arXiv subjects

P. Mimica

Publications and source records attributed to P. Mimica.

At least 19 recordsLinked to original sources

arXiv2018

3D MHD modeling of the expanding remnant of SN 1987A. Role of magnetic field and non-thermal radio emission

Aims. We investigate the role played by a pre-supernova (SN) ambient magnetic field on the dynamics of the expanding remnant of SN 1987A and the origin and evolution of the radio emission from the remnant, in particular, during the interaction of the blast wave with the nebula surrounding the SN. Methods. We model the evolution of SN 1987A from the breakout of the shock wave at the stellar surface to the expansion of its remnant through the surrounding nebula by 3D MHD simulations. The model considers the radiative cooling, the deviations from equilibrium of ionization, the deviation from temperature-equilibration between electrons and ions, and a plausible configuration of the pre-SN ambient magnetic field. From the simulations, we synthesize both thermal X-ray and non-thermal radio emission and compare the model results with observations. Results. The presence of an ambient magnetic field reduces the erosion and fragmentation of the dense equatorial ring after the impact of the SN blast wave. As a result, the ring survives the passage of the blast, at least, during the time covered by the simulations (40 years). Our model is able to reproduce the morphology and lightcurves of SN 1987A in both X-ray and radio bands. The model reproduces the observed radio emission if the flux originating from the reverse shock is heavily suppressed. In this case, the radio emission originates mostly from the forward shock traveling through the H II region. Possible mechanisms for the suppression of emission from the reverse shock are investigated. We find that synchrotron self-absorption and free-free absorption have negligible effects on the emission during the interaction with the nebula. We suggest that the emission from the reverse shock at radio frequencies might be limited by highly magnetized ejecta.

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arXiv2018

A dust-enshrouded tidal disruption event with a resolved radio jet in a galaxy merger

Tidal disruption events (TDEs) are transient flares produced when a star is ripped apart by the gravitational field of a supermassive black hole (SMBH). We have observed a transient source in the western nucleus of the merging galaxy pair Arp 299 that radiated >1.5x10^52 erg in the infrared and radio, but was not luminous at optical or X-ray wavelengths. We interpret this as a TDE with much of its emission re-radiated at infrared wavelengths by dust. Efficient reprocessing by dense gas and dust may explain the difference between theoretical predictions and observed luminosities of TDEs. The radio observations resolve an expanding and decelerating jet, probing the jet formation and evolution around a SMBH.

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arXiv2016

The influence of circumnuclear environment on the radio emission from TDE jets

Dozens of stellar tidal disruption events (TDEs) have been identified at optical, UV and X-ray wavelengths. A small fraction of these, most notably Swift J1644+57, produce radio synchrotron emission, consistent with a powerful, relativistic jet shocking the surrounding circumnuclear gas. The dearth of similar non-thermal radio emission in the majority of TDEs may imply that powerful jet formation is intrinsically rare, or that the conditions in galactic nuclei are typically unfavorable for producing a detectable signal. Here we explore the latter possibility by constraining the radial profile of the gas density encountered by a TDE jet using a one-dimensional model for the circumnuclear medium which includes mass and energy input from a stellar population. Near the jet Sedov radius of 10$^{18}$ cm, we find gas densities in the range of $n_{18} \sim$ 0.1$-$1000 cm$^{-3}$ across a wide range of plausible star formation histories. Using one- and two-dimensional relativistic hydrodynamical simulations, we calculate the synchrotron radio light curves of TDE jets (as viewed both on and off-axis) across the allowed range of density profiles. We find that bright radio emission would be produced across the plausible range of nuclear gas densities by jets as powerful as Swift J1644+57, and we quantify the relationship between the radio luminosity and jet energy. We use existing radio detections and upper limits to constrain the energy distribution of TDE jets. Radio follow up observations several months to several years after the TDE candidate will strongly constrain the energetics of any relativistic flow.

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arXiv2016

Modelling Accretion Disk and Stellar Wind Interactions: the Case of Sgr A*

Sgr A* is an ideal target to study low-luminosity accreting systems. It has been recently proposed that properties of the accretion flow around Sgr A* can be probed through its interactions with the stellar wind of nearby massive stars belonging to the S-cluster. When a star intercepts the accretion disk, the ram and thermal pressures of the disk terminate the stellar wind leading to the formation of a bow shock structure. Here, a semi-analytical model is constructed which describes the geometry of the termination shock formed in the wind. With the employment of numerical hydrodynamic simulations, this model is both verified and extended to a region prone to Kelvin-Helmholtz instabilities. Because the characteristic wind and stellar velocities are in $\sim10^{8}$ cm s$^{-1}$ range, the shocked wind may produce detectable X-rays via thermal bremsstrahlung emission. The application of this model to the pericenter passage of S2, the brightest member of the S-cluster, shows that the shocked wind produces roughly a month long X-ray flare with a peak luminosity of $L\approx 4 \times 10^{33}$ erg s$^{-1}$ for a stellar mass-loss rate, disk number density, and thermal pressure strength of $\dot{M}_{\rm w}= 10^{-7} M_\odot\, {\rm yr}^{-1}$, $n_{\rm d} = 10^{5}$ cm$^{-3}$, and $α=0.1$, respectively. This peak luminosity is comparable to the quiescent X-ray emission detected from Sgr A* and is within the detection capabilities of current X-ray observatories. Its detection could constrain the density and thickness of the disk at a distance of $\sim 3000$ gravitational radii from the supermassive black hole.

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arXiv2016

Numerical simulations of the jetted tidal disruption event Swift J1644+57

In this work we focus on the technical details of the numerical simulations of the non- thermal transient Swift J1644+57, whose emission is probably produced by a two-component jet powered by a tidal disruption event. In this context we provide details of the coupling between the relativistic hydrodynamic simulations and the radiative transfer code. First, we consider the technical demands of one-dimensional simulations of a fast relativistic jet, and show to what extent (for the same physical parameters of the model) do the computed light curves depend on the numerical parameters of the different codes employed. In the second part we explain the difficulties of computing light curves from axisymmetric two dimensonal simulations and discuss a procedure that yields an acceptable tradeoff between the computational cost and the quality of the results.

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arXiv2016

Spectral evolution of flaring blazars from numerical simulations

High resolution Very Long Baseline Interferometry (VLBI) observations of Active Galactic Nuclei (AGN) revealed traveling and stationary or quasi-stationary radio-components in several blazar jets. The traveling ones are in general interpreted as shock waves generated by pressure perturbations injected at the jet nozzle. The stationary features can be interpreted as recollimation shocks in non-pressure matched jets if they show a quasi-symmetric bump in the spectral index distribution. In some jets there may be interactions between the two kinds of shocks. These shock--shock interactions are observable with VLBI techniques, and their signature should also be imprinted on the single--dish light curves. We performed relativistic hydrodynamic (RHD) simulations of over-pressured and pressure-matched jets. To simulate the shock interaction we injected a perturbation at the jet nozzle once a steady-state was reached. We computed the non-thermal emission (including adiabatic and synchotron losses) resulting from the simulation. We show that the injection of perturbations in a jet can produce a bump in emission at GHz frequencies previous to the main flare, which is produced when the perturbation fills the jet in the observer's frame. The detailed analysis of our simulations and the non-thermal emission calculations show that interaction between a recollimation shock and traveling shock produce a typical and clear signature in both the single--dish light curves and in the VLBI observations: the flaring peaks are higher and delayed with respect to the evolution of a perturbation through a conical jet. This fact can allow to detect such interactions for stationary components lying outside of the region in where the losses are dominated by inverse Compton scattering.

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arXiv2015

The Spanish Square Kilometre Array White Book

The Square Kilometre Array (SKA) is called to revolutionise essentially all areas of Astrophysics. With a collecting area of about a square kilometre, the SKA will be a transformational instrument, and its scientific potential will go beyond the interests of astronomers. Its technological challenges and huge cost requires a multinational effort, and Europe has recognised this by putting the SKA on the roadmap of the European Strategy Forum for Research Infrastructures (ESFRI). The Spanish SKA White Book is the result of the coordinated effort of 120 astronomers from 40 different research centers. The book shows the enormous scientific interest of the Spanish astronomical community in the SKA and warrants an optimum scientific exploitation of the SKA by Spanish researchers, if Spain enters the SKA project.

astro-ph.IM↗

arXiv2015

The radio afterglow of Swift J1644+57 reveals a powerful jet with fast core and slow sheath

We model the non-thermal transient Swift J1644+57 as resulting from a relativistic jet powered by the accretion of a tidally-disrupted star onto a super-massive black hole. Accompanying synchrotron radio emission is produced by the shock interaction between the jet and the dense circumnuclear medium, similar to a gamma-ray burst afterglow. An open mystery, however, is the origin of the late-time radio rebrightening, which occurred well after the peak of the jetted X-ray emission. Here, we systematically explore several proposed explanations for this behavior by means of multi-dimensional hydrodynamic simulations coupled to a self-consistent radiative transfer calculation of the synchrotron emission. Our main conclusion is that the radio afterglow of Swift J1644+57 is not naturally explained by a jet with a one-dimensional top-hat angular structure. However, a more complex angular structure comprised of an ultra-relativistic core (Lorentz factor $Γ\sim 10$) surrounded by a slower ($Γ\sim $ 2) sheath provides a reasonable fit to the data. Such a geometry could result from the radial structure of the super-Eddington accretion flow or as the result of jet precession. The total kinetic energy of the ejecta that we infer of $\sim$ few $10^{53}\,$erg requires a highly efficient jet launching mechanism. Our jet model providing the best fit to the light curve of the on-axis event Swift J1644+57 is used to predict the radio light curves for off-axis viewing angles. Implications for the presence of relativistic jets from TDEs detected via their thermal disk emission, as well as the prospects for detecting orphan TDE afterglows with upcoming wide-field radio surveys and resolving the jet structure with long baseline interferometry, are discussed.

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arXiv2014

A method for computing synchrotron and inverse-Compton emission from hydrodynamic simulations of supernova remnants

The observational signature of supernova remnants (SNRs) is very complex, in terms of both their geometrical shape and their spectral properties, dominated by non-thermal synchrotron and inverse-Compton scattering. We propose a post-processing method to analyse the broad-band emission of SNRs based on three-dimensional hydrodynamical simulations. From the hydrodynamical data, we estimate the distribution of non-thermal electrons accelerated at the shock wave and follow the subsequent evolution as they lose or gain energy by adiabatic expansion or compression and emit energy by radiation. As a first test case, we use a simulation of a bipolar supernova expanding into a cloudy medium. We find that our method qualitatively reproduces the main observational features of typical SNRs and produces fluxes that agree with observations to within a factor of a few. allowing for further use in more extended sets of models.

astro-ph.SR↗

arXiv2013

The influence of the magnetic field on the spectral properties of blazars

We explore the signature imprinted by dynamically relevant magnetic fields on the spectral energy distribution (SED) of blazars. It is assumed that the emission from these sources originates from the collision of cold plasma shells, whose magnetohydrodynamic evolution we compute by numerically solving Riemann problems. We compute the SEDs including the most relevant radiative processes and scan a broad parameter space that encompasses a significant fraction of the commonly accepted values of not directly measurable physical properties. We reproduce the standard double hump SED found in blazar observations for unmagnetized shells, but show that the prototype double hump structure of blazars can also be reproduced if the dynamical source of the radiation field is very ultrarelativistic both, in a kinematically sense (namely, if it has Lorentz factors \gtrsim 50) and regarding its magnetization (e.g., with flow magnetizations σ\simeq 0.1). A fair fraction of the {\em blazar sequence} could be explained as a consequence of shell magnetization: negligible magnetization in FSRQs, and moderate or large (and uniform) magnetization in BL Lacs. The predicted photon spectral indices (\gph) in the γ-ray band are above the observed values (Γ_{\rm ph, obs} \lesssim 2.6 for sources with redshifts 0.4\le z \le 0.6) if the magnetization of the sources is moderate (σ\simeq 10^{-2}).

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arXiv2013

Catching the Radio Flare in CTA 102 III. Core-Shift and Spectral Analysis

The temporal and spatial spectral evolution of the jets of AGN can be studied with multi-frequency, multi-epoch VLBI observations. The combination of both, morphological and spectral parameters can be used to derive source intrinsic physical properties such as the magnetic field and the non-thermal particle density. In the first two papers of this series, we analyzed the single-dish light curves and the VLBI kinematics of the blazar CTA 102 and suggested a shock-shock interaction between a traveling and a standing shock wave as a possible scenario to explain the observed evolution of the component associated to the 2006 flare. In this paper we investigate the core-shift and spectral evolution to test our hypothesis of a shock-shock interaction. We used 8 multi-frequency VLBA observations to analyze the temporal and spatial evolution of the spectral parameters during the flare. We observed CTA 102 between May 2005 and April 2007 using the VLBA at six different frequencies spanning from 2 - 86 GHz. After the calibrated VLBA images were corrected for opacity, we performed a detailed spectral analysis. From the derived values we estimated the magnetic field and the density of the relativistic particles. The detailed analysis of the opacity shift reveals that the position of the jet core is proportional to nu^-1 with some temporal variations. The value suggests possible equipartition between magnetic field energy and particle kinetic energy densities at the most compact regions. From the variation of the physical parameters we deduced that the 2006 flare in CTA 102 is connected to the ejection of a new traveling feature (t=2005.9) and the interaction between this shock wave and a stationary structure around 0.1 mas from the core. The source kinematics together with the spectral and structural variations can be described by helical motions in an over-pressured jet.

astro-ph.CO↗

arXiv2013

Core-shift and spectral analysis of the 2006 radio flare in CTA102

The 2006 radio flare in the blazar CTA102 is the largest ever reported for this source. The analysis of the single--dish light curves revealed a possible shock-shock interaction as a scenario for the 2006 outburst. In order to confirm this hypothesis we analyzed eight multi-frequency (2 GHz - 86 GHz) VLBI observations covering the 2006 flare. We detected a radio component ejected around 2005.9 and a stationary feature around 0.1 mas away from the core. We also performed a core-shift and spectral analysis on the fully calibrated VLBI maps. The frequency dependent position of the core slightly deviates from the typical $ν^{-1}$-dependence during the flaring state. This behaviour may reflect a mismatch between magnetic energy density and energy density of the relativistic particles. From the core-shift corrected maps we obtained the spectral parameters and computed the temporal and spatial evolution of the jet intrinsic parameters such as the magnetic field and the particle density. The overall picture of the source could be best interpreted as an over-pressured jet perturbed by traveling shock waves.

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arXiv2012

Catching the radio flare in CTA 102: II. VLBI kinematic analysis

We performed multifrequency multiepoch Very Long Baseline Array (VLBA) observations of the blazar CTA 102 during its 2006 radio flare, the strongest ever reported for this source. These observations provide an excellent opportunity to investigate the evolution of the physical properties of blazars, especially during these flaring events. We want to study the kinematic changes in the source during the strong radio outburst in April 2006 and test the assumption of a shock-shock interaction. This assumption is based on the analysis and modeling of the single-dish observations of CTA\,102 (Paper I). In this paper we study the kinematics of CTA 102 at several frequencies using VLBI observations. From the modeled jet features we derived estimates for the evolution of the physical parameters, such as the particle density and the magnetic field. Furthermore, we combined our observations during the 2006 flare with long-term VLBA monitoring of the source at 15 GHz and 43 GHz. We cross-identified seven features throughout our entire multifrequency observations and find evidence of two possible recollimation shocks around 0.1 mas (deprojected 18 pc at a viewing angle of 2.6 degrees) and 6.0 mas (deprojected 1 kpc) from the core. The 43 GHz observations reveal a feature ejected at epoch $t_\mathrm{ej}=2005.9\pm0.2$, which could be connected to the 2006 April radio flare. Furthermore, this feature might be associated with the traveling component involved in the possible shock-shock interaction, which gives rise to the observed double peak structure in the single-dish light curves reported in Paper I.

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arXiv2012

Radiative signature of magnetic fields in internal shocks

Common models of blazars and gamma-ray bursts assume that the plasma underlying the ob- served phenomenology is magnetized to some extent. Within this context, radiative signatures of dissipation of kinetic and conversion of magnetic energy in internal shocks of relativistic magnetized outflows are studied. We model internal shocks as being caused by collisions of homogeneous plasma shells. We compute the flow state after the shell interaction by solving Riemann problems at the contact surface between the colliding shells, and then compute the emission from the resulting shocks. Under the assumption of a constant flow luminosity we find that there is a clear difference between the models where both shells are weakly magne- tized (σ<\sim0.01) and those where, at least, one shell has a σ>\sim0.01. We obtain that the radiative efficiency is largest for models in which, regardless of the ordering, one shell is weakly and the other strongly magnetized. Substantial differences between weakly and strongly magne- tized shell collisions are observed in the inverse-Compton part of the spectrum, as well as in the optical, X-ray and 1GeV light curves. We propose a way to distinguish observationally between weakly magnetized from magnetized internal shocks by comparing the maximum frequency of the inverse-Compton and synchrotron part of the spectrum to the ratio of the inverse-Compton and synchrotron fluence. Finally, our results suggest that LBL blazars may correspond to barely magnetized flows, while HBL blazars could correspond to moderately magnetized ones. Indeed, by comparing with actual blazar observations we conclude that the magnetization of typical blazars is σ <\sim 0.01 for the internal shock model to be valid in these sources.

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arXiv2011

Efficiency of internal shocks in magnetized relativistic jets

We study the dynamic and radiative efficiency of conversion of kinetic-to-thermal/magnetic energy by internal shocks in relativistic magnetized outflows. A parameter study of a large number of collisions of cylindrical shells is performed. We explore how, while keeping the total flow luminosity constant, the variable fluid magnetization influences the efficiency and find that the interaction of shells in a mildly magnetized jet yields higher dynamic, but lower radiative efficiency than in a non-magnetized flow. A multi-wavelength radiative signature of different shell magnetization is computed assuming that relativistic particles are accelerated at internal shocks.

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arXiv2011

VLBI-selected sample of Compact Symmetric Object candidates and frequency-dependent position of hotspots

The Compact Symmetric Objects (CSOs) are small (<1 kiloparsec) and powerful extragalactic radio sources showing emission on both sides of an active galactic nucleus and no signs of strong relativistic beaming. They may be young radio sources, progenitors of large FRII radio galaxies. We aim to study the statistical properties of CSOs by constructing and investigating a new large sample of CSO candidates on the basis of dual-frequency, parsec-scale morphology. For the candidate selection we utilized VLBI data for 4170 extragalactic objects obtained simultaneously at 2.3 and 8.6 GHz (S and X band) within the VLBA Calibrator Survey 1-6 and the Research and Development - VLBA projects. Properties of their broad-band radio spectra were characterized by using RATAN-600 observations. Numerical modeling was applied in an attempt to explain the observed effects. A sample of 64 candidate CSOs is identified. The median two-point S-X band spectral index of parsec-scale hotspots is found to be -0.52; with the median brightness temperature ~10^9 K at X band. Statistical analysis reveals a systematic difference between positions of brightest CSO components (associated with hotspots) measured in the S and X bands. The distance between these components is found to be on average 0.32+/-0.06 mas greater at 8.6 GHz than at 2.3 GHz. This difference in distances cannot be explained by different resolutions at the S and X bands. It is a manifestation of spectral index gradients across CSO components, which may potentially provide important physical information about them. Despite our detailed numerical modeling of a CSO hotspot, the model was not able to reproduce the magnitude of the observed positional difference. A more detailed modeling may shed light on the origin of the effect.

astro-ph.CO↗

arXiv2011

An unusual stellar death on Christmas Day

Long Gamma-Ray Bursts (GRBs) are the most dramatic examples of massive stellar deaths, usually as- sociated with supernovae (Woosley et al. 2006). They release ultra-relativistic jets producing non-thermal emission through synchrotron radiation as they interact with the surrounding medium (Zhang et al. 2004). Here we report observations of the peculiar GRB 101225A (the "Christmas burst"). Its gamma-ray emission was exceptionally long and followed by a bright X-ray transient with a hot thermal component and an unusual optical counterpart. During the first 10 days, the optical emission evolved as an expanding, cooling blackbody after which an additional component, consistent with a faint supernova, emerged. We determine its distance to 1.6 Gpc by fitting the spectral-energy distribution and light curve of the optical emission with a GRB-supernova template. Deep optical observations may have revealed a faint, unresolved host galaxy. Our proposed progenitor is a helium star-neutron star merger that underwent a common envelope phase expelling its hydrogen envelope. The resulting explosion created a GRB-like jet which gets thermalized by interacting with the dense, previously ejected material and thus creating the observed black-body, until finally the emission from the supernova dominated. An alternative explanation is a minor body falling onto a neutron star in the Galaxy (Campana et al. 2011).

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arXiv2010

A Roe-type Riemann solver based on the spectral decomposition of the equations of Relativistic Magnetohydrodynamics

In a recent paper (Antón et al. 2010) we have derived sets of right and left eigenvectors of the Jacobians of the relativistic MHD equations, which are regular and span a complete basis in any physical state including degenerate ones. We present a summary of the main steps followed in the above derivation and the numerical experiments carried out with the linearized (Roe-type) Riemann solver we have developed, and some note on the (non-)convex character of the relativistic MHD equations.

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