New Potential Class of Long Gamma-Ray Bursts

Intro

Since their discovery, a debate has been sparked over the classification of a new potential class of long gamma-ray bursts (LGRB) that possess ultra-long durations lasting at least 1000s, along with distinctly different x-ray and optical light curves to regular gamma-ray bursts (GRB) (Levan 2014). The search for members of the ultra-long gamma-ray burst (ULGRB) population is currently gaining traction, with some suggesting they may be commonly occurring despite difficulties detecting them (levan 2014) and that their origins may be uniquely different from regular LGRBs (Boer 2015). It is thought that ULGRBs are produced by stars of very large radii evolving into an engine driven super-luminous supernova (SLSN), rather than an envelope stripped compact Wolf-Rayet star which is commonly accepted as a progenitor for LGRBs (levan 2014), however, the connection between SN signatures and ULGRBs is an ambiguous one.

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More recent work has been centred on exploring this partially vague connection, attempting to refine models explaining an engine driven SLSN, the nature of their progenitors and the host galaxy properties (bersten, kann, japlj, gao). The focus of this work is on two papers in this area of astrophysics: “A very luminous magnetar-powered supernova associated with an ultra-long gamma-ray burst,” (Greiner) and “The Diversity of Transients from Magnetar Birth,” (Metzger). The former details the discovery and observational analysis of SN2011k, preceded by GRB111209A, and postulates a magnetar origin, whereas the latter acknowledges the ULGB-SLSN connection and builds a thorough model of magnetar formation in order to explain it.

A very luminous magnetar-powered supernova associated with an ultra-long gamma-ray burst

Summary

The work performed by (greiner) at first focuses on observational data of GRB111209A taken with GROND, a 7-channel imager specifically designed for rapid observations of GRB afterglows that performs simultaneous imaging in the Sloan g’r’I’z’ and near infra-red JHK bands. Use of the GROND imager gives the authors the ability to calculate multiple properties of the GRB, such as photometric redshift, the intrinsic power law slope of the continuum emission, and the galactic host emission, which can all be done in a short window and  monitored as the GRB afterglow evolves (greiner 2008). z΄

GROND was used on 16 epochs with logarithmic temporal spacing until 72 days after the GRB was detected by the Swift satellite on December 9 2011; however, readings were interrupted by the Sun until 280 days after detection when a final epoch for host photometry was obtained. There is also inclusion of supplementary observations of the GRB afterglow: u-band observations from Swift with applied UVOT photometry; HST F336W/F125W data from 11.1 and 35.1 days after the GRB, respectively; two epochs of VLT(X-shooter)/FORS2 g΄RCi΄z΄ data during the SN phase; a late-time Gemini-S u΄-band observation 198 days after the GRB. Data tables for all GROND and Swift data can be found in Appendix A.

GRB111209A was measured to last ~10,000s at a redshift of z=0.677, and to have an equivalent isotropic energy output of Eiso = (5.70.7)1053 erg which is among the brightest of LGRBs. After analysis of the VLT/X-shooter spectrum, obtained near the peak of excess emission and detailed below, (greiner) deduces this emission to be caused by a SN, dubbed SN2011k1, because of similarities in spectral shape to known GRB-SNe and development in time and colour.

Decomposition of the GRB afterglow is shown in figure 1 which displays data from GROND and other instruments. Since the authors accurately determined the host galaxy emission, they were able to subtract it from the GRB afterglow. The optical light curve displays a deviation from its initial power law decay, remaining flat for around 15 days, before decaying again. They also plot the light curve in the u’-band showing a steeper decay slope >10 days where the initial slope is a1.. and the second a2… Since there is no contribution from the associated SN in the u’-band, they explain that they can build a template for the pure afterglow contribution from the SN.

The authors construct a quasi-bolometric light curve for the SN from GROND and supplementary data by first extinction-correcting filter band with Av…, galactic foreground, and Avhost… which was derived from the GRB afterglow spectral energy distribution, then secondly deriving quadratic polynomials for sets of 3 consecutive filters, which were combined to form the light curve.  They then integrate the polynomials over rest-frame wavelength from 3860/(1+z) Å to 13560/(1+z) Å, corresponding to the blue limit of the g’-band filter to the red limit of the J filter, and used k-correction computed from the spectral energy distribution. Finally, they transformed the integrated flux into luminosity (figure 2), employing a luminosity distance of d=4080 Mpc, using concordance cosmology (ï-=0.73, ï-m=0.27, and H0=71 km s1 Mpc1)

Using data from the VLT/X-shooter spectrum, which is the sum of light from the GRB afterglow, the GRB host galaxy, and the supernova SN 2011kl and taken near the SN peak, (greiner) again subtracts the GRB and host contributions followed by rest-frame conversion and correction for intrinsic reddening of E(BV)=0.040.01 mag. They use this data, shown in figure 3, to draw similarities between SN2011k1 and SLSN, noting that it’s flat shape and high UV flux is clearly different from the brightest known GRB-SNe.

When comparing this spectrum (figure 3) to the GRB afterglow, they find that there is no change in absorption lines or redshift and that they are similar to those found in hosts of LGRBs, thus relating SN2011k1 to the same galaxy as GRB111209A. Also mentioned in this comparison is that there is no offset between the GRB afterglow and SN images, which the authors say implies the events are co-spatial to within <200pc.

Spectral analysis of SN2011k1 reveals very low metal content and lack of H or He, unlike typical SNe Ic associated with LGRBs, therefore, (greiner) apply parameterized SN light curve fits to derive an ejecta mass Mej = 31 M⊙ and a 56Ni mass of 1.00.1 M⊙ for photospheric velocity vph = 20,000 km/s, and a grey opacity of 0.070.01 cm2 g1. The high Ni/Mej ratio is reported to be incompatible with the spectrum (figure 3) and thus the authors seek an alternative explanation for the luminosity source.

The likeness of SN2011k1 to SLSNe, its host galaxy properties, low metal abundance, improbable Ni powered luminosity and association with GRB111209A lead to the authors to suggest a magnetar origin for this event, stating that post-birth from the collapse of a massive star it powers the surrounding ejecta for a super-luminous effect. This hypothesis is evidenced by reproducing the SN spectrum via sampling of  magnetar initial spin period Pi, magnetic dipole field strength B, Mej and rotational energy Ek. Best fits to the data found P=12.20.3 ms and B=7.51.51014 G for observed short tpeak (14 rest-frame days) and moderate peak luminosity, as well as Mej=2.4±0.7 M⊙ and energy EK=(5.53.3)1051 erg.

Importance and Implications

The significance of this paper arises from its robust analysis of the GRB111209A afterglow and SN2011k1 using multiple datasets on top of their own data, collected with GROND. It is clear that the development (g2008) and implementation of this instrument is extraordinarily useful in this area of observational astrophysics due to its multiple measurement taking capabilities. Without this device, it is unlikely the authors could have developed their results as well as they have done, despite much of their comparison data coming from other sources, such as the ESO VLT/X-shooter. On the note of observational data analysis, there are a few important points to consider underneath the various statistical methods and rebinning mentioned in this paper.

It is explained that the SN light curve error depends on the decay slope a2 remaining constant after the last afterglow measurement before onset of the SN, and since the GRB light curve is observed to steepen, the authors claim their SN luminosity measurement of 2.8+1.2-1.0 x 1043 erg/s is in fact a lower limit. They also explain that they lack any near-infrared measurements for SN2011k1, acknowledging that the bolometric peak luminosity could be underestimated by 5%-30%.

A crucial achievement from this paper is that a SN with such high bolometric peak luminosity was detected, not only within the same host galaxy, but to within <200pc of an ULGRB. Being able to associate a SLSN candidate to an ULGRB with near any levels of certainty is welcome evidence to the idea that this intrinsically linked subgroup of GRB-SNe exists, especially for those exploring the progenitor and engine powered models of such events.

Evidently the authors are confident that neither the GRB is caused by a tidal disruption event, or that the unusual SN spectrum and luminosity is caused by 56Ni brightening and hence distancing it from SNIc. The former is explained by the lack of ejecta and fall back accretion time being too short to produce an ULGRB, and the latter simply by the overly large amount of 56Ni needed to reproduce the spectrum of SN2011k1. Formation and subsequent accretion of matter on a black hole is also rejected, leaving the authors to suggest the birth of a magnetar as the cause, and as seen in fig 2 their model reproduces the quasi-bolometric spectrum of SN2011k1 rather well without invoking any 56Ni interaction.

Critical Assessment

This paper is difficult to follow for a number of reasons. The layout is most unusual for a scientific report; it presents the abstract, which is well written and informative, then without a section heading proceeds to give a few paragraphs summarising their observational findings, with some values then absent in crucial places throughout the remainder of the work. These paragraphs are followed by a qualitative description of their interpretations for an origin of GRB111209A and SN2011k1 and their modelling of magnetar SN enhancement, in attempt to draw connections. At the end of this section, they suggest that the events could have been caused by formation of a magnetar which then lives on to power SN2011k1, and up to this point they make a good argument.

The problem with this argument is that it is devoid of any mathematical content, and all the data provided is in the three graphs included in this work or essentially contained in references to other papers, making it difficult to decipher which parts (greiner) did themselves, or how they did it. The captions for each figure are also all on separate pages to the figures, located after the end of the untitled section and a collection of references.

Beyond this point is a Methods section with seven 7 subsections, each with their own title and even some supplementary data to explain the origin of their work. Unfortunately, this part of the paper feels cyclical and can often feel at odds with methods they had already discussed. For example, they talk about subtracting the host galaxy emission from most of the light curves, but at multiple points in between mention they firstly have to build a template for the emission, which is detailed in a later section to be constructed from GROND data and using LePHARE . Again, there is a complete lack of mathematical clarity on their methods, however, it appears that the majority is all computational statistics performed on their copious data sources or modelling techniques. Even when discussing the origin for their magnetar properties, they give a simple qualitative explanation which should probably have been accompanied by some mathematics.

When all of this is put together, it makes for an excruciatingly difficult read. There are inconsistencies everywhere in its presentation and in some comments of the work, making one wonder why they would read it again. Despite (greiner) providing some outstanding results and performing brilliant analysis on the afterglow of GRB2011k1 and spectrum of SN2011k1, the details of their work is lost to disorganisation.

The Diversity of Transients from Magnetar Birth

Summary

This paper takes an approach to modelling physical properties of highly magnetized, millisecond rotational neutron stars, known as magnetars, in order to unify ULGRBs and SLSNe in a common framework. The authors intend to show that under their model, both of these events can be explained individually with a magnetar central engine and simultaneously via magnetar formation, as well as stating that the transition from ULGRBs to SLSNe is a natural consequence of the model. They focus on the observations of GRB111209A and SN2011k1 as a benchmark since the pair were found to be closely associated events, whilst also including various observational signatures of magnetar formation. Also discussed is the SLSN, ASASSN-15lh, which has the highest recorded peak luminosity of … (very recently it has been suggested to be a tidal disruption event (leloudas)). This event challenges the magnetar central engine model because of its extreme energy peaks, however, the authors go on to show that the maximum energy output from magnetars was previously underestimated and hence fit ASASSN-15lh within their model.

The proto-magnetar spin-down energetics used in the model were given as follows: firstly, the rotational energy of a magnetar with mass is

where

is the NS moment of inertia and

is the rotational period. The rate which the NS loses rotational energy for an aligned force-free wind is given by

is the spin-down luminosity,

the NS magnetic dipole moment and

is the surface equatorial dipole field, with a radius

12 km and

is the initial spin-down time. The authors note that, although the correction is minor for ULGRBs, the spin-down luminosity given in equation eqref{2} is exceeded for a few seconds after core bounce due to neutrino-heated wind.

(Metzger) secondly consider the constraints on the GRB jet collimation and how the NS properties described by the above equations can be translated to derive opening angle conditions for observed peak luminosity. Though a stable jet may be formed, it may not cleanly escape the star over time, tsd, of peak spin-down power. This is because there is uncertainty over whether the strong toroidal magnetic field in the nebula, separating the magnetar wind from the surrounding star, remains stable in the case of non-asymmetric instabilities.

It is required, for a jet of luminosity Lj and half opening angle …, that tescWhere (Metzger) assume Erot=… since the kinetic energy of the explosion is dominated by the magnetar rotational energy. A cleanly escaping jet will then have a peak spin-down luminosity of Lj=.., implying that the observed isotropic gamma-ray luminosity is Liso=.., where e= is the radiative efficiency and fb=.. the beaming fraction. The authors state that, assuming the propagation time through the star is negligible, we can expect the GRB duration, Ty, to be such that Ty=..

Lastly, the authors describe how SN mass ejecta, Mej, thermalizes over time through adiabatic expansion and 56Ni decay (the standard process for type Ic SNe), which is enhanced by input from the NS to super-luminous levels. The thermal energy E evolution over time t as the ejecta expands is given by

For an initial kinetic energy of Eok… , where the LHS accounts for PdV losses, vej=.. is the ejecta velocity and rej the mean radius. Conservation of energy requires that the kinetic energy Ek=… increases due to the work done from PdV, thus dEk=… Lsn=… is the radiated luminosity, where td=.. is the photon diffusion timescale, with k=… the optical opacity which is set by electron scattering and Doppler broadening lines. Lni=… is the rate of heating due to 56Ni decay where Mni is the 56Ni mass. The final term is the energy input from the magnetar which is assumed to thermalize the ejecta with unity efficiency.