arXiv subjects

Maria Petropoulou

Publications and source records attributed to Maria Petropoulou.

At least 73 records · Page 4Linked to original sources

arXiv2019

On the connection of radio and $γ$-ray emission in blazars

Blazars are a sub-category of radio-loud active galactic nuclei with relativistic jets pointing towards to the observer. They are well-known for their non-thermal variable emission, which practically extends over the whole electromagnetic spectrum. Despite the plethora of multi-wavelength observations, the issue about the origin of the $γ$-ray and radio emission in blazar jets remains unsettled. Here, we construct a parametric leptonic model for studying the connection between the $γ$-ray and radio emission in both steady-state and flaring states of blazars. Assuming that relativistic electrons are injected continuously at a fixed distance from the black hole, we numerically study the evolution of their population as it propagates to larger distances while losing energy due to expansion and radiative cooling. In this framework, $γ$-ray photons are naturally produced at small distances (e.g. $10^{-3}$ pc) when the electrons are still very energetic, whereas the radio emission is produced at larger distances (e.g. $1$ pc), after the electrons have cooled and the emitting region has become optically thin to synchrotron self-absorption due to expansion. We present preliminary results of our numerical investigation for the steady-state jet emission and the predicted time lags between $γ$-rays and radio during flares.

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arXiv2018

A lesson from GW170817: most neutron star mergers result in tightly collimated successful GRB jets

The joint detection of gravitational waves (GWs) and $γ$-rays from a binary neutron-star (NS) merger provided a unique view of off-axis GRBs and an independent measurement of the NS merger rate. Comparing the observations of GRB170817 with those of the regular population of short GRBs (sGRBs), we show that an order unity fraction of NS mergers result in sGRB jets that breakout of the surrounding ejecta. We argue that the luminosity function of sGRBs, peaking at $\approx 2\times 10^{52}\, \mbox{erg s}^{-1}$, is likely an intrinsic property of the sGRB central engine and that sGRB jets are typically narrow with opening angles $θ_0 \approx 0.1$. We perform Monte Carlo simulations to examine models for the structure and efficiency of the prompt emission in off axis sGRBs. We find that only a small fraction ($\sim 0.01-0.1$) of NS mergers detectable by LIGO/VIRGO in GWs is expected to be also detected in prompt $γ$-rays and that GW170817-like events are very rare. For a NS merger rate of $\sim 1500$ Gpc$^{-3}$ yr$^{-1}$, as inferred from GW170817, we expect within the next decade up to $\sim 12$ joint detections with off-axis GRBs for structured jet models and just $\sim 1$ for quasi-spherical cocoon models where $γ$-rays are the result of shock breakout. Given several joint detections and the rates of their discoveries, the different structure models can be distinguished. In addition the existence of a cocoon with a reservoir of thermal energy may be observed directly in the UV, given a sufficiently rapid localisation of the GW source.

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arXiv2018

Blazar Flares as an Origin of High-Energy Cosmic Neutrinos?

We consider implications of high-energy neutrino emission from blazar flares, including the recent event IceCube-170922A and the 2014-2015 neutrino flare that could originate from TXS 0506+056. First, we discuss their contribution to the diffuse neutrino intensity taking into account various observational constraints. Blazars are likely to be subdominant in the diffuse neutrino intensity at sub-PeV energies, and we show that blazar flares like those of TXS 0506+056 could make <1-10 percent of the total neutrino intensity. We also argue that the neutrino output of blazars can be dominated by the flares in the standard leptonic scenario for their gamma-ray emission, and energetic flares may still be detected with a rate of <1 per year. Second, we consider multi-messenger constraints on the source modeling. We show that luminous neutrino flares should be accompanied by luminous broadband cascade emission, emerging also in X-rays and gamma-rays. This implies that not only gamma-ray telescopes like Fermi but also X-ray sky monitors such as Swift and MAXI are critical to test the canonical picture based on the single-zone modeling. We also suggest a two-zone model that can naturally satisfy the X-ray constraints while explaining the flaring neutrinos via either photomeson or hadronuclear processes.

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arXiv2018

The Steady Growth of the High-Energy Spectral Cutoff in Relativistic Magnetic Reconnection

Magnetic reconnection is invoked as an efficient particle accelerator in a variety of astrophysical sources of non-thermal high-energy radiation. With large-scale two-dimensional particle-in-cell simulations of relativistic reconnection (i.e., with magnetization $σ\gg1$) in pair plasmas, we study the long-term evolution of the power-law slope and high-energy cutoff of the spectrum of accelerated particles. We find that the high-energy spectral cutoff does not saturate at $γ_{\rm cut}\sim 4σ$, as claimed by earlier studies, but it steadily grows with time as long as the reconnection process stays active. At late times, the cutoff scales approximately as $γ_{\rm cut}\propto \sqrt{t}$, regardless of the flow magnetization and initial temperature. We show that the particles dominating the high-energy spectral cutoff reside in plasmoids, and in particular in a strongly magnetised ring around the plasmoid core. The growth of their energy is driven by the increase in the local field strength, coupled with the conservation of the first adiabatic invariant. We also find that the power-law slope of the spectrum ($p=-{\rm d}\log N/{\rm d}\log γ$) evolves with time. For $σ\gtrsim10$, the spectrum is hard at early times ($p\lesssim 2$), but it tends to asymptote to $p\sim 2$; the steepening of the power-law slope allows the spectral cutoff to extend to higher and higher energies, without violating the fixed energy budget of the system. Our results demonstrate that relativistic reconnection is a viable candidate for accelerating the high-energy particles emitting in relativistic astrophysical sources.

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arXiv2018

Extreme scattering events from axisymmetric plasma lenses

Frequency-dependent brightness fluctuations of radio sources, the so-called extreme scattering events (ESEs), have been observed over the last three decades. They are caused by Galactic plasma structures whose geometry and origin are still poorly understood. In this paper, we construct axisymmentric two-dimensional (2D) column density profiles for the plasma lens and explore the resulting ESEs for both point-like and extended sources. A quantity that becomes relevant is the impact parameter $b$, namely the distance of the observer's path from the lens symmetry axis. We demonstrate its effects on the shape of ESE light curves and use it for a phenomenological classification of ESEs into four main types. Three of them are unique outcomes of the 2D model and do not show a characteristic U-shaped dip in the light curve, which has been traditionally used as an identification means of ESEs. We apply our model to five well-studied ESEs and show that elongated plasma tubes or quasi-spherical clouds are favoured over plasma sheets for four of them, while the remaining one is compatible with both lens geometries.

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arXiv2018

Detection of an optical/UV jet/counterjet and Multiple Spectral Components in M84

We report an optical/UV jet and counterjet in M84, previously unreported in archival HST imaging. With archival VLA, ALMA, and Chandra imaging, we examine the first well-sampled spectral energy distribution of the inner jet of M84, where we find that multiple co-spatial spectral components are required. In particular, the ALMA data reveal that the radio spectrum of all four knots in the jet turns over at approximately 100 GHz, which requires a second component for the bright optical/UV emission. Further, the optical/UV has a soft spectrum and is inconsistent with the relatively flat X-ray spectrum, which indicates a third component at higher energies. Using archival VLA imaging, we have measured the proper motion of the innermost knots at 0.9+/-0.6 and 1.1+/-0.4 c, which when combined with the low jet-to-counterjet flux ratio yields an orientation angle for the system of 74 (+9,-18) degrees. In the radio, we find high fractional polarization of the inner jet of up to 30% while in the optical no polarization is detected (< 8%). We investigate different scenarios for explaining the particular multi-component SED of the knots. Inverse Compton models are ruled out due to the extreme departure from equipartition and the unrealistically high total jet power required. The multi-component SED can be naturally explained within a leptohadronic scenario, but at the cost of very high power in relativistic protons. A two-component synchrotron model remains a viable explanation, but more theoretical work is needed to explain the origin and properties of the electron populations.

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arXiv2018

Patterns of variability in supercritical hadronic systems

A unique and often overlooked property of a source loaded with relativistic protons is that it can become supercritical, i.e. it can undergo an abrupt transition from a radiatively inefficient to a radiatively efficient state once its proton energy density exceeds a certain threshold. In this paper, we investigate the temporal variability of hadronic systems in this hardly explored regime. We show that there exists a range of proton densities that prevent the system from reaching a steady state, but drive it instead in a quasi-periodic mode. The escaping radiation then exhibits limit cycles, even if all physical parameters are held constant in time. We extend our analysis to cases where the proton injection rate varies with time and explore the variability patterns of escaping radiation as the system moves in and out from the supercritical regime. We examine the relevance of our results to the variability of the prompt gamma-ray burst emission and show that, at least on a phenomenological level, some interesting analogies exist.

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arXiv2017

Collapsar Gamma-Ray Bursts: how the luminosity function dictates the duration distribution

Jets in long-duration $γ$-ray bursts (GRBs) have to drill through the collapsing star in order to break out of it and produce the $γ$-ray signal while the central engine is still active. If the breakout time is shorter for more powerful engines, then the jet-collapsar interaction acts as a filter of less luminous jets. We show that the observed broken power-law GRB luminosity function is a natural outcome of this process. For a theoretically motivated breakout time that scales with jet luminosity as $L^{-χ}$ with $χ\sim 1/3-1/2$, we show that the shape of the $γ$-ray duration distribution can be uniquely determined by the GRB luminosity function and matches the observed one. This analysis has also interesting implications about the supernova-central engine connection. We show that not only successful jets can deposit sufficient energy in the stellar envelope to power the GRB-associated supernovae, but also failed jets may operate in all Type Ib/c supernovae.

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arXiv2017

Point-source and diffuse high-energy neutrino emission from Type IIn supernovae

Type IIn supernovae (SNe), a rare subclass of core collapse SNe, explode in dense circumstellar media that have been modified by the SNe progenitors at their last evolutionary stages. The interaction of the freely expanding SN ejecta with the circumstellar medium gives rise to a shock wave propagating in the dense SN environment, which may accelerate protons to multi-PeV energies. Inelastic proton-proton collisions between the shock-accelerated protons and those of the circumstellar medium lead to multi-messenger signatures. Here, we evaluate the possible neutrino signal of type IIn SNe and compare with IceCube observations. We employ a Monte Carlo method for the calculation of the diffuse neutrino emission from the SN IIn class to account for the spread in their properties. The cumulative neutrino emission is found to be ~ 10 per cent of the observed IceCube neutrino flux above 60 TeV. Type IIn SNe would be the dominant component of the diffuse astrophysical flux, only if 4 per cent of all core collapse SNe were of this type and 20 to 30 per cent of the shock energy was channeled to accelerated protons. Lower values of the acceleration efficiency are accessible by the observation of a single type IIn SN as a neutrino point source with IceCube using up-going muon neutrinos. Such an identification is possible in the first year following the SN shock breakout for sources within 20 Mpc.

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arXiv2017

Radio Emission from Sgr A*: Pulsar Transits Through the Accretion Disc

Radiatively inefficient accretion flow models have been shown to accurately account for the spectrum and luminosity observed from Sgr A* in the X-ray regime down to mm wavelengths. However, observations at a few GHz cannot be explained by thermal electrons alone but require the presence of an additional non-thermal particle population. Here, we propose a model for the origin of such a population in the accretion flow via means of a pulsar orbiting the supermassive black hole in our Galaxy. Interactions between the relativistic pulsar wind with the disc lead to the formation of a bow shock in the wind. During the pulsar's transit through the accretion disc, relativistic pairs, accelerated at the shock front, are injected into the disc. The radio-emitting particles are long-lived and remain within the disc long after the pulsar's transit. Periodic pulsar transits through the disc result in regular injection episodes of non-thermal particles. We show that for a pulsar with spin-down luminosity $L_{\rm sd} \sim 3\times 10^{35}$ erg s$^{-1}$ and a wind Lorentz factor of $γ_{\rm w} \sim 10^4$ a quasi-steady synchrotron emission is established with luminosities in the $1-10$ GHz range comparable to the observed one.

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arXiv2016

The TeV emission of Ap Librae: a hadronic interpretation and prospects for CTA

Ap Librae is one out of a handful of low-frequency peaked blazars to be detected at TeV $γ$-rays and the only one with an identified X-ray jet. Combined observations of Fermi-LAT at high energies (HE) and of H.E.S.S at very high energies (VHE) revealed a striking spectral property of Ap Librae; the presence of a broad high-energy component that extends more than nine orders of magnitude in energy and is, therefore, hard to be explained by the usual single-zone synchrotron self-Compton model. We show that the superposition of different emission components related to photohadronic interactions can explain the $γ$-ray emission of Ap Librae without invoking external radiation fields. We present two indicative model fits to the spectral energy distribution of Ap Librae where the VHE emission is assumed to originate from a compact, sub-pc scale region of the jet. A robust prediction of our model is VHE flux variability on timescales similar to those observed at X-rays and HE $γ$-rays, which can be further used to distinguish between a sub-pc or kpc scale origin of the TeV emission. We thus calculate the expected variability signatures at X-rays, HE and VHE $γ$-rays and show that quasi-simultaneous flares are expected, with larger amplitude flares appearing at $γ$-rays. We assess the detectability of VHE variability from Ap Librae with CTA, next generation of IACTs. We show that $\sim$hr timescale variability at $E_γ>0.1$ TeV could be detectable at high significance with shorter exposure times than current Cherenkov telescopes.

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arXiv2016

Early-time signatures of γ-ray emission from supernovae in dense circumstellar media

We present our results on the γ-ray emission from interaction-powered supernovae (SNe), a recently discovered SN type that is suggested to be surrounded by a circumstellar medium (CSM) with densities 10^7-10^12~ cm^-3. Such high densities favor inelastic collisions between relativistic protons accelerated in the SN blast wave and CSM protons and the production of γ-ray photons through neutral pion decays. Using a numerical code that includes synchrotron radiation, adiabatic losses due to the expansion of the source, photon-photon interactions, proton-proton collisions and proton-photon interactions, we calculate the multi-wavelength non-thermal photon emission soon after the shock breakout and follow its temporal evolution until 100-1000 days. Focusing on the γ-ray emission at >100 MeV, we show that this could be detectable by the Fermi-LAT telescope for nearby (<10 Mpc) SNe with dense CSM (>10^11 cm^-3).

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arXiv2016

Blazar flares powered by plasmoids in relativistic reconnection

Powerful flares from blazars with short ($\sim$ min) variability timescales are challenging for current models of blazar emission. Here, we present a physically motivated ab initio model for blazar flares based on the results of recent particle-in-cell (PIC) simulations of relativistic magnetic reconnection. PIC simulations demonstrate that quasi-spherical plasmoids filled with high-energy particles and magnetic fields are a self-consistent by-product of the reconnection process. By coupling our PIC-based results (i.e., plasmoid growth, acceleration profile, particle and magnetic content) with a kinetic equation for the evolution of the electron distribution function we demonstrate that relativistic reconnection in blazar jets can produce powerful flares whose temporal and spectral properties are consistent with the observations. In particular, our model predicts correlated synchrotron and synchrotron self-Compton flares of duration of several hours--days powered by the largest and slowest moving plasmoids that form in the reconnection layer. Smaller and faster plasmoids produce flares of sub-hour duration with higher peak luminosities than those powered by the largest plasmoids. Yet, the observed fluence in both types of flares is similar. Multiple flares with a range of flux-doubling timescales (minutes to several hours) observed over a longer period of flaring activity (days or longer) may be used as a probe of the reconnection layer's orientation and the jet's magnetization. Our model shows that blazar flares are naturally expected as a result of magnetic reconnection in a magnetically-dominated jet.

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arXiv2016

Time-dependent neutrino emission from Mrk 421 during flares and predictions for IceCube

Blazars are prime candidate sources for the high energy neutrinos recently detected by IceCube. Being one of the brightest sources in the extragalactic X-ray and $γ$-ray sky as well as one of the nearest blazars to Earth, Mrk 421 is an excellent source for testing the scenario of the blazar-neutrino connection. Here, we model the spectral energy distribution of Mrk 421 during a 13-day flare in 2010 with unprecedented multi-wavelength coverage, and calculate the respective neutrino flux. We find a correlation between the $>1$ PeV neutrino and photon fluxes, in all energy bands. Using typical IceCube through-going muon event samples with good angular resolution and high statistics, we derive the mean event rate above 100 TeV ($\sim0.57$ evt/yr) and show that it is comparable to that expected from a four-month quiescent period in 2009. Due to the short duration of the flare, an accumulation of similar flares over several years would be necessary to produce a meaningful signal for IceCube. To better assess this, we apply the correlation between the neutrino and $γ$-ray fluxes to the 6.9 yr Fermi-LAT light curve of Mrk 421. We find that the mean event count above 1 PeV for the full IceCube detector livetime is $3.59\pm0.60$ ($2.73\pm0.38$) $ν_μ+\barν_μ$ with (without) major flares included in our analysis. This estimate exceeds, within the uncertainties, the $95\%$ ($90\%$) threshold value for the detection of one or more muon (anti-)neutrinos. Meanwhile, the most conservative scenario, where no correlation of $γ$-rays and neutrinos is assumed, predicts $1.60\pm0.16$ $ν_μ+\barν_μ$ events. We conclude that a non-detection of high-energy neutrinos by IceCube would probe the neutrino/$γ$-ray flux correlation during major flares or/and the hadronic contribution to the blazar emission.

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arXiv2016

Radio synchrotron emission from secondary electrons in interaction-powered supernovae

Several supernovae (SNe) with an unusually dense circumstellar medium (CSM) have been recently observed at radio frequencies. Their radio emission is powered by relativistic electrons that can be either accelerated at the SN shock (primaries) or injected as a by-product (secondaries) of inelastic proton-proton collisions. We investigate the radio signatures from secondary electrons, by detailing a semi-analytical model to calculate the temporal evolution of the distributions of protons, primary and secondary electrons. With our formalism, we track the cooling history of all the particles that have been injected into the emission region up to a given time, and calculate the resulting radio spectra and light curves. For a SN shock propagating through the progenitor wind, we find that secondary electrons control the early radio signatures, but their contribution decays faster than that of primary electrons. This results in a flattening of the light curve at a given radio frequency that depends only upon the radial profiles of the CSM density and of the shock velocity, $v_0$. The relevant transition time at the peak frequency is $\sim 190 \, K_{\rm ep,-3}^{-1} A_{\rm w, 16} β_{0, -1.5}^{-2}\,{\rm d}$, where $A_{\rm w}$ is the wind mass-loading parameter, $β_0=v_0/c$ and $K_{\rm ep}$ is the electron-to-proton ratio of accelerated particles. We explicitly show that late peak times at 5 GHz (i.e., $t_{\rm pk} \gtrsim 300-1000$ d) suggest a shock wave propagating in a dense wind ($A_{\rm w} \gtrsim 10^{16}-10^{17}$ gr cm$^{-1}$), where secondary electrons are likely to power the observed peak emission.

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arXiv2016

The many faces of blazar emission in the context of hadronic models

We present two ways of modeling the spectral energy distribution of blazars in the hadronic context and discuss the predictions of each "hadronic variant" on the spectral shape, the multi-wavelength variability, the cosmic-ray flux, and the high-energy neutrino emission. Focusing on the latter, we then present an application of the hadronic model to individual BL Lacs that were recently suggested to be the counterparts of some of the IceCube neutrinos, and conclude by discussing the contribution of the whole BL Lac class to the observed neutrino background.

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arXiv2015

The X-ray dust scattered rings of the black hole low mass binary V404 Cyg

We report on the first detection of X-ray dust scattered rings from the Galactic low mass X-ray binary V404 Cyg. The observation of the system with Swift/XRT on June 30 2015 revealed the presence of five concentric ring-like structures centred at the position of V404 Cyg. Follow-up Swift/XRT observations allowed a time-dependent study of the X-ray rings. Assuming that these are the result of small-angle, single X-ray scattering by dust grains along the line of sight, we find that their angular size scales as $θ\propto\sqrt{t}$ in agreement with theoretical predictions. The dust grains are concentrated in five dust layers located at about 2.12, 2.05, 1.63, 1.50 and 1.18 kpc from the observer. These coincide roughly with locations of enhanced extinction as determined by infrared photometry. Assuming that the grain size distribution is described by a generalized Mathis-Rumpl-Nordsieck model, we find that the power-law index of the most distant cloud is $q\sim 4.4$, while $q \sim 3.5-3.7$ in all other clouds. We constrain at a $3σ$ level the maximum grain size of the intermediate dust layers in the range $0.16-0.20\,μ$m and set a lower limit of $\sim 0.2\,μ$m in the other clouds. Hints of an exponential cutoff at the angular intensity profile of the outermost X-ray ring suggest that the smallest grains have sizes $0.01 μ{\rm m}\le α_{\min} \lesssim 0.03\,μ$m. Based on the relative ratios of dust column densities we find the highest dust concentration at $\sim 1.6$ kpc. Our results indicate a gradient in the dust properties within 1 kpc from V404 Cyg.

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arXiv2015

Spectral signatures of compact sources in the inverse Compton catastrophe limit

The inverse Compton catastrophe is defined as a dramatic rise in the luminosity of inverse Compton scattered photons. It is described by a non-linear loop of radiative processes that sets in for high values of the electron compactness and is responsible for the efficient transfer of energy from electrons to photons, predominantly through inverse Compton scatterings. We search for the conditions that drive a magnetized non-thermal source to the inverse Compton catastrophe regime and study its multi-wavelength (MW) photon spectrum. We develop a generic analytical framework and use numerical calculations as a backup to the analytical predictions. We find that the escaping radiation from a source in the Compton catastrophe regime bears some unique features. The MW photon spectrum is a broken power law with a break at $\sim m_e c^2$ due to the onset of the Klein-Nishina suppression. The spectral index below the break energy depends on the electron and magnetic compactnesses logarithmically, while it is independent of the electron power-law index ($s$). The maximum radiating power emerges typically in the $γ$-ray regime, at energies $\sim m_e c^2$ ($\sim γ_{\max} m_e c^2$ ) for $s>2$ ($s\lesssim 2$), where $γ_{\max}$ is the maximum Lorentz factor of the injected electron distribution. We apply the principles of the inverse Compton catastrophe to blazars and $γ$-ray bursts using the analytical framework we developed, and show how these can be used to impose robust constraints on the source parameters.

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