arXiv subjects

Emma de Ona Wilhelmi

Publications and source records attributed to Emma de Ona Wilhelmi.

15 recordsLinked to original sources

arXiv2022

The Crab nebula variability at short timescales with the Cherenkov Telescope Array

Since 2009, several rapid and bright flares have been observed at high energies (>100 MeV) from the direction of the Crab Nebula. Several hypotheses have been put forward to explain this phenomenon, but the origin is still unclear. The detection of counterparts at higher energies with the next generation of Cherenkov telescopes will be determinant to constrain the underlying emission mechanisms. We aim at studying the capability of the Cherenkov Telescope Array (CTA) to explore the physics behind the flares, by performing simulations of the Crab Nebula spectral energy distribution, both in flaring and steady state, for different parameters related to the physical conditions in the nebula. In particular, we explore the data recorded by Fermi during two particular flares that occurred in 2011 and 2013. The expected GeV and TeV gamma-ray emission is derived using different radiation models. The resulting emission is convoluted with the CTA response and tested for detection, obtaining an exclusion region for the space of parameters that rule the different flare emission models. Our simulations show different scenarios that may be favourable for achieving the detection of the flares in Crab with CTA, in different regimes of energy. In particular, we find that observations with low sub-100 GeV energy threshold telescopes could provide the most model-constraining results.

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arXiv2022

The Transients Handler System for the Cherenkov Telescope Array Observatory

The Cherenkov Telescope Array Observatory (CTAO) will be the largest and most advanced ground-based facility for gamma-ray astronomy. Several dozens of telescopes will be operated at both the Northern and Southern Hemisphere. With the advent of multi-messenger astronomy, many new large science infrastructures will start science operations and target-of-opportunity observations will play an important role in the operation of the CTAO. The Array Control and Data Acquisition (ACADA) system deployed on each CTAO site will feature a dedicated sub-system to manage external and internal scientific alerts: the Transients Handler. It will receive, validate, and process science alerts in order to determine if target-of-opportunity observations can be triggered or need to be updated. Various tasks defined by proposal-based configurations are processed by the Transients Handler. These tasks include, among others, the evaluation of observability of targets and their correlation with known sources or objects. This contribution will discuss the concepts and design of the Transients Handler and its integration in the ACADA system.

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arXiv2021

Probing the hadronic nature of the gamma-ray emission associated with Westerlund 2

Star-forming regions have been proposed as potential Galactic cosmic-ray accelerators for decades. Cosmic ray acceleration can be probed through observations of gamma-rays produced in inelastic proton-proton collisions, at GeV and TeV energies. We analyze more than 11 years of Fermi-LAT data from the direction of Westerlund 2, one of the most massive and best-studied star-forming regions in our Galaxy. The spectral and morphological characteristics of the LAT source agree with the ones in the TeV regime (HESS J1023-575), allowing the description of the gamma-ray source from a few hundreds of MeV to a few tens of TeVs. We will present the results and discuss the implications of the identification with the stellar cluster and the radiation mechanism involved.

astro-ph.HE↗

arXiv2021

Probing the hadronic nature of the gamma-ray emission associated with Westerlund 2

Star-forming regions have been proposed as potential Galactic cosmic-ray accelerators for decades. Cosmic-ray acceleration can be probed through observations of gamma-rays produced in inelastic proton-proton collisions, at GeV and TeV energies. In this paper, we analyze more than 11 years of Fermi-LAT data from the direction of Westerlund 2, one of the most massive and best-studied star-forming regions in our Galaxy. In particular, we investigate the characteristics of the bright pulsar PSR J1023-5746 that dominates the gamma-ray emission below a few GeV at the position of Westerlund 2, and the underlying extended source FGES J1023.3-5747. The analysis results in a clear identification of FGES J1023.3-5747 as the GeV counterpart of the TeV source HESS J1023-575, through its morphological and spectral properties. This identification provides new clues about the origin of the HESS J1023-575 gamma-ray emission, favouring a hadronic origin of the emission, powered by Westerlund 2, rather than a leptonic origin related to either the pulsar wind nebula associated with PSR J1023-5746 or the cluster itself. This result indirectly supports the hypothesis that star-forming regions can contribute to the cosmic-ray sea observed in our Galaxy

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arXiv2021

Investigating the nature of MGRO J1908+06 with multiwavelength observations

The unidentified TeV source MGRO J1908+06, with emission extending from hundreds of GeV to beyond 100TeV, is one of the most intriguing sources in the Galactic plane. MGRO J1908+06 spatially associates with an IceCube hotspot of neutrino emission. Although the hotspot is not significant yet, this suggests a possible hadronic origin of the observed gamma-ray radiation. Here we describe a multiwavelength analysis on MGRO J1908+06 to determine its nature. We identify, for the first time, an extended GeV source as the counterpart of MGRO J1908+06, discovering possibly associated molecular clouds (MCs). The GeV spectrum shows two well-differentiated components: a soft spectral component below $\sim10$ GeV, and a hard one ($Γ\sim1.6$) above these energies. The lower-energy part is likely associated with the dense MCs surrounding the supernova remnant SNR G40.5$-$0.5, whereas the higher-energy component, which connects smoothly with the spectrum observed in TeV range, resembles the inverse Compton emission observed in relic pulsar wind nebulae. This simple scenario seems to describe the data satisfactorily, but raises questions about the interpretation of the emission at hundreds of TeV. In this scenario, no detectable neutrino flux would be expected.

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arXiv2020

Gamma-ray heartbeat powered by the microquasar SS 433

Microquasars, the local siblings of extragalactic quasars, are binary systems comprising a compact object and a companion star. By accreting matter from their companions, microquasars launch powerful winds and jets, influencing the interstellar environment around them. Steady gamma-ray emission is expected to rise from their central objects, or from interactions between their outflows and the surrounding medium. The latter prediction was recently confirmed with the detection of SS 433 at high (TeV) energies. In this report, we analyze more than ten years of GeV gamma-ray data from the Fermi Gamma-ray Space Telescope on this source. Detailed scrutiny of the data reveal emission in the SS 433 vicinity, co-spatial with a gas enhancement, and hints for emission possibly associated with a terminal lobe of one of the jets. Both gamma-ray excesses are relatively far from the central binary, and the former shows evidence for a periodic variation at the precessional period of SS 433, linking it with the microquasar. This result challenges obvious interpretations and is unexpected from any previously published theoretical models. It provides us with a chance to unveil the particle transport from SS 433 and to probe the structure of the local magnetic field in its vicinity.

astro-ph.HE↗

arXiv2020

SNR G39.2-0.3, an Hadronic Cosmic Rays Accelerator

Recent results obtained with gamma-ray satellites have established supernova remnants as accelerators of GeV hadronic cosmic rays. In such processes, CRs accelerated in SNR shocks interact with particles from gas clouds in their surrounding. In particular, the rich medium in which core-collapse SNRs explode provides a large target density to boost hadronic gamma-rays. SNR G39.2-0.3 is one of the brightest SNR in infrared wavelengths, and its broad multiwavelength coverage allows detailed modelling of its radiation from radio to high energies. We reanalyzed the Fermi-LAT data on this region and compare it with new radio observations from the MWISP survey. The modelling of the spectral energy distribution from radio to GeV energies favors a hadronic origin of the gamma-ray emission and constrains the SNR magnetic field to be at least ~100 uG. Despite the large magnetic field, the present acceleration of protons seems to be limited to ~10 GeV, which points to a drastic slow down of the shock velocity due to the dense wall traced by the CO observations, surrounding the remnant. Further investigation of the gamma-ray spectral shape points to a dynamically old remnant subjected to severe escape of CRs and a decrease of acceleration efficiency. The low-energy peak of the gamma-ray spectrum also suggests that that the composition of accelerated particles might be enriched by heavy nuclei which is certainly expected for a core-collapse SNR. Alternatively, the contribution of the compressed pre-existing Galactic cosmic rays is discussed, which is, however, found to not likely be the dominant process for gamma-ray production.

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arXiv2018

Observing and modeling the gamma-ray emission from pulsar/pulsar wind nebula complex PSR J0205+6449/3C 58

We present the results of the analysis of 8 years of Fermi-LAT data of the pulsar/pulsar wind nebula complex PSR J0205+6449/3C 58. Using a contemporaneous ephemeris, we carried out a detailed analysis of PSR J0205+6449 both during its off-peak and on-peak phase intervals. 3C 58 is significantly detected during the off-peak phase interval. We show that the spectral energy distribution at high energies is the same disregarding the phases considered, and thus that this part of the spectrum is most likely dominated by the nebula radiation. We present results of theoretical models of the nebula and the magnetospheric emission that confirm this interpretation. Possible high-energy flares from 3C 58 were searched for, but none was unambiguously identified.

astro-ph.HE↗

arXiv2017

NuSTAR Hard X-ray Observation of the Gamma-ray Binary Candidate HESS J1832-093

We present a hard X-ray observation of the TeV gamma-ray binary candidate HESS J1832-093 coincident with supernova remnant (SNR) G22.7-0.2 using the Nuclear Spectroscopic Telescope Array (NuSTAR). Non-thermal X-ray emission from XMMU J183245-0921539, the X-ray source associated with HESS J1832-093, is detected up to ~30 keV and is well-described by an absorbed power-law model with the best-fit photon index $Γ= 1.5\pm0.1$. A re-analysis of archival Chandra and XMM-Newton data finds that the long-term X-ray flux increase of XMMU J183245-0921539 is $50^{+40}_{-20}$% (90% C.L.), much less than previously reported. A search for a pulsar spin period or binary orbit modulation yields no significant signal to a pulse fraction limit of fp < 19% in the range 4 ms < P < 40 ks. No red noise is detected in the FFT power spectrum to suggest active accretion from a binary system. While further evidence is required, we argue that the X-ray and gamma-ray properties of XMMU J183245-0921539 are most consistent with a non-accreting binary generating synchrotron X- rays from particle acceleration in the shock formed as a result of the pulsar and stellar wind collision. We also report on three nearby hard X-ray sources, one of which may be associated with diffuse emission from a fast-moving supernova fragment interacting with a dense molecular cloud.

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arXiv2017

GeV detection of HESS J0632+057

HESS J0632+057 is the only gamma-ray binary that has been detected at TeV energies, but not at GeV energies yet. Based on nearly nine years of Fermi Large Area Telescope (LAT) Pass 8 data, we report here on a deep search for the gamma-ray emission from HESS J0632+057 in the 0.1-300 GeV energy range. We find a previously unknown gamma-ray source, Fermi J0632.6+0548, spatially coincident with HESS J0632+057. The measured flux of Fermi J0632.6+0548 is consistent with the previous flux upper limit on HESS J0632+057 and shows variability that can be related to the HESS J0632+057 orbital phase. We propose that Fermi J0632.6+0548 is the GeV counterpart of HESS J0632+057. Considering the Very High Energy (VHE) spectrum of HESS J0632+057, a possible spectral turnover above 10 GeV may exist in Fermi J0632.6+0548, as appears to be common in other established gamma-ray binaries.

astro-ph.HE↗

arXiv2016

Search for transitions between states in redbacks and black widows using seven years of Fermi-LAT observations

Considering about seven years of Fermi-Large Area Telescope (LAT) data, we present a systematic search for variability possibly related to transitions between states in redbacks and black widow systems. Transitions are characterized by sudden and significant changes in the gamma-ray flux that persist on a timescale much larger than the orbital period. This phenomenology was already detected in the case of two redback systems, PSR J1023+0038 and PSR J1227-4853, for which we present here a dedicated study. We show the existence of only one transition for each of these systems over the past seven years. We determine their spectra, establishing high-energy cutoffs at a few GeV for the high gamma-ray state of PSR J1023+0038 and for both states of PSR J1227-4853. The surveying capability of the Fermi-LAT allows studying whether similar phenomenology has occurred in other sources. Although we have not found any hint for a state transition for most of the studied pulsars, we note two black-widow systems, PSR J2234+0944 and PSR J1446-4701, whose apparent variability is reminiscent of the transitions in PSR J1023+0038 and PSR J1227-4853. For the other systems we set limits on potential transitions in their measured gamma-ray light curves.

astro-ph.HE↗

arXiv2016

Search for gamma-ray emission from AE Aquarii with seven years of FERMI-LAT observations

AE Aquarii (AE Aqr) is a cataclysmic binary hosting one of the fastest rotating (P$_{\rm spin}$ = 33.08 s) white dwarfs known. Based on seven years of Fermi Large Area Telescope (LAT) Pass 8 data, we report on a deep search for gamma-ray emission from AE Aqr. Using X-ray observations from ASCA, XMM-Newton, Chandra, Swift, Suzaku, and NuSTAR, spanning 20 years, we substantially extend and improve the spin ephemeris of AE Aqr. Using this ephemeris, we searched for gamma-ray pulsations at the spin period of the white dwarf. No gamma-ray pulsations were detected above 3 $σ$ significance. Neither phase-averaged gamma-ray emission nor gamma-ray variability of AE Aquarii is detected by Fermi-LAT. We impose the most restrictive upper limit to the gamma-ray flux from AE Aqr to date: $1.3\times 10^{-12}$ erg cm$^{-2}$ s$^{-1}$ in the 100 MeV-300 GeV energy range, providing constraints on models.

astro-ph.HE↗

arXiv2016

Gamma-ray emission from PSR J0007+7303 using 7 years of Fermi Large Area Telescope observations

Based on more than seven years of Fermi Large Area Telescope (LAT) Pass 8 data, we report on a detailed analysis of the bright gamma-ray pulsar (PSR) J0007+7303. We confirm that PSR J0007+7303 is significantly detected as a point source also during the off-peak phases with a TS value of 262 ($\sim$ 16 $σ$). In the description of PSR J0007+7303 off-peak spectrum, a power law with an exponential cutoff at 2.7$\pm$1.2$\pm$1.3 GeV (the first/second uncertainties correspond to statistical/systematic errors) is preferred over a single power law at a level of 3.5 $σ$. The possible existence of a cutoff hints at a magnetospheric origin of the emission. In addition, no extended gamma-ray emission is detected compatible with either the supernova remnant (CTA 1) or the very high energy (> 100 GeV) pulsar wind nebula. A flux upper limit of 6.5$\times$10$^{-12}$ erg cm$^{-2}$ s$^{-1}$ in the 10-300 GeV energy range is reported, for an extended source assuming the morphology of the VERITAS detection. During on-peak phases, a sub-exponential cutoff is significantly preferred ($\sim$11 $σ$) for representing the spectral energy distribution, both in the phase-averaged and in the phase-resolved spectra. Three glitches are detected during the observation period and we found no flux variability at the time of the glitches or in the long-term behavior. We also report the discovery of a previously unknown gamma-ray source in the vicinity of PSR J0007+7303, Fermi J0020+7328, which we associate with the z = 1.781 quasar S5 0016+73. A concurrent analysis of this source is needed to correctly characterize the behavior of CTA 1 and it is also presented in the paper.

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arXiv2015

The high-energy gamma-ray detection of G73.9+0.9, a supernova remnant interacting with a molecular cloud

We have analysed the Fermi LAT data on the SNR G73.9+0.9. We have confirmed a previous detection of high-energy gamma-rays from this source at a high significance of $\simeq 12σ$. The observed spectrum shows a significant curvature, peaking in $E F_E$ at $\sim$1 GeV. We have also calculated the flux upper limits in the mm-wavelength and X-ray ranges from Planck and XMM-Newton, respectively. We have inspected the intensity of the CO (1$\rightarrow $0) emission line and found a large peak at a velocity range corresponding to the previously estimated source distance of $\sim$4 kpc, which may indicate an association between a molecular cloud and the SNR. The gamma-ray emission appears due to interaction of accelerated particles within the SNR with the matter of the cloud. The most likely radiative process responsible for the gamma-ray emission is decay of neutral pions produced in ion-ion collisions. While a dominant leptonic origin of this emission can be ruled out, the relativistic electron population related to the observed radio flux will necessarily lead to a certain level of bremsstrahlung gamma-ray emission. Based on this broad-band modelling, we have developed a method to estimate the magnetic field, yielding $B\geq 80\,μ$G at our best estimate of the molecular cloud density (or less at a lower density). G73.9+0.9 appears similar, though somewhat weaker, to other SNRs interacting with a local dense medium detected by the LAT.

astro-ph.HE↗

arXiv2013

Deep Observation of the Giant Radio Lobes of Centaurus A with the Fermi Large Area Telescope

The detection of high energy (HE) γ-ray emission up to about 3 GeV from the giant lobes of the radio galaxy Centaurus A has been recently reported by the Fermi-LAT Collaboration based on ten months of all-sky survey observations. A data set more than three times larger is used here to study the morphology and photon spectrum of the lobes with higher statistics. The larger data set results in the detection of HE γ-ray emission (up to about 6 GeV) from the lobes with a significance of more than 10 and 20 σ for the North and the South lobe, respectively. Based on a detailed spatial analysis and comparison with the associated radio lobes, we report evidence for a substantial extension of the HE γ-ray emission beyond the WMAP radio image in the case of the Northern lobe of Cen A. We reconstruct the spectral energy distribution (SED) of the lobes using radio (WMAP) and Fermi-LAT data from the same integration region. The implications are discussed in the context of hadronic and leptonic scenarios.

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