthe central engine of GRBs (Ioka et al., 2005).The optical and infrared observations of afterglows are im-portant to evaluate the models for the X-ray flare because theexternal shock model predicts that an optical–infrared flare is as-sociated with an X-ray flare. No optical–infrared flare has beenreported during the X-ray flares in previous observations, while apart of those observations were too sparse to investigate the de-tailed behavior of optical–infrared afterglows during the X-rayflares (Stanek et al. 2007; Krimm et al. 2007).Little is known about the temporal variation of spectralenergy distributions (SEDs) associated with the X-ray flare.This is because simultaneous multi-wavelength observations are T. Uehara et al.: Infrared / optical — X-ray simultaneous observations of X-ray flares in GRB 071112C and GRB 080506 required with a high time-resolution. In addition, it is prob-lematic to correct the interstellar extinction in host galaxiesof GRBs which is highly uncertain even if such a densely-sampled multi-wavelength data is available (e.g. see Stratta et al.2004; Kann et al. 2006; Chen et al. 2006; Schady et al. 2007;Starling et al. 2007; Watson et al. 2007).GRB 071112C and 080506 were detected by Swift / BATat 18:32:57.54 UT 12 November 2007 (Perri et al., 2007) and17:46:21.22 UT 6 May 2008 (Baumgartner et al., 2008), respec-tively. X-ray flares were detected in both GRBs (Stratta et al.2007; Sbarufatti et al. 2008). Here we report on our optical andNIR observations of those two GRBs using the Kanata 1.5-mtelescope. Combined with X-ray spectra, these simultaneous op-tical and NIR data allowed us to investigate the interstellar ex-tinction in the host galaxies, and thereby, variations of SEDs. Wedescribe the details of our observations in §
2. We report on thetemporal evolution of the optical and X-ray afterglows and SEDsin §
3. In §
4, we discuss the origin of the X-ray flare using theinternal and external shock models. Finally, we summarize ourresults in § Our observations of GRB 071112C and GRB 080506 started at18:36:21 (UT) 12 November 2007 and 17:49:51 (UT) 6 May2008, which were ∼
324 s and ∼
210 s after the GRB trig-ger times, respectively. Both observations were performed withTRISPEC attached to the Kanata 1.5-m telescope at Higashi-Hiroshima Observatory of Hiroshima University. TRISPEC is asimultaneous imager and spectrograph with polarimetry cover-ing both optical and NIR wavelengths (Watanabe et al., 2005).We used the imaging mode of TRISPEC with the V , J , and K s band filters for GRB 071112C. Instead of the V -band fil-ter, we used the R C -band filter for GRB 080506. The obser-vations continued for 4.5 and 5.7 ks, and we obtained 40 and35 sets of three photometric band images for GRB 071112Cand GRB 080506, respectively. The central wavelength of theTRISPEC’s R C system is ∼
620 nm, slightly shifted from thestandard one ( =
645 nm). The di ff erence of the photometric sys-tems is so small that we neglect it in our following discussionabout spectral energy distributions.We obtained di ff erential magnitudes of the afterglows us-ing a Java-based PSF photometry package after making dark-subtracted and flat-fielded images. We used a nearby field-starlocated at R.A. = h m s .
46, Dec. = ◦ ′ ′′ . V , J , and K s -band magni-tudes of the comparison star were quoted from the Guide StarCatalog Version 2.3.2 ( V = .
43) and 2MASS All-Sky Catalogof Point Sources ( J = .
238 and K s = . V , J , and K s magnitudes de-pending on comparison stars, and found that it is smaller than0.28, 0.02, and 0.02 mag, using neighboring stars. Uemura et al.(2007b) contains di ff erential magnitudes obtained by our opticalobservations of GRB 071112C.For the di ff erential photometry of GRB 080506, we usedaverages of magnitudes of USNO B1.0 1289-0511223, 1289-0511261, 1289-0511235, 1289-0511157, and 1289-0511139 forthe R C band, 2MASS 1289-0511197, 1289-0511261, 1289-0511235, 1289-0511157, and 1289-0511139 for the J band, and2MASS 1289-0511197 and 1289-0511261 for the K s band. Wechecked systematic errors of R C , J , and K s magnitudes depend-ing on comparison stars, and found that it is smaller than 0.13,0.01, and 0.06 mag, using neighboring USNO B1.0 and 2MASSstars. Table 1 contains magnitudes obtained by our observations of GRB 080506. In this table, the magnitudes are averages inequally spaced bins in the logarithmic scale of time. The errorsinclude both statistical and systematic ones. XRT began observing GRB 071112C and GRB 080506 at 2007November 12 18:34:27 UT, i.e. at T +
90 s and 2008 May 617:48:47 UT, i.e. at T +
146 s, respectively ( T represents aGRB trigger time). Both XRT data were processed using theHEASOFT package. We extracted both source data with a rect-angular 40 ′′ × ′′ region for the Windowed Timing mode(WT), and 40 ′′ radius region for the Photon Counting mode (PC)from the processed data. Both GRB backgrounds were also ex-tracted from 40 ′′ × ′′ source of both ends region for the WT,and 192 ′′ internal and 231 ′′ outer annulus radius region for thePC, far from the source. Light curves were binned with a re-quirement of a minimum of 30 photons per bin of WT and 20photons per bin of PC for GRB 071112C, while 30 photons perbin of WT and 40 photons per bin of PC for GRB 080506,
2. Results
In figure 1, we show the X-ray and NIR—UV light curvesof afterglows of GRB 071112C (left) and GRB 080506(right). The X-ray observations by XRT are indicated by thecrosses. The other symbols indicate the UV, optical, and NIRobservations as described in the figure. In addition to ourobservations by the Kanata telescope, this figure includesobservations reported in GCN Circular:Yuan et al. (2007);Uemura et al. (2007a); Klotz et al. (2007); Burenin et al.(2007); Chen et al. (2007a); Nugent & Bloom (2007);Dintinjana et al. (2007); Oates & Stratta (2007); Chen et al.(2007b); Updike et al. (2007); Ishimura et al. (2007);Greco et al. (2007); Sposetti (2007); Yoshida et al. (2007);Uemura et al. (2007b); van der Horst & Wijers (2007);Chandra & Frail (2007); Minezaki et al. (2007); Huang et al.(2008) for GRB 071112C and Baumgartner et al. (2008);Kawabata et al. (2008); Osborne et al. (2008); Kann et al.(2008b); Kocka et al. (2008); de Postigo Ugarte et al. (2008);McLean et al. (2008); Oates & Stratta (2008); Sbarufatti et al.(2008); Kann et al. (2008a); Chandra & Frail (2008); Sahu et al.(2008); Maeno et al. (2008) for GRB 080506. According toJakobsson et al. (2007a), the host galaxy of GRB 071112C isdetected at R c = . ± .
5. The host galaxy is so faint that itscontribution to the afterglow is negligible in our analysis. Thehost galaxy of GRB 080506 was not detected.The X-ray light curve of GRB 071112C can be describedwith a broken power-law form; f ∝ t α ( t ≤ t break ) and , f ∝ t α ( t > t break ) with a weak X-ray flare between T +
450 s and T + α and α ), and show them in table 2. The breaktime ( t break ) was estimated to be 7 . ± . T +
365 s. After the maximum, the flux decayed as a singlepower-law without a break as observed in the X-ray afterglow.We confirmed that the decay indexes are same in all NIR to UVbands within errors. The decay index was, hence, calculated withall NIR—UV observations, and shown in table 3. . Uehara et al.: Infrared / optical — X-ray simultaneous observations of X-ray flares in GRB 071112C and GRB 080506 3 Table 1.
Kanata optical photometry of GRB 080506
Filter Time (s) ∗ Exposure (s) mag error R C ± R C ± R C ± R C ± R C ± R C ± R C ± R C ± R C ± R C ± R C ± R C ± R C ± R C ± R C ± R C ± R C ± R C ± R C ± R C ± R C ± R C ± R C ± R C ± R C ± J ± J ± J ± J ± J ± J ± J ± K s ± K s ± K s ± Table 2.
Decay and spectral index of the X-ray afterglow of GRB 071112C
Region T (s) α X † ( χ / d.o.f) β X ‡ Normal Decay < T < < T < − ± / − ± Flare Rise < T <
650 2.51 ± / − ± Flare Decay < T < − ± / − ± Post-break < T < − ± / − + . − . The uncertainties show the 90% confidence levels of the parameters. † X-ray decay index. ‡ X-ray spectral index( χ / d.o.f = / The X-ray light curve of GRB 080506 (the right panel offigure 1) is more complex than that of GRB 071112C. Before T +
200 s, the X-ray flux remained at a high level, whichmay be related to the prompt emission (McLean et al. 2008;Sbarufatti et al. 2008). A steep decay was observed from T +
200 s to T +
350 s. Between T +
350 s and T +
672 s, it showed anX-ray flare centered at T + ± T + . Table 3.
Decay indexes of the optical light curves.
GRB T (s) α O † χ / d.o.fGRB 071112C 250 < T < − ± / < T < − ± / < T < × − ± † Optical decay index.
GRB 080506. At the X-ray flare maxima, the X-ray flux in-creased by a factor of 3.5 and 15 in GRB 071112C andGRB 080506, respectively. On the other hand, the NIR—UV
T. Uehara et al.: Infrared / optical — X-ray simultaneous observations of X-ray flares in GRB 071112C and GRB 080506 Fig. 1.
X-ray and NIR—UV light curves of afterglows of GRB 071112C (left) and GRB 080506 (right). The flux density ofthe NIR—UV afterglows are shifted by 2 in XRT, 10 . in uvw1- (Oates & Stratta, 2007), 10 . in U - (Oates & Stratta, 2007),10 − . in B - (Oates & Stratta, 2007), 10 − . in g ′ - (Ishimura et al. 2007; Yoshida et al. 2007), 10 − . in V ∗ (UVOT)- (Oates & Stratta,2007), 10 − . in V -, 10 − . in R - (Klotz et al. 2007; Burenin et al. 2007; Chen et al. 2007a; Greco et al. 2007; Minezaki et al. 2007),10 − . in R C - (Dintinjana et al. 2007; Ishimura et al. 2007; Yoshida et al. 2007), 10 − . in I - (Minezaki et al., 2007), 10 − . in I C -(Ishimura et al. 2007; Yoshida et al. 2007), 10 − . in J - (Minezaki et al., 2007), 10 − . in H - (Minezaki et al., 2007), and 10 − . in K s -band (Minezaki et al., 2007) in the left panel. In the right panel, they are shifted by 10 − . in B - (Oates & Stratta, 2008), 10 − . in V - (Oates & Stratta, 2008), 10 − . in R - (Kocka et al. 2008; de Postigo Ugarte et al. 2008; Sahu et al. 2008; Maeno et al. 2008),10 − . in R C - (Kann et al. 2008b,a), 10 − . in J -, 10 − . in K s -band.light curves exhibit no significant variations associated with theX-ray flares in both GRBs. The variation amplitudes are < V for GRB 071112C, and <
14% in R C for GRB 080506. In this subsection, we report the result of our analysis on X-rayspectra obtained with XRT, in particular, about the temporal vari-ation of the X-ray spectral index ( β X ). Based on the light curveanalysis shown in the last section, we defined 4 and 6 phases forGRB 071112C and GRB 080506, as described in table 2 and 4,respectively. The following analysis was performed for averagedX-ray spectra of each phase. The average spectra of each phasewere fitted with an absorbed power-law model with two absorp-tion components. The first component is the absorption in ourgalaxy. According to Dickey & Lockman (1990), the galactichydrogen column density is N g al H = . × and 1 . × cm for the direction of GRB 071112C and GRB 080506, respec-tively. We used those N g al H to estimate the galactic absorptionsfor the GRBs. The second component is the absorption in the host galaxy. The column density, N ext H , was a free parameter inour analysis. N ext H can be estimated from the absorption model, whichdepends on the redshift. The redshift correction of the ob-served energy band is, hence, essential to correct the ab-sorption. We performed the redshift correction to the energyband for GRB 071112C using a reported redshift of z = .
82 (Jakobsson et al. 2007b; Cucchiara et al. 2007). AboutGRB 080506, the redshift correction was not performed sincethe redshift is not determined. We note that β X is independent ofthe redshift correction, while N ext H depends on it. Using the ab-sorbed power-law model, we confirmed that the column density N ext H was constant in all phases for each GRB, while β X changed.We obtained the temporal variation of β X in 0.3–8.0 keV bandby simultaneously fitting the spectra of all the phases by lettingthe N ext H to be common among phases and β X to be free for eachphase. The obtained β X are summarized in table 2 and 4, wherethe χ / do f of the fitting is 31 /
44 and 59 /
76 for GRB 071112Cand GRB 080506, respectively. As shown in the table, the X-ray spectra in the rising phase of the X-ray flares became harder . Uehara et al.: Infrared / optical — X-ray simultaneous observations of X-ray flares in GRB 071112C and GRB 080506 5 Table 4.
Decay and spectral index of the X-ray afterglow of GRB 080506
Region T (s) α X † ( χ / d.o.f) β X ‡ Prompt Emission − ± Steep Decay − ± / − ± Flare Rise ± / − ± Flare Peak − ± Flare Decay − ± / − ± Normal Decay − ± / − + . − . The uncertainties show the 90% confidence levels of the parameters. † X-ray decay index. ‡ X-ray spectral index( χ / d.o.f = /
15 16 17 18 − − l og ( F l u x den s i t y ) ( m Jy ) (a)
15 16 17 18 (b)
15 16 17 18 log(Frequency) (Hz) (c)
15 16 17 18 (d)
15 16 17 18 (e)
15 16 17 18 (f)
Fig. 2.
SEDs of the NIR—X-ray regime of GRB 071112C. The 6 panels show the SED on (a) T +
100 — T +
365 s, (b) T +
365 — T +
450 s, (c) T +
450 — T +
650 s, (d) T +
650 — T + T + T + T + T + N ext H of GRB 071112Cwas estimated to be 7 . ± . × cm − . α - β relationin theX-ray afterglows In the standard external shock model, the temporal decay index, α , is related to the spectral slope, β (e.g. Zhang et al. 2006a).For example, α and β in the X-ray regime have a relation of α X = . β X in the case that the X-ray band is between the syn-chrotron cooling frequency ν c and the typical frequency ν m un-der a homogeneous circum-burst medium. The α and β duringthe normal decay phase of GRB 071112C satisfies this relation. The electron energy distribution index, p , was estimated to be p = .
7, inferred from p = − α X and / or p = − β X (Meszaros & Rees 1997; Sari et al. 1998). This is similar to atypical p in previously observed afterglows (Liang et al., 2007).On the other hand, the α X and β X in GRB 080506 do not followthe α - β relation. Figure 2 shows NIR—X-ray SEDs of GRB 071112C. The fig-ure contains SEDs at 6 epochs, that is, (a) the rising phase ofthe optical afterglow ( T +
100 — T +
365 s), (b) the normal de-cay phase ( T +
36 — T +
450 s), (c) the rising phase of the X-
T. Uehara et al.: Infrared / optical — X-ray simultaneous observations of X-ray flares in GRB 071112C and GRB 080506 ray flare ( T +
450 — T +
650 s), (d) the decay phase of the X-ray flare ( T +
650 — T + T + T + T + T + V -band, A V , from N H ob-tained from the X-ray spectral analysis. The ratio, N H / A V , hasbeen reported in our and nearby galaxies, that is, N H / A V = . × cm − for Milky Way (MW), 7 . × cm − for the LargeMagellanic Cloud (LMC), and 1 . × cm − for the SmallMagellanic Cloud (SMC) (Pei 1992; Weingartner & Draine2000). Using those 3 models, we corrected the extinction in the V -band. The conversion from A V to A λ of the other bands wasperformed based on Cardelli et al. (1989). In figure 2, the dotted,dashed, and dash-dotted lines indicate the best-fitted power-lawmodels for the corrected NIR—UV SEDs with the MW, LMC,and SMC models, respectively. In the case that the NIR—UV ex-tinction was corrected with the LMC or SMC model, the X-rayflux has an excess over the SED extrapolated from the NIR—UVregime in panel (a) of figure 2. Such an X-ray excess is inconsis-tent with the standard external shock model. In the case that theextinction model is the MW model, the SED should have a breakbetween the optical and UV band in panel (f), while such a breakis not clearly seen in the observed NIR—UV SED. However, thebreak could be in UV–X-ray SED, when considering the error ofX-ray spectral index.In the framework of the GRB external shock model, NIR—X-ray SEDs can be described with a single power-law model ifthe cooling frequency, ν c , is below the NIR band. Alternatively,SEDs have a break in the case that ν c lies between the NIR—X-ray bands. As mentioned above and shown in figure 2, a sin-gle power-law model cannot reproduce the observed SED of theNIR–X-ray bands regardless of the ambiguity of the extinction.Therefore, the standard external shock model can explain the ob-served SED only when ν c lies between the NIR—X-ray bands.Then, the SED should have a break between the NIR—X-raybands. This break actually appears in the case that N ext H / A V inthe host galaxy of GRB 071112C is between the values of theMW and LMC models. These epochs correspond to the phases defined in table 4, thatis, (a) the prompt emission phase ( T +
145 — T +
200 s), (b) thesteep decay phase ( T +
200 — T +
350 s), (c) the flare rise phase( T +
350 — T +
474 s), (d) the flare peak phase ( T +
474 — T +
503 s), (e) the flare decay phase ( T +
503 — T +
672 s), and(f) the normal decay phase ( T + T + N H based on the X-ray spectrum, we cannot apply the same approach for theabsorption correction to GRB 080506, as in GRB 071112C.In the framework of the external shock model, the conditionof α opt = α X means that ν c is below the optical frequency, andhence, an optical—X-ray SED should be described with a sin-gle power-law, namely β opt = β X . In the case of GRB 080506,the observed α opt was in agreement with α X within errors af-ter T + . β opt = β X in GRB 080506 after T + . β opt = . ± .
05 shown by dotted line in panel (f) of figure 3,using the NIR–UV data after T + . ff erent from β X in the normal decay phase,as shown in table 4. This is possibly due to a significant redden-ing of the optical afterglow in the host galaxy of the GRB.Then, we can estimate the extinction in the host galaxy inthe NIR—UV range, assuming a single power-law SED be-tween the NIR—X-ray range. We defined the extinction, A λ ,as A ν = F λ / F obs λ , where F λ is the flux extrapolated from theX-ray power-law spectrum, F ν = F X ( ν/ν X ) β where F X the ob-served X-ray band( ν X ) flux. F obs λ is the observed NIR–UV fluxes.Figure 4 shows the obtained extinction curve. The extinction A λ was normalized at A V . The extinction curves in the MW is alsoshown for comparison in figure 4 (Cardelli et al., 1989). In gen-eral, the extinction is larger in the UV region rather than theNIR one. In the case of GRB 080506, however, the extinction islarger in the NIR than in the UV region. This result is problem-atic for the standard extinction model with dusts (Cardelli et al.,1989). Thus, the extraordinary extinction curve in figure 4 in-dicates β opt , β X in the normal decay phase of GRB 080506,while α opt = α X . In the case that we correct the flux with a gen-eral extinction law, the SED definitely has a break between theNIR and X-ray bands since the observed β opt is smaller than β X .As well as the α X – β X relation reported in §
3. Discussion
In this section, we discuss whether the X-ray flare originates inthe external or internal shocks.If they arise from external shocks, optical and NIR flaresshould be contemporaneously detected during the X-ray flares(Fan et al., 2005). In addition, the spectral index in the opti-cal band should also change (e.g. Zhang et al. 2006a). Our ob-servations unambiguously show no optical variation associatedwith the X-ray flares. The lack of optical variations is consis-tent with previously reported observations (e.g. Burrows et al.2005; Krimm et al. 2007; Butler & Kocevski 2007). The opticalspectral index also unchanged during the X-ray flares in bothGRB 071112C and 080506. Therefore, the optical behavior dur-ing these X-ray flares is inconsistent with the prediction fromthe standard external shock model. On the other hand, the lateinternal shock model can explain the X-ray flare even if no con-temporaneous flares are observed in the optical and NIR bands(Burrows et al., 2005). In fact, the lack of the NIR–optical varia-tions in both GRB 071112C and GRB 080506 is consistent withthis model.In the following, we propose an alternative model for the X-ray flare. We consider two components of the external shocks;one originates in the main shell which produces the normal af-terglow, and the other originates in a delayed shell which is re-sponsible for the X-ray flare. The observed sharp decay of the . Uehara et al.: Infrared / optical — X-ray simultaneous observations of X-ray flares in GRB 071112C and GRB 080506 7 Fig. 3.
SEDs of the NIR—X-ray regime of GRB 080506. These epochs correspond to the phases defined in table 4, that is, (a)Prompt Emission on T +
145 — T +
200 s, (b) Steep Decay on T +
200 — T +
350 s, (c) Flare Rise on T +
350 — T +
474 s, (d)Flare Peak on T +
474 — T +
503 s, (e) Flare Decay on T +
503 —- T +
672 s, and (f) Normal Decay on T + T + Table 5. ν c estimated for possible cases on ν ′ m and ν ′ c in the two-component external shock model. The checks ( √ ) show the casesthat the present model can reproduce the observed SED variations. case condition ν c (Hz)formula GRB 071112C GRB 080506S1 ν ′ m < ν O < ν ′ c < ν X (cid:16) aA (cid:17) p − p ′ ) ν ′ c < × < × S2 ν ′ m < ν O < ν X < ν ′ c (cid:16) aA (cid:17) p − p ′ ) + < × √ < × S3 ν O < ν ′ m < ν ′ c < ν X (cid:16) aA (cid:17) p − p ′ + ν ′ p ′ − m ν ′ / c see figure 5 √ see figure 5S4 ν O < ν ′ m < ν X < ν ′ c (cid:16) aA (cid:17) p − p ′ + ν ′ c < × √ < × √ S5 ν ′ m < ν ′ c < ν O < ν X (cid:16) aA (cid:17) p − p ′ ) + < × − < ν ′ c < ν O < ν ′ m < ν X (cid:16) aA (cid:17) p − p ′ + ν ′ ( p ′ − c < × √ < × √ F2 ν ′ c < ν O < ν X < ν ′ m (cid:16) aA (cid:17) p + < × < × √ F3 ν O < ν ′ c < ν ′ m < ν X (cid:16) aA (cid:17) p − p ′ + ν ′ p ′ − / m ν ′ c see figure 5 √ see figure 5F4 ν O < ν ′ c < ν X < ν ′ m (cid:16) aA (cid:17) p − p ′ + ν ′ p ′ + / c < × √ < × F5 ν ′ c < ν ′ m < ν O < ν X (cid:16) aA (cid:17) p − p ′ ) + ν ′ m < × − < × X-ray flare can be explained by this scenario because the decayslope of the emission from the delayed shell can be apparentlysteep in the time frame of the normal afterglow (Zhang et al.2006b; Yamazaki et al. 2006). The delayed shell could generatea prominent X-ray emission under the condition that it passes aregion containing enough interstellar medium. Such a condition is probably achieved, for example, if the opening angle of thedelayed jet is larger than that of the main jet, or if the axis of thedelayed jet is o ff to that of the main jet. These conditions wereoriginally proposed to explain ‘X-ray flashes’, which are anal-ogous to GRBs except for their softer emission and less ener-getics (Yamazaki et al., 2002, 2003, 2004; Lamb et al., 2005; Li, T. Uehara et al.: Infrared / optical — X-ray simultaneous observations of X-ray flares in GRB 071112C and GRB 080506 Fig. 4.
Extinction curves (normalized at the V band) for the host galaxy of GRB 080506 estimated with a simple power-law SEDextrapolated from the X-ray region. The bar and filled circles show the curves for GRB 080506 and MW. Fig. 5.
Allowed region of ν ′ c and ν ′ m . The red and gray shaded regions show the areas in case F3 and S3, respectively. The bluelines show the assumed condition for ν ′ c and ν ′ m . These frequencies are 6 × Hz–2 × Hz and 5 × Hz–2 × Hz forGRB 071112C and GRB 080506, respectively. Left and right panels show the cases of GRB 071112C and GRB 080506, respectively.2006). Piro et al. (2005) and Galli & Piro (2006) have proposedmodels for the X-ray flare, in which the delayed shell interactswith the reverse shock region of the preceding jet.In our model, we assume that the temporal evolution of theemission from the delayed shell is the same as that of the mainshell, following the standard external shock model (Sari et al.,1998). For the decay phase of the X-ray flare, we assumed thatthe spectrum of delayed shell overlaps that of the main shell. Wedenote the maximum and cooling frequencies of the synchrotronemission as ν m and ν c for the main shell, and ν ′ m and ν ′ c for thedelayed shell. Then, we investigated several conditions of ν ′ m and ν ′ c , which satisfy the observed amplitudes of variations and elec-tron energy distribution index.We have mentioned in § ν c lies in this bands. There is no spec-tral break in the observed NIR–optical SEDs and X-ray spectra.Therefore, ν c are constrained as 6 × Hz < ν c < × Hzand 5 × Hz < ν c < × Hz for GRB 071112C and GRB 080506, respectively. The above frequencies are those inthe observer’s frame. Because we focus on the ratio of the fre-quency in the following discussion, the redshift correction to theenergy band is not important.The cooling frequency of the normal afterglow ν c , is alsoconstrained using the observed amplitudes of the X-ray flares.In this procedure, we consider ten cases depending on the re-lationship among ν ′ m , ν ′ c , and the observation frequencies of ν O and ν X . As listed in table 5, possible conditions are divided intofive cases in the slow cooling regime ( ν ′ m < ν ′ c ) and five casesin the fast cooling regime ( ν ′ c < ν ′ m ). Since we discuss the SEDof the decay phase of the X-ray flare, the conditions are limitedto those ten cases; the conditions of ν ′ m < ν X and ν ′ c < ν X arerequired for the decay phase in the slow and fast cooling regime,respectively. The cases for the slow and fast cooling regimes areindicated by the characters ‘S’ and ‘F’, respectively.For example, in case S1 in table 5, the X-ray and opticalflux densities of the normal decay component, F n , X and F n , O are . Uehara et al.: Infrared / optical — X-ray simultaneous observations of X-ray flares in GRB 071112C and GRB 080506 9 Table 6.
Observational parameters used in the two-componentexternal shock model. results GRB 071112C GRB 080506 A a < < p ± ± p ′ ± ± related as; F n , X = F n , O ν c ν O ! − p − ν X ν c ! − p . (1)Here we assumed an SED predicted by the standard externalshock model (Sari et al., 1998). Similarly, the optical and X-rayflux densities of the X-ray flare component, F flare , O and F flare , X are related as; F flare , O = F flare , X ν ′ c ν X ! − p ′ ν O ν ′ c ! − p ′− , (2)where p ′ is the electron distribution index of the X-ray flare com-ponent. We define the ratios of the flare to the normal fluxes inthe X-ray and optical bands as A = F flare , X / F n , X , (3) a = F flare , O / F n , O . (4)Using Eqs. (1), (2), (3), and (4), we obtain ν c = (cid:18) aA (cid:19) p − p ′ ) ν ′ c , (5)where we take ν X /ν O ≈ . In this case S1, the cooling fre-quency of the flare component, ν ′ c , is assumed to lie between ν O and ν X . Given this fact, and substituting observed quanti-ties summarized in table 6 into Eq. (5), we derive the conditionfor the cooling frequency of the normal afterglow componentas ν c < × and < × Hz for GRB 071112C andGRB 080506, respectively. Those ν c was calculated taking intoaccount the uncertainties of p and p ′ in order to obtain firm es-timates. Both for GRB 071112C and 080506, two independentconditions give no allowed regions for ν c , so that case S1 fails toexplain the observed results.Similar to case S1, ν c in the other nine cases are also evalu-ated. Table 5 shows the results. Upper limits of ν c are given inthe table if it is independent of ν ′ c and ν ′ m or if it is a functionof either of them (cases S1, S2, S4, F1, F2, and F4). If ν c de-pends on both ν ′ c and ν ′ m (cases S3 and F3), then in the ν ′ c – ν ′ m plane we search for allowed regions to satisfy the conditions of6 × Hz < ν c < × Hz and 5 × Hz < ν c < × Hzfor GRB 071112C and GRB 080506, respectively. Figure 5shows the results. The left and right panels of Figure 5 are forGRB 071112C and GRB 080506, respectively. The blue solidand dotted lines indicate the assumed conditions for ν ′ c and ν ′ m .The red and gray shaded regions indicate the allowed region for ν c in cases F3 and S3, respectively. Thus, both cases F3 andS3 can explain the observation with the two-component externalshock model for only GRB 071112C. On the other hand, thereis no allowed region in the case of GRB 080506, as can be seenin the right panel of Figure 5. The ‘check’ symbols are given in table 5 in the case that there are allowed values of parame-ters that can reproduce the observed SED variations. For bothGRBs, there are several cases in which the observations can beexplained by our model.Note that all parameters needed for the above discussion arethe variation amplitudes during the X-ray flares in the optical andX-ray ranges, a and A , and electron energy distribution index, p and p ′ . Thus, the discussion was independent of the uncertaintyin the correction of the dust extinction in the GRB host galaxies.
4. Conclusion
We have observed GRB 071112C and GRB 080506 before thestart of X-ray flares. In conjunction with published X-ray andoptical data, we analyzed densely sampled light curves of theearly afterglows and SEDs of the NIR–X-ray range. We foundthat the SEDs in the normal decay phase had a break betweenthe UV and soft X-ray regions. No significant variation in theoptical-NIR range was detected contemporaneous to the X-rayflares. The lack of the optical–NIR variation suggests that thelate internal shock is a reasonable origin for the X-ray flare. Inaddition, we found that two-component external shock modelcan also explain the observed variations of SEDs during the X-ray flares.The authors thank the referee for careful reading and manyuseful comments. The authors also thank the S w i f t team for de-velopment of hardware / software and operation. SN is supportedby Research Fellowships of the Japan Society for the Promotionof Science for Young Scientists. References
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