Abstract
Long-duration, spectrally soft gamma-ray bursts (GRBs) are associated with Type Ic core collapse (CC) supernovae (SNe), and thus arise from the death of massive stars. In the collapsar ...model, the jet launched by the central engine must bore its way out of the progenitor star before it can produce a GRB. Most of these jets do not break out, and are instead ‘choked’ inside the star, as the central engine activity time, t
e, is not long enough. Modelling the long-soft GRB duration distribution assuming a power-law distribution for their central engine activity times,
$\propto\! t_{\rm e}^{-\alpha }$
for t
e > t
b, we find a steep distribution (α ∼ 4) and a typical GRB jet breakout time of t
b ∼ 60s in the star's frame. The latter suggests the presence of a low-density, extended envelope surrounding the progenitor star, similar to that previously inferred for low-luminosity GRBs. Extrapolating the range of validity of this power law below what is directly observable, to t
e < t
b, by only a factor of ∼4–5 produces enough events to account for all Type Ib/c SNe. Such extrapolation is necessary to avoid fine-tuning the distribution of central engine activity times with the breakout time, which are presumably unrelated. We speculate that central engines launching relativistic jets may operate in all Type Ib/c SNe. In this case, the existence of a common central engine would imply that (i) the jet may significantly contribute to the energy of the SN; (ii) various observational signatures, like the asphericity of the explosion, could be directly related to jet's interaction with the star.
The recent discovery of the 'weak-field, old magnetar' soft gamma repeater (SGR) J0418+5729, whose dipole magnetic field, B
dip, is less than 7.5 × 1012 G, has raised perplexing questions: how can ...the neutron star produce SGR-like bursts with such a low magnetic field? What powers the observed X-ray emission when neither the rotational energy nor the magnetic dipole energy is sufficient? These observations, which suggest either a much larger energy reservoir or a much younger true age (or both), have renewed the interest in the evolutionary sequence of magnetars. We examine here a phenomenological model for the magnetic field decay:
and compare its predictions with the observed period, P, the period derivative,
, and the X-ray luminosity, L
X, of magnetar candidates. We find a strong evidence for a dipole field decay on a time-scale of ∼103 yr for the strongest (B
dip∼ 1015 G) field objects, with a decay index within the range 1 ≤α < 2 and more likely within 1.5 ≲α≲ 1.8. The decaying field implies a younger age than what is implied by
. Surprisingly, even with the younger age, the energy released in the dipole field decay is insufficient to power the X-ray emission, suggesting the existence of a stronger internal field, B
int. Examining several models for the internal magnetic field decay, we find that it must have a very large (≳ 1016 G) initial value. Our findings suggest two clear distinct evolutionary tracks - the SGR/anomalous X-ray pulsar branch and the transient branch, with a possible third branch involving high-field radio pulsars that age into low-luminosity X-ray dim isolated neutron stars.
We analyze the prompt emission of GRB 100724B and GRB 160509A, two of the brightest gamma-ray bursts (GRBs) observed by Fermi at MeV energies but surprisingly faint at 100 MeV energies. Time-resolved ...spectroscopy reveals a sharp high-energy cutoff at energies Ec ∼ 20-60 MeV for GRB 100724B and Ec ∼ 80-150 MeV for GRB 160509A. We first characterize phenomenologically the cutoff and its time evolution. We then fit the data to two models where the high-energy cutoff arises from intrinsic opacity to pair production within the source (τγγ): (i) a Band spectrum with τγγ from the internal-shocks-motivated model of Granot et al. (2008) and (ii) the photospheric model of Gill & Thompson (2014). Alternative explanations for the cutoff, such as an intrinsic cutoff in the emitting electron energy distribution, appear to be less natural. Both models provide a good fit to the data with very reasonable physical parameters, providing an estimate of bulk Lorentz factors in the range Γ ∼ 100-400, on the lower end of what is generally observed in Fermi GRBs. Surprisingly, their lower cutoff energies Ec compared to other Fermi/LAT GRBs arise not only predominantly from the lower Lorentz factors, but also at a comparable level from differences in variability time, luminosity, and high-energy photon index. Finally, particularly low Ec values may prevent detection by Fermi/LAT, thus introducing a bias in the Fermi/LAT GRB sample against GRBs with low Lorentz factors or variability times.
ABSTRACT We report on the analysis of two deep XMM-Newton observations of the magnetar Swift J1834.9−0846 and its surrounding extended emission taken in 2014 March and October, 2.5 and 3.1 yr after ...the source went into outburst. The magnetar is only weakly detected in the first observation, with an absorption-corrected flux erg s−1 cm−2 and a upper limit during the second observation of about 3 × 10−14 erg s−1 cm−2. This flux level is more than 3 orders of magnitude lower than the flux measured at the outburst onset in 2011 September. The extended emission, centered at the magnetar position and elongated toward the southwest, is clearly seen in both observations; it is best fit by a highly absorbed power law (PL), with a hydrogen column density of cm−2 and PL photon index . Its flux is constant between the two observations at erg s−1 cm−2. We find no statistically significant changes in the spectral shape or the flux of this extended emission over a period of 9 yr from 2005 to 2014. These new results strongly support the extended emission nature as a wind nebula and firmly establish Swift J1834.9−0846 as the first magnetar to show a surrounding wind nebula. Further, our results imply that such nebulae are no longer exclusive to rotation-powered pulsars and narrow the gap between these two subpopulations of isolated neutron stars. The size and spectrum of the nebula are compatible with those of pulsar-wind nebulae, but its radiative efficiency is markedly high, possibly pointing to an additional wind component in Swift J1834.9−0846.
GRB 130427A was extremely bright as a result of occurring at low redshift whilst the energetics were more typical of high-redshift gamma-ray bursts (GRBs). We collected well-sampled light curves at ...1.4 and 4.8 GHz of GRB 130427A with the Westerbork Synthesis Radio Telescope (WSRT); and we obtained its most accurate position with the European Very Long Baseline Interferometry Network (EVN). Our flux density measurements are combined with all the data available at radio, optical and X-ray frequencies to perform broad-band modelling in the framework of a reverse–forward shock model and a two-component jet model, and we discuss the implications and limitations of both models. The low density inferred from the modelling implies that the GRB 130427A progenitor is either a very low metallicity Wolf–Rayet star, or a rapidly rotating, low-metallicity O star. We also find that the fraction of the energy in electrons is evolving over time, and that the fraction of electrons participating in a relativistic power-law energy distribution is less than 15 per cent. We observed intraday variability during the earliest WSRT observations, and the source sizes inferred from our modelling are consistent with this variability being due to interstellar scintillation effects. Finally, we present and discuss our limits on the linear and circular polarization, which are among the deepest limits of GRB radio polarization to date.
The discovery of quasi-periodic oscillations (QPOs) in magnetar giant flares has opened up prospects for neutron star asteroseismology. The scarcity of giant flares makes a search for QPOs in the ...shorter, far more numerous bursts from soft gamma repeaters (SGRs) desirable. In Huppenkothen et al., we developed a Bayesian method for searching for QPOs in short magnetar bursts, taking into account the effects of the complicated burst structure, and have shown its feasibility on a small sample of bursts. Here we apply the same method to a much larger sample from a burst storm of 286 bursts from SGR J1550-5418. We report a candidate signal at 260 Hz in a search of the individual bursts, which is fairly broad. We also find two QPOs at ~93 Hz, and one at 127 Hz, when averaging periodograms from a number of bursts in individual triggers, at frequencies close to QPOs previously observed in magnetar giant flares. Finally, for the first time, we explore the overall burst variability in the sample and report a weak anti-correlation between the power-law index of the broadband model characterizing aperiodic burst variability and the burst duration: shorter bursts have steeper power-law indices than longer bursts. This indicates that longer bursts vary over a broader range of timescales and are not simply longer versions of the short bursts.
Abstract
We present the results of our X-ray, ultraviolet, and optical follow-up campaigns of 1RXS J165424.6-433758, an X-ray source detected with the Swift Deep Galactic Plane Survey. The source ...X-ray spectrum (Swift and NuSTAR) is described by thermal bremsstrahlung radiation with a temperature of
kT
= 10.1 ± 1.2 keV, yielding an X-ray (0.3–10 keV8) luminosity
L
X
= (6.5 ± 0.8) × 10
31
erg s
−1
at a Gaia distance of 460 pc. Spectroscopy with the Southern African Large Telescope revealed a flat continuum dominated by emission features, demonstrating an inverse Balmer decrement, the
λ
4640 Bowen blend, almost a dozen He
i
lines, and He
ii
λ
4541,
λ
4686, and
λ
5411. Our high-speed photometry demonstrates a preponderance of flickering and flaring episodes, and revealed the orbital period of the system,
P
orb
= 2.87 hr, which fell well within the cataclysmic variable (CV) period gap between 2 and 3 hr. These features classify 1RXS J165424.6-433758 as a nearby polar magnetic CV.
Most gamma-ray bursts (GRBs) observed by the Swift satellite show an early steep decay phase (SDP) in their X-ray light curve, which is usually a smooth continuation of the prompt gamma-ray emission, ...strongly suggesting that it is its tail. However, the mechanism behind it is still not clear. The most popular model for this SDP is high-latitude emission (HLE), in which after the prompt emission from a (quasi-) spherical shell stops photons from increasingly large angles relative to the line of sight still reach the observer, with a smaller Doppler factor. This results in a simple relation between the temporal and spectral indexes, α= 2 +β where Fν∝t−αν−β. While HLE is expected in many models for the prompt GRB emission, such as the popular internal shocks model, there are models in which it is not expected, such as sporadic magnetic reconnection events. Therefore, testing whether the SDP is consistent with HLE can help distinguish between different prompt emission models. In order to adequately address this question in a careful quantitative manner we develop a realistic self-consistent model for the prompt emission and its HLE tail, which can be used for combined temporal and spectral fits to GRB data that would provide strict tests for the HLE model. We model the prompt emission as the sum of its individual pulses with their HLE tails, where each pulse arises from an ultrarelativistic uniform thin spherical shell that emits isotropically in its own rest frame over a finite range of radii. Analytic expressions for the observed flux density are obtained for the internal shock case with a Band function emission spectrum. We find that the observed instantaneous spectrum is also a Band function. Our model naturally produces, at least qualitatively, the observed spectral softening and steepening of the flux decay as the peak photon energy sweeps across the observed energy range. The observed flux during the SDP is initially dominated by the tail of the last pulse, but the tails of one or more earlier pulses can become dominant later on. A simple criterion is given for the dominant pulse at late times. The relation α= 2 +β holds also as β and α change in time. Modelling several overlapping pulses as a single wider pulse would overpredict the emission tail.
Abstract
The Swift Deep Galactic Plane Survey (DGPS) is a Swift Key Project consisting of 380 tiled pointings covering ∼40 deg
2
of the Galactic plane between longitude 10 < ∣
l
∣ < 30 deg and ...latitude ∣
b
∣ < 0.5 deg. Each pointing has a 5 ks exposure, yielding a total of 1.9 Ms spread across the entire survey footprint. Phase I observations were carried out between 2017 March and 2021 May. The survey is complete to depth
L
X
> 10
34
erg s
−1
to the edge of the Galaxy. The main survey goal is to produce a rich sample of new X-ray sources and transients, while also covering a broad discovery space. Here, we introduce the survey strategy and present a catalog of sources detected during Phase I observations. In total, we identify 928 X-ray sources, of which 348 are unique to our X-ray catalog. We report on the characteristics of sources in our catalog and highlight sources newly classified and published by the DGPS team.
ABSTRACT In 2015 June, the black hole X-ray binary (BHXRB) V404 Cygni went into outburst for the first time since 1989. Here, we present a comprehensive search for quasi-periodic oscillations (QPOs) ...of V404 Cygni during its recent outburst, utilizing data from six instruments on board five different X-ray missions: Swift/XRT, Fermi/GBM, Chandra/ACIS, INTEGRAL's IBIS/ISGRI and JEM-X, and NuSTAR. We report the detection of a QPO at 18 mHz simultaneously with both Fermi/GBM and Swift/XRT, another example of a rare but slowly growing new class of mHz-QPOs in BHXRBs linked to sources with a high orbital inclination. Additionally, we find a duo of QPOs in a Chandra/ACIS observation at 73 mHz and 1.03 Hz, as well as a QPO at 136 mHz in a single Swift/XRT observation that can be interpreted as standard Type-C QPOs. Aside from the detected QPOs, there is significant structure in the broadband power, with a strong feature observable in the Chandra observations between 0.1 and 1 Hz. We discuss our results in the context of current models for QPO formation.
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