arXiv subjects

G. E. Romero

Publications and source records attributed to G. E. Romero.

At least 55 records · Page 3Linked to original sources

arXiv2015

Spectral energy distribution, polarization, and synthetic radio maps of Cygnus X-1: a lepto-hadronic jet model

In this work we combine SEDs, radio maps and polarization observations to understand the emission mechanisms in Cygnus X-1. Our radiative model indicates that the MeV emission originates in the jet and that all the very high-energy emission is from hadronic origin. We also performed a synthetic radio map that suggests that our description of the magnetic field should be improved, since it leads to a very compact emission region. In order to choose the most suitable magnetic field geometry, we investigated the polarization in X-rays and we found that very simple geometries can explain the high levels of polarization reported by other authors.

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arXiv2014

Coronal origin of the polarization of the high-energy emission of Cygnus X-1

Cygnus X-1 is the candidate with the highest probability of containing a black hole among the X-ray binary systems in the Galaxy. It is also by far the most often studied of these objects. Recently, the International Gamma-Ray Astrophysics Laboratory Imager onboard then Integral satellite ({\it INTEGRAL}/IBIS) detected strong polarization in the high-energy radiation of this source, between 400 keV and 2 MeV. This radiation has been attributed to a jet launched by the black hole. We consider whether the corona around the black hole might be the site of production of the polarized emission instead of the jet. We studied self-consistently the injection of nonthermal particles in the hot, magnetized plasma around the black hole. We show that both the high-energy spectrum and polarization of Cygnus X-1 in the low-hard state can originate in the corona, without needing to invoke a jet. We estimate the degree of polarization in the intermediate state, where there is no jet, to provide a tool to test our model. Contrary to the commonly accepted view, the jet might not be the source of the MeV polarized tail in the spectrum of Cygnus X-1.

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arXiv2013

Multimessenger astronomy with gravitational waves and high-energy neutrinos

Many of the astrophysical sources and violent phenomena observed in our Universe are potential emitters of gravitational waves (GW) and high-energy neutrinos (HEN). Both GWs and HENs may escape very dense media and travel unaffected over cosmological distances, carrying information from the innermost regions of the astrophysical engines. Such messengers could also reveal new, hidden sources that have not been observed by conventional photon-based astronomy. Coincident observation of GWs and HENs may thus play a critical role in multimessenger astronomy. This is particularly true at the present time owing to the advent of a new generation of dedicated detectors: IceCube, ANTARES, VIRGO and LIGO. Given the complexity of the instruments, a successful joint analysis of this data set will be possible only if the expertise and knowledge of the data is shared between the two communities. This review aims at providing an overview of both theoretical and experimental state-of-the-art and perspectives for such a GW+HEN multimessenger astronomy.

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arXiv2012

Is the bowshock of the runaway massive star HD 195592 a Fermi source?

HD 195592 is an O-type super-giant star, known as a well-established runaway. Recently, a Fermi gamma-ray source (2FGL J2030.7+4417) with a position compatible with that of HD 195592 has been reported. Our goal is to explore the scenario where HD 195592 is the counterpart of the Fermi gamma-ray source modeling the non-thermal emission produced in the bowshock of the runaway star. We calculate the spectral energy distribution of the radiation produced in the bowshock of HD 195592 and we compare it with Fermi observations of 2FGL J2030.7+4417. We present relativistic particle losses and the resulting radiation of the bowshock of HD 195592 and show that the latter is compatible with the detected gamma-ray emission. We conclude that the Fermi source 2FGL J2030.7+4417 might be produced, under some energetic assumptions, by inverse Compton up-scattering of photons from the heated dust in the bowshock of the runaway star. HD 195592 might therefore be the very first object detected belonging to the category of gamma-ray emitting runaway massive stars, whose existence has been recently predicted.

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arXiv2012

Nonthermal processes and neutrino emission from the black hole GRO J0422+32 in a bursting state

GRO J0422+32 is a member of the class of low-mass X-ray binaries (LMXBs). It was discovered during an outburst in 1992. During the entire episode a persistent power-law spectral component extending up to $\sim 1$ MeV was observed, which suggests that nonthermal processes should have occurred in the system. We study relativistic particle interactions and the neutrino production in the corona of GRO J0422+32, and explain the behavior of GRO J0422+32 during its recorded flaring phase. We have developed a magnetized corona model to fit the spectrum of GRO J0422+32 during the low-hard state. We also estimate neutrino emission and study the detectability of neutrinos with 1 km$^3$ detectors, such as IceCube. The short duration of the flares ($\sim$ hours) and an energy cutoff around a few TeV in the neutrino spectrum make neutrino detection difficult. There are, however, many factors that can enhance neutrino emission. The northern-sky coverage and full duty cycle of IceCube make it possible to detect neutrino bursts from objects of this kind through time-dependent analysis.

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arXiv2012

AE Aurigae: first detection of non-thermal X-ray emission from a bow shock produced by a runaway star

Runaway stars produce shocks when passing through interstellar medium at supersonic velocities. Bow shocks have been detected in the mid-infrared for several high-mass runaway stars and in radio waves for one star. Theoretical models predict the production of high-energy photons by non-thermal radiative processes in a number sufficiently large to be detected in X-rays. To date, no stellar bow shock has been detected at such energies. We present the first detection of X-ray emission from a bow shock produced by a runaway star. The star is AE Aur, which was likely expelled from its birthplace by the encounter of two massive binary systems and now is passing through the dense nebula IC 405. The X-ray emission from the bow shock is detected at 30" to the northeast of the star, coinciding with an enhancement in the density of the nebula. From the analysis of the observed X-ray spectrum of the source and our theoretical emission model, we confirm that the X-ray emission is produced mainly by inverse Compton upscattering of infrared photons from dust in the shock front.

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arXiv2012

Dark matter and dark energy accretion onto intermediate-mass black holes

In this work we investigate the accretion of cosmological fluids onto an intermediate-mass black hole at the centre of a globular cluster, focusing on the influence of the parent stellar system on the accretion flow. We show that the accretion of cosmic background radiation and the so-called dark energy onto an intermediate-mass black hole is negligible. On the other hand, if cold dark matter has a nonvanishing pressure, the accretion of dark matter is large enough to increase the black hole mass well beyond the present observed upper limits. We conclude that either intermediate-mass black holes do not exist, or dark matter does not exist, or it is not strictly collisionless. In the latter case, we set a lower limit for the parameter of the cold dark matter equation of state.

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arXiv2011

Polarization and photometric observations of the gamma-ray blazar PG 1553+113

We present the results of an observational photo-polarimetry campaign of the blazar PG 1553+113 at optical wavelengths. The blazar was recently detected at very high energies (> 100 GeV) by the H.E.S.S and MAGIC gamma-ray Cherenkov telescopes. Our high-temporal resolution data show significant variations in the linear polarization percentage and position angle at inter-night time-scales, while at shorter (intra-night) time-scales both parameters varied less significantly, if at all. Changes in the polarization angle seem to be common in gamma-ray emitting blazars. Simultaneous differential photometry (through the B and R bands) shows no significant variability in the total optical flux. We provide B and R magnitudes, along with a finding chart, for a set of field stars suitable for differential photometry.

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arXiv2011

A lepto-hadronic model for high-energy emission from FR I radiogalaxies

The well know radiogalaxy Cen A has been recently detected as a source of very high energy (VHE) gamma-rays by the HESS experiment just before Fermi/LAT detected it at high energies (HE). The detection, together with that of M87, establishes radiogalaxies as VHE gamma-ray emitters. The aim of this work is to present a lepto-hadronic model for the VHE emission from the relativistic jets in FR I radiogalaxies. We consider that protons and electrons are accelerated in a compact region near the base of the jet, and they cool emitting multi wavelength radiation as they propagate along the jet. The proton and electron distributions are obtained through an inhomogeneous steady-state transport equation taking into account acceleration, radiative and non-radiative cooling processes, as well as particle transport by convection. Considering the effects of photon absorption at different wavelengths, we calculate the radiation emitted by the primary protons and electrons, as well as the contribution of secondaries particles (e+/-, pions and muons). The expected high-energy neutrino signal is also obtained and the possibility of detections with KM3Net and IceCube is discussed. The spectral energy distribution obtained in our model with an appropriate set of parameters for an extended emission zone can account for much of the observed spectrum for both AGNs.

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arXiv2010

XMM-Newton detection of the supernova remnant G304.6+0.1 (Kes 17)

Aims. We report the first detailed X-ray study of the supernova remnant (SNR) G304.6+0.1, achieved with the XMM-Newton mission. Methods. The powerful imaging capability of XMM-Newton was used to study the X-ray characteristics of the remnant at different energy ranges. The X-ray morphology and spectral properties were analyzed. In addittion, radio and mid-infrared data obtained with the Molonglo Observatory Synthesis Telescope and the Spitzer Space Telescope were used to study the association with the detected X-ray emission and to understand the structure of the SNR at differents wavelengths. Results. The SNR shows an extended and arc-like internal structure in the X-ray band with out a compact point-like source inside the remnant. We find a high column density of NH in the range 2.5-3.5x1022 cm-2, which supports a relatively distant location (d $\geq$ 9.7 kpc). The X-ray spectrum exhibits at least three emission lines, indicating that the X-ray emission has a thin thermal plasma origin, although a non-thermal contribution cannot be discarded. The spectra of three different regions (north, center and south) are well represented by a combination of a non-equilibrium ionization (PSHOCK) and a power-law (PL) model. The mid-infrared observations show a bright filamentary structure along the north-south direction coincident with the NW radio shell. This suggests that Kes 17 is propagating in a non-uniform environment with high density and that the shock front is interacting with several adjacent massive molecular clouds. The good correspondence of radio and mid-infrared emissions suggests that the filamentary features are caused by shock compression. The X-ray characteristics and well-known radio parameters indicate that G304.6+0.1 is a middle-aged SNR (2.8-6.4)x104 yr old and a new member of the recently proposed group of mixed-morphology SNRs.

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arXiv2010

An X-ray study of the SNR G344.7-0.1 and the central object CXOU J170357.8-414302

Aims. We report results of an X-ray study of the supernova remnant (SNR) G344.7-0.1 and the point-like X-ray source located at the geometrical center of the SNR radio structure. Methods. The morphology and spectral properties of the remnant and the central X-ray point-like source were studied using data from the XMM-Newton and Chandra satellites. Archival radio data and infrared Spitzer observations at 8 and 24 $μ$m were used to compare and study its multi-band properties at different wavelengths. Results. The XMM-Newton and Chandra observations reveal that the overall X-ray emission of G344.7-0.1 is extended and correlates very well with regions of bright radio and infrared emission. The X-ray spectrum is dominated by prominent atomic emission lines. These characteristics suggest that the X-ray emission originated in a thin thermal plasma, whose radiation is represented well by a plane-parallel shock plasma model (PSHOCK). Our study favors the scenario in which G344.7-0.1 is a 6 x 10^3 year old SNR expanding in a medium with a high density gradient and is most likely encountering a molecular cloud on the western side. In addition, we report the discovery of a soft point-like X-ray source located at the geometrical center of the radio SNR structure. The object presents some characteristics of the so-called compact central objects (CCO). However, its neutral hydrogen absorption column (N_{H}) is inconsistent with that of the SNR. Coincident with the position of the source, we found infrared and optical objects with typical early-K star characteristics. The X-ray source may be a foreground star or the CCO associated with the SNR. If this latter possibility were confirmed, the point-like source would be the farthest CCO detected so far and the eighth member of the new population of isolated and weakly magnetized neutron stars.

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arXiv2010

Radio and IR study of the massive star-forming region IRAS 16353-4636

Context. With the latest infrared surveys, the number of massive protostellar candidates has increased significantly. New studies have posed additional questions on important issues about the formation, evolution, and other phenomena related to them. Complementary to infrared data, radio observations are a good tool to study the nature of these objects, and to diagnose the formation stage. Aims. Here we study the far-infrared source IRAS 16353-4636 with the aim of understanding its nature and origin. In particular, we search for young stellar objects (YSOs), possible outflow structure, and the presence of non-thermal emission. Methods. Using high-resolution, multi-wavelength radio continuum data obtained with the Australia Telescope Compact Array, we image IRAS 16353-4636 and its environment from 1.4 to 19.6 GHz, and derive the distribution of the spectral index at maximum angular resolution. We also present new JHKs photometry and spectroscopy data obtained at ESO NTT. 13 CO and archival HI line data, and infrared databases (MSX, GLIMPSE, MIPSGal) are also inspected. Results. The radio continuum emission associated with IRAS 16353-4636 was found to be extended (~10 arcsec), with a bow-shaped morphology above 4.8 GHz, and a strong peak persistent at all frequencies. The NIR photometry led us to identify ten near-IR sources and classify them according to their color. We used the HI line data to derive the source distance, and analyzed the kinematical information from the CO and NIR lines detected. Conclusions. We have identified the source IRAS 16353-4636 as a new protostellar cluster. In this cluster we recognized three distinct sources: a low-mass YSO, a high-mass YSOs, and a mildly confined region of intense and non-thermal radio emission. We propose the latter corresponds to the terminal part of an outflow.

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arXiv2010

A systematic cross-search for radio/infrared counterparts of XMM-Newton sources

We present a catalog of cross-correlated radio, infrared and X-ray sources using a very restrictive selection criteria with an IDL-based code developed by us. The significance of the observed coincidences was evaluated through Monte Carlo simulations of synthetic sources following a well-tested protocol. We found 3320 coincident radio/X-ray sources with a high statistical significance characterized by the sum of error-weighted coordinate differences. For 997 of them, 2MASS counterparts were found. The percentage of chance coincidences is less than 1%. X-ray hardness ratios of well-known populations of objects were used to provide a crude representation of their X-ray spectrum and to make a preliminary diagnosis of the possible nature of unidentified X-ray sources. The results support the fact that the X-ray sky is largely dominated by Active Galactic Nuclei at high galactic latitudes (|b| >= 10^\circ). At low galactic latitudes (|b| <= 10^\circ) most of unidentified X-ray sources (~94%) lie at |b| <= 2^\circ. This result suggests that most of the unidentified sources found toward the Milky Way plane are galactic objects. Well-known and unidentified sources were classified in different tables with their corresponding radio/infrared and X-ray properties. These tables are intended as a useful tool for researchers interested in particular identifications.

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arXiv2010

Non-thermal processes around accreting galactic black holes

Accreting black holes in galactic X-ray sources are surrounded by hot plasma. The innermost part of these systems is likely a corona with different temperatures for ions and electrons. In the so-called low-hard state, hot electrons Comptonize soft X-ray photons from the disk that partially penetrates the corona, producing emission up to $\sim 150$ keV, well beyond the expectations for an optically thick disk of maximum temperature $\sim 10^{7}$ K. However, sources such as Cygnus X-1 produce steady emission up to a few MeV, which is indicative of a non-thermal contribution to the spectral energy distribution. We study the radiative output produced by the injection of non-thermal (both electron and proton) particles in a magnetized corona around a black hole. Energy losses and maximum energies are estimated for all types of particles in a variety of models, characterized by different kinds of advection and relativistic proton content. Transport equations are solved for primary and secondary particles, and spectral energy distributions are determined and corrected by internal absorption. We show that a local injection of non-thermal particles can account for the high energy excess observed in some sources, and we predict the existence of a high-energy bump at energies above 1 TeV, and typical luminosities of $\sim 10^{33}$ erg s$^{-1}$. High-energy instruments such as the future Cherenkov Telescope Array (CTA) can be used to probe the relativistic particle content of the coronae around galactic black holes.

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arXiv2010

Non-thermal radiation from Cygnus X-1 corona

Cygnus X-1 was the first X-ray source widely accepted to be a black hole candidate and remains among the most studied astronomical objects in its class. The detection of non-thermal radio, hard X-rays and gamma rays reveals the fact that this kind of objects are capable of accelerating particles up to very high energies. In order to explain the electromagnetic emission from Cygnus X-1 in the low-hard state we present a model of a black hole corona with both relativistic lepton and hadron content. We characterize the corona as a two-temperature hot plasma plus a mixed non-thermal population in which energetic particles interact with magnetic, photon and matter fields. Our calculations include the radiation emitted by secondary particles (pions, muons and electron/positron pairs). Finally, we take into account the effects of photon absorption. We compare the results obtained from our model with data of Cygnus X-1 obtained by the COMPTEL instrument.

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arXiv2009

Massive protostars as gamma-ray sources

Massive protostars have associated bipolar outflows with velocities of hundreds of km s$^{-1}$. Such outflows can produce strong shocks when interact with the ambient medium leading to regions of non-thermal radio emission. We aim at exploring under which conditions relativistic particles are accelerated at the terminal shocks of the protostellar jets and can produce significant gamma-ray emission. We estimate the conditions necessary for particle acceleration up to very high energies and gamma-ray production in the non-thermal hot spots of jets associated with massive protostars embedded in dense molecular clouds. We show that relativistic Bremsstrahlung and proton-proton collisions can make molecular clouds with massive young stellar objects detectable by the {\it Fermi}{} satellite at MeV-GeV energies and by Cherenkov telescope arrays in the GeV-TeV range. Gamma-ray astronomy can be used to probe the physical conditions in star forming regions and particle acceleration processes in the complex environment of massive molecular clouds.

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arXiv2009

Radio continuum and near-infrared study of the MGRO J2019+37 region

(abridged) MGRO J2019+37 is an unidentified extended source of VHE gamma-rays originally reported by the Milagro Collaboration as the brightest TeV source in the Cygnus region. Its extended emission could be powered by either a single or several sources. The GeV pulsar AGL J2020.5+3653, discovered by AGILE and associated with PSR J2021+3651, could contribute to the emission from MGRO J2019+37, although extrapolation of the GeV spectrum does not explain the detected multi-TeV flux. Our aim is to identify radio and NIR sources in the field of the extended TeV source MGRO J2019+37, and study potential counterparts that could contribute to its emission. We surveyed a region of about 6 square degrees with the Giant Metrewave Radio Telescope (GMRT) at the frequency 610 MHz. We also observed the central square degree of this survey in the NIR Ks-band using the 3.5 m telescope in Calar Alto. Archival X-ray observations of some specific fields are included. VLBI observations of an interesting radio source were performed. We explored possible scenarios to produce the multi-TeV emission from MGRO J2019+37 and studied which of the sources could be the main particle accelerator. We present a catalogue of 362 radio sources detected with the GMRT in the field of MGRO J2019+37, and the results of a cross-correlation of this catalog with one obtained at NIR wavelengths, as well as with available X-ray observations of the region. Some peculiar sources inside the ~1 degree uncertainty region of the TeV emission from MGRO J2019+37 are discussed in detail, including the pulsar PSR J2021+3651 and its pulsar wind nebula PWN G75.2+0.1, two new radio-jet sources, the HII region Sh 2-104 containing two star clusters, and the radio source NVSS J202032+363158.

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arXiv2009

A synchrotron self-Compton model for the VHE gamma-ray emission from Cen A

The H.E.S.S. experiment has reported the detection of very high energy (VHE: E>100 GeV) gamma-ray emission from the well-known radio-galaxy Cen A. Following this discovery, we refine the previous multi-blob synchrotron self-Compton (SSC) model applied to the multi-wavelength emission from this source using the new H.E.S.S. data. The prediction of the VHE gamma-ray level of emission for Cen A presented in Lenain et al. paper agrees very well in the view of this recent data. Further VHE observations of Cen A might detect variability, which would comfort our inner jet modeling. The next generation of imaging atmospheric Cherenkov telescopes, will help to reveal possible multiple sources of VHE emission in the complex structure of Cen A.

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