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Ajello, M.ORCID iD iconorcid.org/0000-0002-6584-1703
Publications (10 of 14) Show all publications
Ajello, M., Atwood, W. B., Axelsson, M., Bagagli, R., Bagni, M., Baldini, L., . . . Zaharijas, G. (2021). Fermi Large Area Telescope Performance after 10 Years of Operation. Astrophysical Journal Supplement Series, 256(1), Article ID 12.
Open this publication in new window or tab >>Fermi Large Area Telescope Performance after 10 Years of Operation
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2021 (English)In: Astrophysical Journal Supplement Series, ISSN 0067-0049, E-ISSN 1538-4365, Vol. 256, no 1, article id 12Article in journal (Refereed) Published
Abstract [en]

The Large Area Telescope (LAT), the primary instrument for the Fermi Gamma-ray Space Telescope (Fermi) mission, is an imaging, wide field-of-view, high-energy gamma-ray telescope, covering the energy range from 30 MeV to more than 300 GeV. We describe the performance of the instrument at the 10 yr milestone. LAT performance remains well within the specifications defined during the planning phase, validating the design choices and supporting the compelling case to extend the duration of the Fermi mission. The details provided here will be useful when designing the next generation of high-energy gamma-ray observatories.
National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-198243 (URN)10.3847/1538-4365/ac0ceb (DOI)000692877400001 ()2-s2.0-85115774062 (Scopus ID)
Available from: 2021-11-08 Created: 2021-11-08 Last updated: 2022-11-10Bibliographically approved
Ajello, M., Baldini, L., Ballet, J., Barbiellini, G., Bastieri, D., Bellazzini, R., . . . Zrake, J. (2021). Gamma Rays from Fast Black-hole Winds. Astrophysical Journal, 921(2), Article ID 144.
Open this publication in new window or tab >>Gamma Rays from Fast Black-hole Winds
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2021 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 921, no 2, article id 144Article in journal (Refereed) Published
Abstract [en]

Massive black holes at the centers of galaxies can launch powerful wide-angle winds that, if sustained over time, can unbind the gas from the stellar bulges of galaxies. These winds may be responsible for the observed scaling relation between the masses of the central black holes and the velocity dispersion of stars in galactic bulges. Propagating through the galaxy, the wind should interact with the interstellar medium creating a strong shock, similar to those observed in supernovae explosions, which is able to accelerate charged particles to high energies. In this work we use data from the Fermi Large Area Telescope to search for the gamma-ray emission from galaxies with an ultrafast outflow (UFO): a fast (v similar to 0.1 c), highly ionized outflow, detected in absorption at hard X-rays in several nearby active galactic nuclei (AGN). Adopting a sensitive stacking analysis we are able to detect the average gamma-ray emission from these galaxies and exclude that it is due to processes other than UFOs. Moreover, our analysis shows that the gamma-ray luminosity scales with the AGN bolometric luminosity and that these outflows transfer similar to 0.04% of their mechanical power to gamma-rays. Interpreting the observed gamma-ray emission as produced by cosmic rays (CRs) accelerated at the shock front, we find that the gamma-ray emission may attest to the onset of the wind-host interaction and that these outflows can energize charged particles up to the transition region between galactic and extragalactic CRs.
Keywords
Active galactic nuclei, Galactic winds, Gamma-rays
National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-199550 (URN)10.3847/1538-4357/ac1bb2 (DOI)000716730500001 ()
Available from: 2021-12-14 Created: 2021-12-14 Last updated: 2022-02-25Bibliographically approved
Ajello, M., Arimoto, M., Axelsson, M., Baldini, L., Barbiellini, G., Bastieri, D., . . . De Pasquale, M. (2020). Fermi and Swift Observations of GRB 190114C: Tracing the Evolution of High-energy Emission from Prompt to Afterglow. Astrophysical Journal, 890(1), Article ID 9.
Open this publication in new window or tab >>Fermi and Swift Observations of GRB 190114C: Tracing the Evolution of High-energy Emission from Prompt to Afterglow
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2020 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 890, no 1, article id 9Article in journal (Refereed) Published
Abstract [en]

We report on the observations of gamma-ray burst (GRB) 190114C by the Fermi Gamma -ray Space Telescope and the Neil Gehrels Swift Observatory. The prompt gamma-ray emission was detected by the Fermi GRB Monitor (GBM), the Fermi Large Area Telescope (LAT), and the Swift Burst Alert Telescope (BAT) and the long-lived afterglow emission was subsequently observed by the GBM, LAT, Swift X-ray Telescope (XRT), and Swift UV Optical Telescope. The early -time observations reveal multiple emission components that evolve independently, with a delayed power-law component that exhibits significant spectral attenuation above 40 MeV in the first few seconds of the burst. This power-law component transitions to a harder spectrum that is consistent with the afterglow emission observed by the XRT at later times. This afterglow component is clearly identifiable in the GBM and BAT light curves as a slowly fading emission component on which the rest of the prompt emission is superimposed. As a result, we are able to observe the transition from internal-shock- to external-shock-dominated emission. We find that the temporal and spectral evolution of the broadband afterglow emission can be well modeled as synchrotron emission from a forward shock propagating into a wind -like circumstellar environment. We estimate the initial bulk Lorentz factor using the observed high-energy spectral cutoff. Considering the onset of the afterglow component, we constrain the deceleration radius at which this forward shock begins to radiate in order to estimate the maximum synchrotron energy as a function of time. We find that even in the LAT energy range, there exist high-energy photons that are in tension with the theoretical maximum energy that can be achieved through synchrotron emission from a shock. These violations of the maximum synchrotron energy are further compounded by the detection of very high-energy (VHE) emission above 300 GeV by MAGIC concurrent with our observations. We conclude that the observations of VHE photons from GRB 190114C necessitates either an additional emission mechanism at very high energies that is hidden in the synchrotron component in the LAT energy range, an acceleration mechanism that imparts energy to the particles at a rate that is faster than the electron synchrotron energy -loss rate, or revisions of the fundamental assumptions used in estimating the maximum photon energy attainable through the synchrotron process.
National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-181090 (URN)10.3847/1538-4357/ab5b05 (DOI)000522095800009 ()
Available from: 2020-04-29 Created: 2020-04-29 Last updated: 2022-03-23Bibliographically approved
Abdollahi, S., Acero, F., Ackermann, M., Ajello, M., Atwood, W. B., Axelsson, M., . . . Zaharijas, G. (2020). Fermi Large Area Telescope Fourth Source Catalog. Astrophysical Journal Supplement Series, 247(1), Article ID 33.
Open this publication in new window or tab >>Fermi Large Area Telescope Fourth Source Catalog
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2020 (English)In: Astrophysical Journal Supplement Series, ISSN 0067-0049, E-ISSN 1538-4365, Vol. 247, no 1, article id 33Article in journal (Refereed) Published
Abstract [en]

We present the fourth Fermi Large Area Telescope catalog (4FGL) of gamma-ray sources. Based on the first eight years of science data from the Fermi Gamma-ray Space Telescope mission in the energy range from 50 MeV to 1 TeV, it is the deepest yet in this energy range. Relative to the 3FGL catalog, the 4FGL catalog has twice as much exposure as well as a number of analysis improvements, including an updated model for the Galactic diffuse gamma-ray emission, and two sets of light curves (one-year and two-month intervals). The 4FGL catalog includes 5064 sources above 4 sigma significance, for which we provide localization and spectral properties. Seventy-five sources are modeled explicitly as spatially extended, and overall, 358 sources are considered as identified based on angular extent, periodicity, or correlated variability observed at other wavelengths. For 1336 sources, we have not found plausible counterparts at other wavelengths. More than 3130 of the identified or associated sources are active galaxies of the blazar class, and 239 are pulsars.
Keywords
Catalogs, Gamma-ray astronomy, Sky surveys
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-181064 (URN)10.3847/1538-4365/ab6bcb (DOI)000520200200001 ()2-s2.0-85091172627 (Scopus ID)
Available from: 2020-05-01 Created: 2020-05-01 Last updated: 2022-11-08Bibliographically approved
Ackermann, M., Ajello, M., Baldini, L., Ballet, J., Barbiellini, G., Bastieri, D., . . . Biteau, J. (2018). The Search for Spatial Extension in High-latitude Sources Detected by the Fermi Large Area Telescope. Astrophysical Journal Supplement Series, 237(2), Article ID 32.
Open this publication in new window or tab >>The Search for Spatial Extension in High-latitude Sources Detected by the Fermi Large Area Telescope
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2018 (English)In: Astrophysical Journal Supplement Series, ISSN 0067-0049, E-ISSN 1538-4365, Vol. 237, no 2, article id 32Article in journal (Refereed) Published
Abstract [en]

We present a search for spatial extension in high-latitude (vertical bar b vertical bar > 5 degrees) sources in recent Fermi point source catalogs. The result is the Fermi High-Latitude Extended Sources Catalog, which provides source extensions (or upper limits thereof) and likelihood profiles for a suite of tested source morphologies. We find 24. extended sources, 19 of which were not previously characterized as extended. These include sources that are potentially associated with supernova remnants and star-forming regions. We also found extended.-ray emission in the vicinity of the Cen. A radio lobes and-at GeV energies for the first time-spatially coincident with the radio emission of the SNR CTA 1, as well as from the Crab Nebula. We also searched for halos around active galactic nuclei, which are predicted from electromagnetic cascades induced by the e(+)e(-) pairs that are deflected in intergalactic magnetic fields. These pairs are produced when gamma-rays interact with background radiation fields. We do not find evidence for extension in individual sources or in stacked source samples. This enables us to place limits on the flux of the extended source components, which are then used to constrain the intergalactic magnetic field to be stronger than 3 x 10(-16) G for a coherence length lambda greater than or similar to 10 kpc, even when conservative assumptions on the source duty cycle are made. This improves previous limits by several orders of magnitude.
Keywords
BL Lacertae objects: general, catalogs, gamma rays: general, ISM: H II Regions, ISM: supernova remnants
National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-158910 (URN)10.3847/1538-4365/aacdf7 (DOI)000441093400001 ()
Available from: 2018-08-20 Created: 2018-08-20 Last updated: 2022-02-26Bibliographically approved
Ajello, M., Atwood, W. B., Baldini, L., Ballet, J., Barbiellini, G., Bastieri, D., . . . Wood, M. (2017). 3FHL: The Third Catalog of Hard Fermi-LAT Sources. Astrophysical Journal Supplement Series, 232(2), Article ID 18.
Open this publication in new window or tab >>3FHL: The Third Catalog of Hard Fermi-LAT Sources
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2017 (English)In: Astrophysical Journal Supplement Series, ISSN 0067-0049, E-ISSN 1538-4365, Vol. 232, no 2, article id 18Article in journal (Refereed) Published
Abstract [en]

We present a catalog of sources detected above 10 GeV by the Fermi Large Area Telescope (LAT) in the first 7 years of data using the Pass 8 event-level analysis. This is the Third Catalog of Hard Fermi-LAT Sources (3FHL), containing 1556 objects characterized in the 10 GeV-2 TeV energy range. The sensitivity and angular resolution are improved by factors of 3 and 2 relative to the previous LAT catalog at the same energies (1FHL). The vast majority of detected sources (79%) are associated with extragalactic counterparts at other wavelengths, including 16 sources located at very high redshift (z > 2). Of the sources, 8% have Galactic counterparts and 13% are unassociated (or associated with a source of unknown nature). The high-latitude sky and the Galactic plane are observed with a flux sensitivity of 4.4 to 9.5 x 10(-11) ph cm(-2) s(-1), respectively (this is approximately 0.5% and 1% of the Crab Nebula flux above 10 GeV). The catalog includes 214 new gamma-ray sources. The substantial increase in the number of photons (more than 4 times relative to 1FHL and 10 times to 2FHL) also allows us to measure significant spectral curvature for 32 sources and find flux variability for 163 of them. Furthermore, we estimate that for the same flux limit of 10(-12) erg cm(-2) s(-1), the energy range above 10 GeV has twice as many sources as the range above 50 GeV, highlighting the importance, for future Cherenkov telescopes, of lowering the energy threshold as much as possible.
Keywords
catalogs, gamma rays: general
National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-148868 (URN)10.3847/1538-4365/aa8221 (DOI)000411892800001 ()2-s2.0-85032822161 (Scopus ID)
Available from: 2017-11-22 Created: 2017-11-22 Last updated: 2022-10-19Bibliographically approved
Desai, A., Ajello, M., Omodei, N., Hartmann, D. H., Dominguez, A., Paliya, V. S., . . . Meyer, M. (2017). Probing the EBL Evolution at High Redshift Using GRBs Detected with the Fermi-LAT. Astrophysical Journal, 850(1), Article ID 73.
Open this publication in new window or tab >>Probing the EBL Evolution at High Redshift Using GRBs Detected with the Fermi-LAT
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2017 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 850, no 1, article id 73Article in journal (Refereed) Published
Abstract [en]

The extragalactic background light (EBL), from ultraviolet to infrared wavelengths, is predominantly due to emission from stars, accreting black holes and reprocessed light due to Galactic dust. The EBL can be studied through the imprint it leaves, via gamma-gamma absorption of high-energy photons, in the spectra of distant.-ray sources. The EBL has been probed through the search for the attenuation it produces in the spectra of BL Lacertae (BL Lac) objects and individual.-ray bursts (GRBs). GRBs have significant advantages over blazars for the study of the EBL especially at high redshifts. Here we analyze a combined sample of 22 GRBs, detected by the Fermi Large Area Telescope between 65 MeV and 500 GeV. We report a marginal detection (at the similar to 2.8 sigma level) of the EBL attenuation in the stacked spectra of the source sample. This measurement represents a first constraint of the EBL at an effective redshift of similar to 1.8. We combine our results with prior EBL constraints and conclude that Fermi-LAT is instrumental to constrain the UV component of the EBL. We discuss the implications on existing empirical models of EBL evolution.
Keywords
cosmology: observations, galaxies: high-redshift, gamma-ray burst: general, gamma-rays: observations, gamma-rays: theory
National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-149980 (URN)10.3847/1538-4357/aa917c (DOI)000415832400016 ()2-s2.0-85037675694 (Scopus ID)
Available from: 2017-12-22 Created: 2017-12-22 Last updated: 2022-10-20Bibliographically approved
Ackermann, M., Ajello, M., Asano, K., Baldini, L., Barbiellini, G., Baring, M. G., . . . Littlejohns, O. (2013). MULTIWAVELENGTH OBSERVATIONS OF GRB 110731A: GeV EMISSION FROM ONSET TO AFTERGLOW. Astrophysical Journal, 763(2), Article ID 71.
Open this publication in new window or tab >>MULTIWAVELENGTH OBSERVATIONS OF GRB 110731A: GeV EMISSION FROM ONSET TO AFTERGLOW
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2013 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 763, no 2, article id 71Article in journal (Refereed) Published
Abstract [en]

We report on the multiwavelength observations of the bright, long gamma-ray burst GRB 110731A, by the Fermi and Swift observatories, and by the MOA and GROND optical telescopes. The analysis of the prompt phase reveals that GRB 110731A shares many features with bright Large Area Telescope bursts observed by Fermi during the first three years on-orbit: a light curve with short time variability across the whole energy range during the prompt phase, delayed onset of the emission above 100 MeV, extra power-law component and temporally extended high-energy emission. In addition, this is the first GRB for which simultaneous GeV, X-ray, and optical data are available over multiple epochs beginning just after the trigger time and extending for more than 800 s, allowing temporal and spectral analysis in different epochs that favor emission from the forward shock in a wind-type medium. The observed temporally extended GeV emission is most likely part of the high-energy end of the afterglow emission. Both the single-zone pair transparency constraint for the prompt signal and the spectral and temporal analysis of the forward-shock afterglow emission independently lead to an estimate of the bulk Lorentz factor of the jet Gamma similar to 500-550.
Keywords
gamma-ray burst: individual (GRB110731A)
National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-161706 (URN)10.1088/0004-637X/763/2/71 (DOI)000313869800002 ()2-s2.0-84872690981 (Scopus ID)
Available from: 2018-11-09 Created: 2018-11-09 Last updated: 2023-12-06Bibliographically approved
Ackermann, M., Ajello, M., Atwood, W. B., Baldini, L., Barbiellini, G., Bastieri, D., . . . Zimmer, S. (2012). CONSTRAINTS ON THE GALACTIC HALO DARK MATTER FROM FERMI-LAT DIFFUSE MEASUREMENTS. Astrophysical Journal, 761(2), Article ID 91.
Open this publication in new window or tab >>CONSTRAINTS ON THE GALACTIC HALO DARK MATTER FROM FERMI-LAT DIFFUSE MEASUREMENTS
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2012 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 761, no 2, article id 91Article in journal (Refereed) Published
Abstract [en]

We have performed an analysis of the diffuse gamma-ray emission with the Fermi Large Area Telescope (LAT) in the Milky Way halo region, searching for a signal from dark matter annihilation or decay. In the absence of a robust dark matter signal, constraints are presented. We consider both gamma rays produced directly in the dark matter annihilation/decay and produced by inverse Compton scattering of the e(+)/e(-) produced in the annihilation/decay. Conservative limits are derived requiring that the dark matter signal does not exceed the observed diffuse gamma-ray emission. A second set of more stringent limits is derived based on modeling the foreground astrophysical diffuse emission using the GALPROP code. Uncertainties in the height of the diffusive cosmic-ray halo, the distribution of the cosmic-ray sources in the Galaxy, the index of the injection cosmic-ray electron spectrum, and the column density of the interstellar gas are taken into account using a profile likelihood formalism, while the parameters governing the cosmic-ray propagation have been derived from fits to local cosmic-ray data. The resulting limits impact the range of particle masses over which dark matter thermal production in the early universe is possible, and challenge the interpretation of the PAMELA/Fermi-LAT cosmic ray anomalies as the annihilation of dark matter.
Keywords
dark matter, Galaxy: halo, gamma rays: diffuse background, methods: statistical
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-161716 (URN)10.1088/0004-637X/761/2/91 (DOI)000312090300010 ()2-s2.0-84870842972 (Scopus ID)
Available from: 2018-11-13 Created: 2018-11-13 Last updated: 2024-03-01Bibliographically approved
Ackermann, M., Ajello, M., Albert, A., Baldini, L., Barbiellini, G., Bechtol, K., . . . Zimmer, S. (2012). Fermi LAT search for dark matter in gamma-ray lines and the inclusive photon spectrum. Physical Review D: covering particles, fields, gravitation, and cosmology, 86(2), Article ID 022002.
Open this publication in new window or tab >>Fermi LAT search for dark matter in gamma-ray lines and the inclusive photon spectrum
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2012 (English)In: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 86, no 2, article id 022002Article in journal (Refereed) Published
Abstract [en]

Dark matter particle annihilation or decay can produce monochromatic gamma-ray lines and contribute to the diffuse gamma-ray background. Flux upper limits are presented for gamma-ray spectral lines from 7 to 200 GeV and for the diffuse gamma-ray background from 4.8 GeV to 264 GeV obtained from two years of Fermi Large Area Telescope data integrated over most of the sky. We give cross-section upper limits and decay lifetime lower limits for dark matter models that produce gamma-ray lines or contribute to the diffuse spectrum, including models proposed as explanations of the PAMELA and Fermi cosmic-ray data.
National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-162402 (URN)10.1103/PhysRevD.86.022002 (DOI)000306193000001 ()2-s2.0-84863837831 (Scopus ID)
Available from: 2019-02-13 Created: 2019-02-13 Last updated: 2022-09-30Bibliographically approved
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-6584-1703

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