Magnetar Central Engine Research Articles

Jets launched at magnetar birth cannot be ignored

I question models for powering super energetic supernovae (SESNe) with a magnetar central engine that do not include jets that are expected to be launched by the magnetar progenitor. I show that under reasonable assumptions the outflow that is expected during the formation of a magnetar can carry an amount of energy that does not fall much below, and even surpasses, the energy that is stored in the newly born spinning neutron star (NS). The rapidly spinning NS and the strong magnetic fields attributed to magnetars require that the accreted mass onto the newly born NS possesses high specific angular momentum and strong magnetic fields. These ingredients are expected, as in many other astrophysical objects, to form collimated outflows/jets. I argue that the bipolar outflow in the pre-magnetar phase transfers a substantial amount of energy to the supernova (SN) ejecta, and it cannot be ignored in models that attribute SESNe to magnetars. I conclude that jets launched by accretion disks and accretion belts are more likely to power SESNe than magnetars are. This conclusion is compatible with the notion that jets might power all core collapse SNe (CCSNe).

A self-consistent analytical magnetar model: the luminosity of γ-ray burst supernovae is powered by radioactivity

We present an analytical model that considers energy arising from a magnetar central engine. The results of fitting this model to the optical and X-ray light curves (LCs) of five long-duration $\gamma$-ray bursts (LGRBs) and two ultra-long GRBs (ULGRBs), including their associated supernovae (SNe), show that emission from a magnetar central engine cannot be solely responsible for powering an LGRB-SN. While the early AG-dominated phase can be well described with our model, the predicted SN luminosity is underluminous by a factor of $3-17$. We use this as compelling evidence that additional sources of heating must be present to power an LGRB-SN, which we argue must be radioactive heating. Our self-consistent modelling approach was able to successfully describe all phases of ULGRB 111209A / SN 2011kl, from the early afterglow to the later SN, where we determined for the magnetar central engine a magnetic field strength of $1.1-1.3\times10^{15}$ G, an initial spin period of $11.5-13.0$ ms, a spin-down time of $4.8-6.5$ d, and an initial energy of $1.2-1.6\times10^{50}$ erg. These values are entirely consistent with those determined by other authors. The luminosity of a magnetar-powered SN is directly related to how long the central engine is active, where central engines with longer durations give rise to brighter SNe. The spin-down timescales of superluminous supernovae (SLSNe) are of order months to years, which provides a natural explanation as to why SN 2011kl was less luminous than SLSNe that are also powered by emission from magnetar central engines.

POLARIZATION EVOLUTION OF EARLY OPTICAL AFTERGLOWS OF GAMMA-RAY BURSTS

ABSTRACT The central engine and jet composition of gamma-ray bursts (GRBs) remain mysterious. Here we suggest that observations on the polarization evolution of early optical afterglows may shed light on these questions. We first study the dynamics of a reverse shock and a forward shock that are generated during the interaction of a relativistic jet and its ambient medium. The jet is likely magnetized with a globally large-scale magnetic field from the central engine. The existence of the reverse shock requires that the magnetization degree of the jet should not be high (σ ≤ 1), so that the jet is mainly composed of baryons and leptons. We then calculate the light curves and polarization evolution of early optical afterglows and find that when the polarization position angle changes by 90° during the early afterglow, the polarization degree is zero for a toroidal magnetic field but is very likely to be nonzero for an aligned magnetic field. This result would be expected to provide a probe for the central engine of GRBs because an aligned field configuration could originate from a magnetar central engine and a toroidal field configuration could be produced from a black hole via the Blandford–Znajek mechanism. Finally, for such two kinds of magnetic field configurations, we fit the observed data of the early optical afterglow of GRB 120308A equally well.

A trio of gamma-ray burst supernovae:

We present optical and near-infrared (NIR) photometry for three gamma-ray burst supernovae (GRB-SNe): GRB 120729A, GRB 130215A / SN 2013ez and GRB 130831A / SN 2013fu. In the case of GRB 130215A / SN 2013ez, we also present optical spectroscopy at t-t0=16.1 d, which covers rest-frame 3000-6250 Angstroms. Based on Fe II (5169) and Si (II) (6355), our spectrum indicates an unusually low expansion velocity of 4000-6350 km/s, the lowest ever measured for a GRB-SN. Additionally, we determined the brightness and shape of each accompanying SN relative to a template supernova (SN 1998bw), which were used to estimate the amount of nickel produced via nucleosynthesis during each explosion. We find that our derived nickel masses are typical of other GRB-SNe, and greater than those of SNe Ibc that are not associated with GRBs. For GRB 130831A / SN 2013fu, we use our well-sampled R-band light curve (LC) to estimate the amount of ejecta mass and the kinetic energy of the SN, finding that these too are similar to other GRB-SNe. For GRB 130215A, we take advantage of contemporaneous optical/NIR observations to construct an optical/NIR bolometric LC of the afterglow. We fit the bolometric LC with the millisecond magnetar model of Zhang & Meszaros (2001), which considers dipole radiation as a source of energy injection to the forward shock powering the optical/NIR afterglow. Using this model we derive an initial spin period of P=12 ms and a magnetic field of B=1.1 x 10^15 G, which are commensurate with those found for proposed magnetar central engines of other long-duration GRBs.

Constraining properties of GRB magnetar central engines using the observed plateau luminosity and duration correlation

An intrinsic correlation has been identified between the luminosity and duration of plateaus in the X-ray afterglows of Gamma-Ray Bursts (GRBs; Dainotti et al. 2008), suggesting a central engine origin. The magnetar central engine model predicts an observable plateau phase, with plateau durations and luminosities being determined by the magnetic fields and spin periods of the newly formed magnetar. This paper analytically shows that the magnetar central engine model can explain, within the 1$\sigma$ uncertainties, the correlation between plateau luminosity and duration. The observed scatter in the correlation most likely originates in the spread of initial spin periods of the newly formed magnetar and provides an estimate of the maximum spin period of ~35 ms (assuming a constant mass, efficiency and beaming across the GRB sample). Additionally, by combining the observed data and simulations, we show that the magnetar emission is most likely narrowly beamed and has $\lesssim$20% efficiency in conversion of rotational energy from the magnetar into the observed plateau luminosity. The beaming angles and efficiencies obtained by this method are fully consistent with both predicted and observed values. We find that Short GRBs and Short GRBs with Extended Emission lie on the same correlation but are statistically inconsistent with being drawn from the same distribution as Long GRBs, this is consistent with them having a wider beaming angle than Long GRBs.

A TEST OF THE MILLISECOND MAGNETAR CENTRAL ENGINE MODEL OF GAMMA-RAY BURSTS WITHSWIFTDATA

A rapidly spinning, strongly magnetized neutron star (magnetar) has been proposed as one possible candidate of the central engine of gamma-ray bursts (GRBs). We systematically analyze the Swift/XRT light curves of long GRBs detected before 2013 August, and characterize them into four categories based on how likely they may harbor a magnetar central engine: Gold, Silver, Aluminum, and Non-magnetar. We also independently analyze the data of short GRBs with a putative magnetar central engine. We then perform a statistical study of various properties of the magnetar samples and the non-magnetar sample, and investigate whether the data are consistent with the hypothesis that there exist two types of central engines. By deriving the physical parameters of the putative magnetars, we find that the observations of the Gold and Silver samples are generally consistent with the predictions of the magnetar model. For a reasonable beaming factor for long GRBs, the derived magnetar surface magnetic field $B_p$ and initial spin period $P_0$ fall into the reasonable range. Magnetar winds in short GRBs, on the other hand, are consistent with being isotropic. No GRB in the magnetar sample has a beam-corrected total energy exceeding the maximum energy budget defined by the initial spin energy of the magnetar, while some non-magnetar GRBs do violate such a limit. With beaming correction, on average the non-magnetar sample is more energetic and luminous than the magnetar samples. Our analysis hints that millisecond magnetars are likely operating in a good fraction, but probably not all, GRBs.

A DOUBLE NEUTRON STAR MERGER ORIGIN FOR THE COSMOLOGICAL RELATIVISTIC FADING SOURCE PTF11agg?

The Palomar Transient Factory (PTF) team recently reported the discovery of a rapidly fading optical transient source, PTF11agg. A long-lived scintillating radio counterpart was identified, but the search for a high energy counterpart showed negative results. The PTF team speculated that PTF11agg may represent a new class of relativistic outbursts. Here we suggest that a neutron star (NS)-NS merger system with a supra-massive magnetar central engine could be a possible source to power such a transient, if our line of sight is not on the jet axis direction of the system. These systems are also top candidates for gravitational wave sources to be detected in the advanced LIGO/Virgo era. We find that the PTF11agg data could be explained well with such a model, suggesting that at least some gravitational wave bursts due to NS-NS mergers may be associated with such a bright electromagnetic counterpart without a \gamma-ray trigger.

A SUPRAMASSIVE MAGNETAR CENTRAL ENGINE FOR GRB 130603B

We show that the peculiar early optical and in particular X-ray afterglow emission of the short duration burst GRB 130603B can be explained by continuous energy injection into the blastwave from a supra-massive magnetar central engine. The observed energetics and temporal/spectral properties of the late infrared bump (i.e., the "kilonova") are also found consistent with emission from the ejecta launched during an NS-NS merger and powered by a magnetar central engine. The isotropic-equivalent kinetic energies of both the GRB blastwave and the kilonova are about $E_{\rm k}\sim 10^{51}$ erg, consistent with being powered by a near-isotropic magnetar wind. However, this relatively small value demands that most of the initial rotational energy of the magnetar $(\sim {\rm a~ few \times 10^{52}~ erg})$ is carried away by gravitational wave radiation. Our results suggest that (i) the progenitor of GRB 130603B would be a NS-NS binary system, whose merger product would be a supra-massive neutron star that lasted for about $\sim 1000$ seconds; (ii) the equation-of-state of nuclear matter would be stiff enough to allow survival of a long-lived supra-massive neutron star, so that it is promising to detect bright electromagnetic counterparts of gravitational wave triggers without short GRB associations in the upcoming Advanced LIGO/Virgo era.

Signatures of magnetar central engines in short GRB light curves

A significant fraction of the Long Gamma-ray Bursts (LGRBs) in the Swift sample have a plateau phase showing evidence of ongoing energy injection. We suggest that many Short Gamma-ray Bursts (SGRBs) detected by the Swift satellite also show evidence of energy injection. Explaining this observation within the typical SGRB progenitor model is challenging as late time accretion, often used to explain plateaus in LGRBs, is likely to be absent from the SGRB population. Alternatively, it is predicted that the remnant of NS-NS mergers may not collapse immediately to a BH (or even collapse at all), forming instead an unstable millisecond pulsar (magnetar) which powers a plateau phase in the X-ray lightcurve. By fitting the magnetar model to all of the Swift SGRBs observed until May 2012, we find that about half can be clearly fitted with a magnetar plateau phase while the rest are consistent with forming a magnetar but the data are insufficient to prove a plateau phase. More data, both at early times and a larger sample, are required to confirm this. This model can be tested by detecting the gravitational wave emission from events using the next generation gravitational wave observatories.

High-energy emission in short GRBs and the role of magnetar central engines

A significant number of long Gamma-ray Bursts (GRBs) detected by the Swift Satellite have a plateau phase signifying ongoing energy injection. Using BAT and XRT observations, we find that many short GRBs show similar behavior which challenges the typical short GRB progenitor model. We suggest the remnant of neutron star - neutron star mergers may not collapse immediately to a black hole (or even collapse at all) forming instead a magnetar. This model predicts that there would be a plateau phase in the X-ray lightcurve followed by a shallow decay phase, if it is a stable magnetar, or a steep decay if the magnetar collapses to a black hole within a few hundred seconds. By fitting this model to all short GRB BAT-XRT lightcurves, we show that a magnetar could power the observed energy injection. This model can be tested using the next generation gravitational wave observatories