A Comprehensive Analysis of Swift XRT Data. II. Diverse Physical Origins of the Shallow Decay Segment

By E. Liang, Binbin Zhang, Bing Zhang. 2007

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The origin of the shallow decay segment in Swift XRT light curves remains a puzzle. We analyze the properties of this segment with a sample of 53 long Swift GRBs detected before 2007 February. We show that the distributions of the sample's characteristics are lognormal or normal, and its isotropic X-ray energy (Eiso,X) is linearly correlated with the prompt gamma-ray energy but with a steeper photon spectrum, aside from some X-ray flashes. No significant spectral evolution is observed from this phase to the following phase, and the latter is usually consistent with external-shock models, implying that the shallow decay is also of external-shock origin, likely a refreshed external shock. Within the refreshed-shock model, the data are generally consistent with a roughly constant injection luminosity up to the end of this phase, tb. A positive correlation between Eiso,X and tb also favors this scenario. Among the 13 bursts that have well-sampled optical light curves, six have an optical break around tb and the breaks are consistent with being achromatic. However, the other seven either do not show an optical break or have a break at an epoch different from tb. This raises a concern for the energy injection scenario, suggesting that the optical and X-ray emission may not be the same component, at least for some bursts. There are four significant outliers in the sample, GRBs 060413, 060522, 060607A, and 070110. The shallow decay phase in these bursts is immediately followed by a very steep decay after tb, which is inconsistent with any external-shock model. The optical data for these bursts evolve independently from the X-ray data. These X-ray plateaus likely have an internal origin and demand continuous operation of a long-term central engine. We conclude that the observed shallow decay phase likely has diverse physical origins.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/9ede3bb6218fb1e47b79cf36785d74da29a79161

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In Astronomy and Astrophysics

1. Bing Zhang
2. Pawan Kumar
3. E. Liang
4. Binbin Zhang
5. Xuefeng Wu
6. E. Nakar
7. E. Berger
8. D. Burrows

Citations in the corpus

• THE MILLISECOND MAGNETAR CENTRAL ENGINE IN SHORT GRBs (2015, methodology): “The details of the data analysis method can be found in several previous papers (Zhang et al. 2007c; Liang et al. 2007; Lü et al. 2014) in our group, and Sakamoto et al. (2008).”
• CONSTRAINING GAMMA-RAY BURST INITIAL LORENTZ FACTOR WITH THE AFTERGLOW ONSET FEATURE AND DISCOVERY OF A TIGHT Γ0–Eγ,iso CORRELATION (2009, methodology): “The physics origin of the X-ray emission is still a mystery (e.g. Zhang 2007 for a review), although the consensus is that there might be diverse origins (Liang et al. 2007).”
• The physics of gamma-ray bursts & relativistic jets (2014, background): “…− 1049 erg s−1, typically with long durations, soft spectra, and single-peaked smooth lightcurves) as a separate population, which has a distinct bump in the luminosity function, then it is found that α1 ∼ 0.5 for the high-luminosity GRB component (e.g. Liang et al., 2007a; Virgili et al., 2009).”
• A Comprehensive Analysis of Swift XRT Data. III. Jet Break Candidates in X-Ray and Optical Afterglow Light Curves (2007, background): “The GRB tail and the shallow decay segments are usually seen in the XRT light curves (O’Brien et al. 2006b; Liang et al. 2006; Willingale et al. 2007; Zhang et al. 2007c, hereafter Paper I; Liang et al. 2007, hereafter Paper II).”
• A complete reference of the analytical synchrotron external shock models of gamma-ray bursts (2013, methodology): “It is likely that some X-ray plateaus followed by steep decays (internal plateaus) are also caused by late central engine activities (Troja et al., 2007; Liang et al., 2007; Lyons et al., 2010).”
• Mass fall-back and accretion in the central engine of gamma-ray bursts (2008, background): “Such a model can explain GRBs like 060413, 060607A, 070110 (Liang et al. 2007), which show a sudden drop at the end of the plateau.”
• The prompt-afterglow connection in gamma-ray bursts: a comprehensive statistical analysis of Swift X-ray light curves (2012, background): “…al. 2007): a first steep decay phase tightly connected to the prompt γ-ray emission (Tagliaferri et al. 2005; Goad et al. 2006); and a second phase characterized by a flattening of the LC (with limited evidence for spectral evolution, see e.g. Liang et al. 2007) followed by a “normal decay” phase.”
• JET BREAKS AND ENERGETICS OF Swift GAMMA-RAY BURST X-RAY AFTERGLOWS (2008, background): “The distributions are not nearly as narrow as those suggested by the canonical light curve form (Nousek et al. 2006; Zhang et al. 2006), but do suggest a spread to larger break times in the contaminated ambiguous II-III/III-IV sample and the segments I-II-III sample.”
• A POSSIBLE CONNECTION BETWEEN FAST RADIO BURSTS AND GAMMA-RAY BURSTS (2013, background): “The degeneracy between the two models was broken when the so-called “internal plateaus” were discovered in the early X-ray afterglow lightcurves of some GRBs (Fig.1 upper panel) (Troja et al. 2007; Liang et al. 2007).”
• A RADIO-SELECTED SAMPLE OF GAMMA-RAY BURST AFTERGLOWS (2011, background): “A population of dark bursts has been identified (Rol et al. 2005), as has a population of nearby, low-luminosity events (Sazonov et al. 2004; Soderberg et al. 2006b; Pian et al. 2006; Liang et al. 2007; Kann et al. 2010).”
• THE AFTERGLOWS OF SWIFT-ERA GAMMA-RAY BURSTS. II. TYPE I GRB VERSUS TYPE II GRB OPTICAL AFTERGLOWS (2008, background): “Analyses of Swift GRB X-ray afterglows however suggest that most bursts are consistent with a constant-density medium (Zhang et al. 2007b; Liang et al. 2007b).”