Comparison with other short GRBs

GRB150101B stands out of the short GRB sample for its prompt emission and afterglow properties. Its gamma-ray phase is weak and very short in duration (~20 ms, Fig. 1), an order of magnitude shorter than GRB170817A. The burst fluence is ~10−7 erg cm−2 (10–1000 keV) corresponding to a total isotropic-equivalent gamma-ray energy release E γ,iso ~4.7 × 1048 erg at z = 0.1341 (see Section Environment), one of the lowest ever detected by Swift15 (Fig. 1, inset). Since this event was not found by the on-board software, no prompt localization was available. Follow-up observations started with a delay of 1.5 days, after a ground-based analysis found the transient gamma-ray source in the Swift data. Nonetheless, bright optical and X-ray counterparts were found. This is highly unusual as delayed follow-up observations of short GRBs typically fail to detect the counterpart, especially in the case of weak gamma-ray events.

Fig. 1 Prompt phase of GRB15010B. a Gamma-ray light curve of GRB150101B as seen by Swift BAT. The time bin is 2 ms. The background contribution is subtracted through the mask-weighting procedure. The shaded area shows the duration T 90 ~ 12 ms of the emission in BAT. Vertical error bars are 1 σ. The inset shows the distribution of isotropic-equivalent gamma-ray energy for short GRBs detected by Swift15. The positions of GRB150101B and, for comparison, GRB170817A are marked by the vertical lines. b Background subtracted gamma-ray light curve of GRB150101B above 300 keV, as seen by Fermi GBM. The time bin is 4 ms. It shows that at high energies the prompt phase starts a few milliseconds earlier Full size image

In the standard GRB model, the broadband afterglow emission is produced by the interaction of the relativistic fireball with the ambient medium16. It is therefore expected that the afterglow brightness depends, among other variables, on the total energy release of the explosion17. Indeed, typical GRB afterglows are found to be correlated with the total energy radiated in the gamma-rays, being brighter afterglows associated on average to the most luminous GRBs. This is valid for both long duration and short duration bursts18,19. In this context, two surprising features of GRB170817A were that, despite its weak gamma-ray emission, the burst was followed by a bright optical transient20,7, AT2017gfo, and a long-lived X-ray afterglow8,12. The observed optical luminosity of AT2017gfo (L pk ~ 1041 erg s−1) lies within the range of optical afterglows from short GRBs (1040 erg s−1 < L opt < 1044 erg s−1 at 12 h). However, when normalizing for the gamma-ray energy release, the optical emission stands out of the afterglow population (Fig. 2a). Indeed, this early UV/optical component is widely interpreted as the kilonova emission from the merger ejecta8,9,13, and its luminosity is not directly related to the gamma-ray burst as in the case of a standard afterglow. The optical afterglow of GRB170817A was instead much fainter than AT2017gfo at early times and became visible >100 days after the merger21. Figure 2a singles out GRB150101B as another event with an optical counterpart brighter than the average afterglow population. Its luminosity (L opt ~ 2 × 1041 erg s−1 at 1.5 d) is ~2 times brighter than AT2017gfo at the same epoch and appears to decay at a slower rate (0.5 ± 0.3 mag d−1), although we caution that residual light from the underlying galaxy may be affecting this estimate.

Fig. 2 Comparison of GRB150101B with GW and GRB afterglows. a Optical light curves of short GRBs normalized to the observed gamma-ray energy release. Downward triangles are 3 σ upper limits. Vertical error bars are 1 σ. The optical afterglow of GRB170817A became visible >100 days after the merger21 and is not reported in the plot. b X-ray light curves of short GRBs normalized to the observed gamma-ray energy release. The shaded area shows the 68% dispersion region. In both cases, the counterparts of GW170817 (AT2017gfo and GRB170817A) and GRB150101B stand out of the sample of standard afterglows. They can be described as a bright kilonova (dotted lines) followed by a late-peaking off-axis afterglow (dashed lines) Full size image

Figure 2b shows the X-ray light curves of short GRBs normalized by the isotropic-equivalent gamma-ray energy release. In this case too, the X-ray counterparts of GRB170817A and GRB150101B differ from the general population of short GRBs and are consistent with the predictions of off-axis afterglows22. In the off-axis scenario, the post-peak afterglow reveals the total blastwave energy and is therefore as bright as standard on-axis afterglows at a similar epoch, whereas only a small fraction of the total prompt energy is visible in gamma-rays. Off-axis afterglows are therefore expected to have a L X /E γ,iso ratio higher than the average population of bursts seen on-axis, as observed for GRB150101B and GRB170817A.

Temporal analysis

The earliest follow-up observations of GRB150101B were performed by the Swift satellite starting 1.5 days after the burst. Swift monitoring lasted for 4 weeks and shows a persistent X-ray source (Fig. 3, top panel). The study of GRB150101B at X-ray energies is complicated by its proximity to a low-luminosity AGN, which contaminates the Swift measurements. Observations with the Chandra X-ray Observatory (PI: E. Troja, A. Levan) were critical to resolve the presence of the two nearby sources, and to characterize their properties. The brighter X-ray source, coincident with the galaxy nucleus, shows no significant flux or spectral variations between the two epochs (7.9 d and 39.6 d after the burst). Its observed flux is ~3.7 × 10−13 erg cm−2 s−1 in the 0.3–10 keV energy band, thus accounting for most of the emission measured by Swift. The fainter X-ray source, coincident with the position of the GRB optical counterpart23, instead dropped by a factor of 7 between the two epochs (Fig. 3, bottom panel). By assuming a power-law decay, F X ∝ t-α, we derive a temporal decay slope α ~ 1.2, typical of standard afterglows. A previous study of this event23, based only on the Chandra dataset, used a simple power-law decay to describe the afterglow temporal evolution from early (~1.5 d) to late (~40 d) times. However, Fig. 3 (top panel) shows that such decay (dashed line) would violate the early Swift measurements, not considered in past analyses.

Fig. 3 X-ray observations of GRB150101B. Swift observations (top panel) found a single steady X-ray source. Chandra observations (bottom panel) revealed the presence of two nearby X-ray sources: a brighter constant source at the location of the galaxy nucleus (AGN), and a fainter variable source at the location of the GRB optical counterpart. Its X-ray behavior is consistent with a standard power-law decay (dashed line) or a rising afterglow from either an off-axis jet (solid line) or a cocoon (dot-dashed line). However, the superposition of a constant source with a power-law model violates the early Swift observations. These are instead well described by the superposition of a constant source with a late-peaking afterglow. Vertical error bars are 1 σ. The horizontal gray line shows the average flux level of the AGN Full size image

The flat Swift light curve, although dominated by the AGN contribution, provides an important indication on the behavior of the early GRB afterglow, which had to remain sub-dominant over the observed period. Figure 3 shows that this is consistent with the onset of a delayed afterglow, as observed for GRB170817A8. Two leading models are commonly adopted to describe the broadband afterglow evolution of GRB170817A: a highly relativistic structured jet seen off-axis8,12,14, and a choked jet with a nearly isotropic mildly relativistic cocoon11,24. We fit both models to the GRB150101B afterglow with a Bayesian MCMC parameter estimation scheme, using the same priors and afterglow parameters as in ref. 12. For the structured jet, we assumed that the energy follows a Gaussian angular profile E(θ) = E 0 exp(−θ)−/2θ c 2) where θ c is the width of the energy distribution. The fit results are summarized in Table 1 and shown in Fig. 3 as a solid line (off-axis jet), and a dot-dashed line (cocoon). These models can reproduce both the Chandra data (bottom panel) and the Swift light curve (top panel). In the cocoon model, the predicted post-peak temporal slope is α ~ 1.0, consistent with the result from the simple power-law fit. This implies that the afterglow peak must precede the first Chandra observation at 8 d, although not by much as the Swift light curve constraints t pk » 1 d. In the jet scenario, the post-peak temporal decay tends to α ~ 2.5, and constrains the range of possible peak times to t pk ~10–15 d.

Table 1 Afterglow parameters for GRB150101B Full size table

Other scenarios, such as a long-lasting (~8 d) X-ray plateau followed by a standard afterglow decay, are consistent with the Swift and Chandra constraints. However, the required timescales far exceed the typical lifetime (<104 s) of X-ray plateaus and would not follow the time-luminosity relation25 usually observed in GRB afterglows.

Spectral analysis

The afterglow X-ray spectrum is well described by a simple power-law, F ν ∝ ν−β, with spectral index β = 0.64 ± 0.17 (68% confidence level). This implies a non-thermal electron energy distribution with power law slope p = 2.28 ± 0.34 seen below the cooling break, similar to GRB170817A12,26. The first Swift observation at 1.5 days is likely dominated by the AGN contribution (Fig. 3) and can place a 3 σ upper limit on the early X-ray afterglow of ~1.5 × 10−13 erg cm−2 s−1 (Methods section), if one assumes the same spectral shape as the later Chandra epoch. This is a plausible assumption as, for example, continued monitoring of GRB170817A shows no significant variations in its afterglow spectral shape during the first 12 months12,26.

In Fig. 4, we show the spectral energy distribution of the GRB counterpart. At early times, the optical luminosity exceeds the afterglow extrapolation based on the X-ray limits. This optical excess is consistent with the emergence of a kilonova slightly brighter (by a factor of ~2) than AT2017gfo. Given the limited dataset, we cannot exclude that the optical excess is due to an intrinsic variability of the afterglow (e.g., flares). However, these types of chromatic features are usually observed at X-ray rather than at optical wavelengths, typically occur within a few hours after the GRB, and are more frequent in long GRBs than in short GRBs27,28. The luminosity and timescales of the observed optical excess more naturally fit within the kilonova scenario6,29,30. The observed color (r–J < 1.2 at 2.5 d) is somewhat bluer than the color of AT2017gfo at the same epoch, possibly indicating a higher temperature of the ejecta. A slower cooling rate is also consistent with the shallower temporal decay of the optical light.

Fig. 4 Spectral energy distribution of GRB150101B. Downward triangles are 3 σ upper limits. At early times, the optical light is brighter than expected based on the extrapolation of the X-ray constraints (dot-dashed line), suggesting that it belongs to a different component of emission. The blue area shows the spectrum of AT2017gfo8,10,20 at 1.5 days, rescaled by a factor 2 in order to match the optical brightness of GRB150101B. At later times, the lack of optical detection is consistent with the X-ray afterglow behavior. At both epochs, our model is consistent with the lack of radio detection at 5.7 d23 Full size image

At later times, this excess is no longer visible. The deep upper limit from Gemini shows that at ~10 days the afterglow became the dominant component and the kilonova already faded away. This is consistent with the behavior of AT2017gfo8,10,20, and predicted in general by kilonova models13,29,30.

Environment

Only a minor fraction (<30%) of short GRBs is associated to an early-type host galaxy31. This number decreases to ~18% if one considers only bona-fide associations, i.e., those with a low probability of being spurious. Notably, both GRB150101B and GW170817 were harbored in a luminous elliptical galaxy (Fig. 5).

Fig. 5 The host galaxy of GRB150101B. Color image of the host galaxy of GRB150101B created from the HST/WFC3 observations in filters F606W (blue, green) and F160W (red). The two intersecting lines mark the location of the GRB afterglow. The galaxy optical spectrum (inset) displays several absorption lines (Ca II H & K, G-band, Mg, and Na at z = 0.1341) and a red continuum Full size image