arXiv subjects

Fulvio Melia

Publications and source records attributed to Fulvio Melia.

At least 145 records · Page 8Linked to original sources

arXiv2007

A Possible Link Between the Galactic Center HESS Source and Sgr A*

Recently, HESS and other air Cerenkov telescopes have detected a source of TeV gamma-rays coincident with the Galactic center. It is not yet clear whether the gamma-rays are produced via leptonic or hadronic processes, so it is important to consider possible acceleration sites for the charged particles which produce the gamma-rays. One exciting possibility for the origin of these particles is the central black hole, Sgr A*, where the turbulent magnetic fields close to the event horizon can accelerate protons to TeV energies. Using a realistic model of the density distribution in a 6 pc x 6 pc x 6pc cube at the Galactic center, we here calculate the trajectories followed by these TeV protons as they gyrate through the turbulent medium surrounding Sgr A*. Diffusing out from the black hole, the protons produce TeV gamma-rays via pi^0 decay following a collision with a proton in the surrounding medium. After following over 222,000 such trajectories, we find that the circumnuclear ring around Sgr A* can reproduce the observed 0.1-100 TeV HESS spectrum and flux if the protons are injected into this medium with an effective power-law index of 0.75, significantly harder than the observed photon index of 2.25. The total energy in the steady-state 1-40 TeV proton population surrounding Sgr A* is inferred to be approx 5x10^{45} ergs. Only 31% of the emitted 1-100 TeV protons encounter the circumnuclear torus, leaving a large flux of protons that diffuse outward to contribute to the Galactic ridge emission observed by HESS on scales of >~ 1 degree.

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arXiv2006

MHD Simulations of Accretion onto Sgr A*: Quiescent Fluctuations, Outbursts, and Quasi-Periodicity

High resolution observations of Sgr A* have revealed a wide variety of phenomena, ranging from intense rapid flares to quasi-periodic oscillations, making this object an ideal system to study the properties of low luminosity accreting black holes. In this paper, we use a pseudo-spectral algorithm to construct and evolve a three-dimensional magnetohydrodynamic model of the accretion disk in Sgr A*. Assuming a hybrid thermal-nonthermal emission scheme, we show that the MHD turbulence can by itself only produce factor of two fluctuations in luminosity. These amplitudes in variation cannot explain the magnitude of flares observed in this system. However, we also demonstrate that density perturbations in the disk do produce outbursts qualitatively similar to those observed by XMM-Newton in X-rays and ground-based facilities in the near infrared. Quasi-periodic oscillations emerge naturally in the simulated lightcurves. We attribute these to non-axisymmetric density perturbations that emerge as the disk evolves back toward its quiescent state.

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arXiv2006

Stochastic Acceleration in the Galactic Center HESS Source

Stochastic acceleration of electrons interacting resonantly with a turbulent magnetic field in a small accretion torus appears to be the likely mechanism responsible for much of Sagittarius A*'s millimeter and shorter wavelength spectrum. The longer wavelength radiation is produced at larger radii by electrons either diffusing from smaller scales or accelerated in situ. An important prediction of this model is the ejection of a significant flux of relativistic protons from a magnetic-field-dominated acceleration site into the wind-shocked medium surrounding the black hole. Recently, several air Cerenkov telescopes, notably HESS, have detected TeV emission from the Galactic center, with characteristics hinting at a p-p-induced pion decay process for the γ-ray emission. Given (1) the size of this acceleration region measured in the radio band and (2) the wind-injected ISM mapped with Chandra using the diffuse X-rays, it is feasible to test the idea that protons accelerated within \~20 Schwarzschild radii of the black hole produce the TeV emission farther out. We show a fraction of TeV protons scattering about once within ~3 pc of Sagittarius A* and the proton power (~10^37 erg s^-1) produced in concert with the 7 mm radio emission matches the TeV luminosity well. This model explains why the TeV source does not vary on a timescale of a year or less. The particle cascade generated by the p-p scatterings also produces bremsstrahlung, inverse Compton, and synchrotron emission at longer wavelengths from secondary particles. We compare these with current measurements and demonstrate that GLAST will detect this source during its one-year all-sky survey.

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arXiv2006

A Testable the Stochastic Acceleration Model for Flares in Sagittarius A*

The near-IR and X-ray flares in Sagittarius A* are believed to be produced by relativistic electrons via synchrotron and synchrotron self-Comptonization, respectively. These electrons are likely energized by turbulent plasma waves through second-order Fermi acceleration that, in combination with the radiative cooling processes, produces a relativistic Maxwellian distribution in the steady state. This model has four principal parameters, namely the magnetic field B, the electron density n and temperature γ_c mec2, and the size of the flare region R. In the context of stochastic acceleration, the quantities Rn^1/2B and γ_cRn should remain nearly constant in time. Therefore, simultaneous spectroscopic observations in the NIR and X-ray bands can readily test the model, which, if proven to be valid, may be used to determine the evolution of the plasma properties during an eruptive event with spectroscopic observations in either band or simultaneous flux density measurements in both bands. The formulae can be applied to other isolated or confined systems, where electrons are accelerated to relativistic energies by plasma wave turbulence and produce most of the emission via synchrotron processes.

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arXiv2006

Periodic Modulations in an X-ray Flare from Sagittarius A*

We present the highly significant detection of a quasi-periodic flux modulation with a period of 22.2 min seen in the X-ray data of the Sgr A* flare of 2004 August 31. This flaring event, which lasted a total of about three hours, was detected simultaneously by EPIC on XMM-Newton and the NICMOS near-infrared camera on the HST. Given the inherent difficulty in, and the lack of readily available methods for quantifying the probability of a periodic signal detected over only several cycles in a data set where red noise can be important, we developed a general method for quantifying the likelihood that such a modulation is indeed intrinsic to the source and does not arise from background fluctuations. We here describe this Monte Carlo based method, and discuss the results obtained by its application to a other XMM-Newton data sets. Under the simplest hypothesis that we witnessed a transient event that evolved, peaked and decayed near the marginally stable orbit of the supermassive black hole, this result implies that for a mass of 3.5 x 10^{6} Msun, the central object must have an angular momentum corresponding to a spin parameter of a=0.22.

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arXiv2005

A Possible Rossby Wave Instability Origin for the Flares in Sagittarius A*

In recent years, near-IR and X-ray flares have been detected from the Galaxy's central radio point source, Sagittarius A* (Sgr A*), believed to be a \~3.10^6 solar masses supermassive black hole. In some cases, the transient emission appears to be modulated with a (quasi-)periodic oscillation (QPO) of ~ 17-20 minutes. The implied ~ 3 r_S size of the emitter (where r_S = 2GM/c^2 is the Schwarzschild radius) points to an instability - possibly induced by accretion - near the Marginally Stable Orbit (MSO) of a slowly spinning object. But Sgr A* is not accreting via a large, `standard' disk; instead, the low density environment surrounding it apparently feeds the black hole with low angular momentum clumps of plasma that circularize within ~ 10-300 r_S and merge onto a compact, hot disk. In this Letter, we follow the evolution of the disk following such an event, and show that a Rossby wave instability, particularly in its magnetohydrodynamic (MHD) form, grows rapidly and produces a period of enhanced accretion lasting several hours. Both the amplitude of this response, and its duration, match the observed flare characteristics rather well.

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arXiv2005

Repeated X-ray Flaring Activity in Sagittarius A*

Investigating the spectral and temporal characteristics of the X-rays coming from Sagittarius A* (Sgr A*) is essential to our development of a more complete understanding of the emission mechanisms in this supermassive black hole located at the center of our Galaxy. Several X-ray flares with varying durations and spectral features have already been observed from this object. Here we present the results of two long XMM-Newton observations of the Galactic nucleus carried out in 2004, for a total exposure time of nearly 500 ks. During these observations we detected two flares from Sgr A* with peak 2-10 keV luminosities about 40 times (L ~ 9x10^34 erg s−1) above the quiescent luminosity: one on 2004 March 31 and another on 2004 August 31. The first flare lasted about 2.5 ks and the second about 5 ks. The combined fit on the Epic spectra yield photon indeces of about 1.5 and 1.9 for the first and second flare respectively. This hard photon index strongly suggests the presence of an important population of non-thermal electrons during the event and supports the view that the majority of flaring events tend to be hard and not very luminous.

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arXiv2005

A Persistent High-Energy Flux from the Heart of the Milky Way : Integral's view of the Galactic Center

The Ibis/Isgri imager on Integral detected for the first time a hard X-ray source, IGR J17456-2901, located within 1' of Sgr A* over the energy range 20-100 keV. Here we present the results of a detailed analysis of ~7 Ms of Integral observations of the GC. With an effective exposure of 4.7 Ms we have obtained more stringent positional constraints on this HE source and constructed its spectrum in the range 20-400 keV. Furthermore, by combining the Isgri spectrum with the total X-ray spectrum corresponding to the same physical region around SgrA* from XMM data, and collected during part of the Integral observations, we constructed and present the first accurate wide band HE spectrum for the central arcmins of the Galaxy. Our complete analysis of the emission properties of IGR shows that it is faint but persistent with no variability above 3 sigma contrary to what was alluded to in our first paper. This result, in conjunction with the spectral characteristics of the X-ray emission from this region, suggests that the source is most likely not point-like but, rather, that it is a compact, yet diffuse, non-thermal emission region. The centroid of IGR is estimated to be R.A.=17h45m42.5, decl.=-28deg59'28'', offset by 1' from the radio position of Sgr A* and with a positional uncertainty of 1'. Its 20-400 keV luminosity at 8 kpc is L=5.4x10^35 erg/sec. Very recently, Hess detected of a source of ~TeV g-rays also located within 1' of Sgr A*. We present arguments in favor of an interpretation according to which the photons detected by Integral and Hess arise from the same compact region of diffuse emission near the central BH and that the supernova remnant Sgr A East could play an important role as a contributor of very HE g-rays to the overall spectrum from this region.

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arXiv2005

Spin-Induced Disk Precession in Sagittarius A*

In Sgr A* at the Galactic center, by far the closest and easiest supermassive black hole we can study, the observational evidence is increasingly pointing to the presence of a compact, hot, magnetized disk feeding the accretor. In such low-Mach-number plasmas, forces arising, e.g., from pressure gradients in the plasma, can altogether negate the warping of disks around Kerr black holes caused by the Bardeen-Petterson effect and can lead to coherent precession of the entire disk. In this Letter, we present for the first time highly detailed 3D SPH simulations of the accretion disk evolution in Sgr A*, guided by observational constraints on its physical characteristics, and conclude that indeed the Bardeen-Petterson effect is probably absent in this source. Given what we now understand regarding the emission geometry in this object, we suggest that a ~ 50-500-day modulation in Sgr A*'s spectrum, arising from the disk precession, could be an important observational signature; perhaps the ~ 106-day period seen earlier in its radio flux, if confirmed, could be due to this process. On the other hand, if future observations do not confirm this long modulation in Sgr A*'s spectrum, this would be an indication that either the disk size or orientation is very different from current estimates, or that the black hole is not spinning at all (unlikely), or that our current understanding of how it produces its radiative output is incorrect.

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arXiv2005

Long-Term Radio Modulation in Sagittarius A* from Spin-Induced Disk Precession

There is some evidence, though yet unconfirmed, that Sagittarius A*--the supermassive black hole at the Galactic center--emits its radio waves modulated with a ~100-day period. What is intriguing about this apparent quasi-periodicity is that, though the amplitude of the modulation increases with decreasing wavelength (from 3.6 to 1.3 cm), the quasi-period itself does not seem to depend on the frequency of the radiation. It is difficult to imagine how a binary companion, were that the cause of this modulation, could have escaped detection until now. Instead, it has been suggested that the spin-induced precession of a disk surrounding a slowly rotating black hole could have the right period to account for this behavior. In this paper, we examine how Sagittarius A*'s light curve could be modulated by this mechanism. We demonstrate that the partial occultation of a nonthermal halo by a compact, radio-opaque disk does indeed produce the observed frequency-dependent amplitude. This appears to be in line with other observational arguments suggesting that Sagittarius A*'s mm/sub-mm spectrum is produced by a ~10 Schwarzschild-radius disk, whereas its cm-waves originate from a nonthermal particle distribution in a halo extending out to over 20 Schwarzschild radii. Interestingly, this model suggests that the observed period corresponds to half the precession period and that a non-axisymmetric disk could produce a second period roughly twice as long as the first.

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arXiv2005

The X-ray Ridge Surrounding Sgr A* at the Galactic Center

We present the first detailed simulation of the interaction between the supernova explosion that produced Sgr A East and the wind-swept inner ~ 2-pc region at the Galactic center. The passage of the supernova ejecta through this medium produces an X-ray ridge ~ 9'' to 15'' to the NE of the supermassive black hole Sagittarius A* (Sgr A*). We show that the morphology and X-ray intensity of this feature match very well with recently obtained Chandra images, and we infer a supernova remnant age of less than 2,000 years. This young age--a factor 3--4 lower than previous estimates--arises from our inclusion of stellar wind effects in the initial (pre-explosion) conditions in the medium. The supernova does not clear out the central ~ 0.2-pc region around Sgr~A* and does not significantly alter the accretion rate onto the central black hole upon passage through the Galactic center.

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arXiv2005

Stochastic Electron Acceleration During the NIR and X-ray Flares in Sagittarius A*

Recent near-IR (NIR) and X-ray observations of Sagittarius A*'s spectrum have yielded several strong constraints on the transient energization mechanism, justifying a re-examination of the stochastic acceleration model proposed previously for these events. We here demonstrate that the new results are fully consistent with the acceleration of electrons via the transit-time damping process. But more importantly, these new NIR and X-ray flares now can constrain the source size, the gas density, the magnetic field, and the wave energy density in the turbulent plasma. Future simultaneous multi-wavelength observations with good spectral information will, in addition, allow us to study their temporal evolution, which will eventually lead to an accurate determination of the behavior of the plasma just minutes prior to its absorption by the black hole.

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arXiv2005

Transrelativistic Synchrotron Emissivity, Cross-Section, and Polarization

The spectrum and polarization produced by particles spiraling in a magnetic field undergo dramatic changes as the emitters transition from nonrelativistic to relativistic energies. However, none of the currently available methods for calculating the characteristics of this radiation field are adequate for the purpose of sustaining accuracy and speed of computation in the intensity, and none even attempt to provide a means of determining the polarization fraction other than in the cyclotron or synchrotron limits. But the transrelativistic regime, which we here find to lie between $5\times 10^7$ K and $5\times 10^9$ K for a thermal plasma, is becoming increasingly important in high-energy astrophysical environments, such as in the intra-cluster medium, and in the accretion flows of supermassive black holes. In this paper, we present simple, yet highly accurate, fitting formulae for the magnetobremsstrahlung (also known as cyclo-synchrotron) emissivity, its polarization fraction (and content), and the absorption cross-section. We demonstrate that both the harmonic and high-energy limiting behavior are well represented, incurring at most an error of $\sim 5%$ throughout the transition region.

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arXiv2005

Covariant Kinetic Theory with an Application to the Coma Cluster

In this paper, we introduce a novel solution to the covariant Landau equation for a pure electron plasma. The method conserves energy and particle number, and reduces smoothly to the Rosenbluth potentials of non-relativistic theory. In addition, we find that a fully relativistic plasma equilibrates in only 1/100th of a Spitzer time--much faster than in the non-relativistic limit--a factor of significant import to situations in which distortions to a Maxwellian distribution are produced by anomalous methods of acceleration. To demonstrate the power of our solution in dealing with hot, astrophysical plasmas, we use this technique to show that one of the currently considered models--continuous stochastic acceleration--for the hard X-ray emission in the Coma cluster actually cannot work because the energy gained by the particles is distributed to the {\it whole} plasma on a time scale much shorter than that of the acceleration process itself.

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arXiv2005

Neutrinos from the Galactic Center in the Light of its Gamma-Ray Detection at TeV Energy

We re-evaluate the event rate expected in km^3-scale detectors for neutrinos from the direction of the Galactic Center (GC) in light of recent spectral measurements obtained by the HESS instrument for ~TeV gamma-radiation from this direction. In the most plausible scenario the re-evaluated event rate is smaller than that previously calculated--and here re-calculated--on the basis of EGRET data. However, the GC TeV gamma-ray detections by the Whipple, CANGAROO, and HESS instruments, together with the strong indications for an overabundance of cosmic rays coming from the GC at EeV energies, strengthen the expectation for a detectable, TeV-PeV GC neutrino signal from proton-proton interactions in that region. If the TeV gamma-ray--EeV cosmic ray anisotropy connection is correct, this signal will be detectable within a year and half for km^3-scale neutrino detectors in the Northern Hemisphere at super-TeV energies and, significantly, should also be detectable in 1.6 years by the South Polar IceCube detector at energies > 10^14 eV. The GC neutrino signal should also produce a detectable signal from neutrino showering and resonant W^- production by anti-electron-neutrinos in the volume of a km^3-scale detector.

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arXiv2005

Diffuse X-rays from the Arches and Quintuplet clusters

The origin and initial mass function of young stellar clusters near the Galactic center are still poorly understood. Two of the more prominent ones, the Arches and Quintuplet clusters, may have formed from a shock-induced burst of star formation, given their similar age and proximity to each other. Their unusual mass distribution, however, may be evidence of a contributing role played by other factors, such as stellar capture from regions outside the clusters themselves. Diffuse X-ray emission from these sources provides us with a valuable, albeit indirect, measure of the stellar mass-loss rate from their constituents. Using recent data acquired with Chandra, we can study the nature and properties of the outflow to not only probe the pertinent physical conditions, such as high metallicity, the magnetic field, and so forth, but also to better constrain the stellar distribution within the clusters, in order to identify their formative history. In this paper, we present a set of three-dimensional smoothed particle hydrodynamics simulations of the wind-wind interactions in both the Arches and Quintuplet clusters. We are guided primarily by the currently known properties of the constituent stars, though we vary the mass-loss rates in order to ascertain the dependence of the measured X-ray flux on the assumed stellar characteristics. Our results are compared with the latest observations of the Arches cluster. Our analysis of the Quintuplet cluster may be used as a basis for comparison with future X-ray observations of this source.

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arXiv2004

Primary Versus Secondary Leptons in the EGRET SNR's

The EGRET supernova remnants (SNR's) are all expanding into nearby dense molecular clouds, powering a shock at the interface where protons and electrons accelerate to relativistic energies. A viable mechanism for the emission of gamma$-rays in these sources is the decay of neutral pions created in collisions between the relativistic hadrons and protons in the ambient medium. But neutral pion decay alone cannot reproduce the whole high-energy spectrum, particularly below 100 MeV. A pion-decay scenario thus requires a lepton population to "fill in" the lower part of the spectrum via bremsstrahlung emission. This population, however, is constrained by the SNR radio spectrum. Taking our cue from the behavior of Sgr A East, an energetic EGRET SNR at the Galactic center, we here examine the role played in these sources by secondary leptons--electrons and positrons produced in proton-proton scattering events and the ensuing particle cascades. We show that while secondary leptons cannot account for the gamma-rays below 100 MeV, they can account for the hard radio spectra observed from the EGRET SNR's. Thus, it appears that both primary and secondary leptons may be important contributors to the overall broadband emission from these sources, but if so, must radiate most of their energy in different parts of the SNR-cloud environment. We show that shock acceleration in dense cores being overtaken by the expanding SNR shell can naturally lead to such a scenario.

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arXiv2004

The AGASA/SUGAR Anisotropies and TeV Gamma Rays from the Galactic Center: A Possible Signature of Extremely High-energy Neutrons

Recent analysis of data sets from two extensive air shower cosmic ray detectors shows tantalizing evidence of an anisotropic overabundance of cosmic rays towards the Galactic Center (GC) that ``turns on'' around $10^{18}$ eV. We demonstrate that the anisotropy could be due to neutrons created at the Galactic Center through charge-exchange in proton-proton collisions, where the incident, high energy protons obey an $\sim E^{-2}$ power law associated with acceleration at a strong shock. We show that the normalization supplied by the gamma-ray signal from EGRET GC source 3EG J1746-2851 -- ascribed to pp induced neutral pion decay at GeV energies -- together with a very reasonable spectral index of 2.2, predicts a neutron flux at $\sim 10^{18}$ eV fully consistent with the extremely high energy cosmic ray data. Likewise, the normalization supplied by the very recent GC data from the HESS air-Cerenkov telescope at TeV energies is almost equally-well compatible with the $\sim 10^{18}$ eV cosmic ray data. Interestingly, however, the EGRET and HESS data appear to be themselves incompatible. We consider the implications of this discrepancy. We discuss why the Galactic Center environment can allow diffusive shock acceleration at strong shocks up to energies approaching the ankle in the cosmic ray spectrum. Finally, we argue that the shock acceleration may be occuring in the shell of Sagittarius A East, an unusual supernova remnant located very close to the Galactic Center. If this connection between the anisotropy and Sagittarius A East could be firmly established it would be the first direct evidence for a particular Galactic source of cosmic rays up to energies near the ankle.

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