arXiv subjects

S. Guiriec

Publications and source records attributed to S. Guiriec.

At least 73 records · Page 4Linked to original sources

arXiv2013

Constraints on the Galactic Halo Dark Matter from Fermi-LAT Diffuse Measurements

We have performed an analysis of the diffuse gamma-ray emission with the Fermi Large Area Telescope 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 annihilation of dark matter.

astro-ph.CO↗

arXiv2013

Identifying the Location in the Host Galaxy of the Short GRB 111117A with the Chandra Sub-arcsecond Position

We present our successful Chandra program designed to identify, with sub-arcsecond accuracy, the X-ray afterglow of the short GRB 111117A, which was discovered by Swift and Fermi. Thanks to our rapid target of opportunity request, Chandra clearly detected the X-ray afterglow, though no optical afterglow was found in deep optical observations. The host galaxy was clearly detected in the optical and near-infrared band, with the best photometric redshift of z=1.31_{-0.23}^{+0.46} (90% confidence), making it one of the highest known short GRB redshifts. Furthermore, we see an offset of 1.0 +- 0.2 arcseconds, which corresponds to 8.4 +- 1.7 kpc, between the host and the afterglow position. We discuss the importance of using Chandra for obtaining sub-arcsecond X-ray localizations of short GRB afterglows to study GRB environments.

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arXiv2012

Binary Millisecond Pulsar Discovery via Gamma-Ray Pulsations

Millisecond pulsars, old neutron stars spun-up by accreting matter from a companion star, can reach high rotation rates of hundreds of revolutions per second. Until now, all such "recycled" rotation-powered pulsars have been detected by their spin-modulated radio emission. In a computing-intensive blind search of gamma-ray data from the Fermi Large Area Telescope (with partial constraints from optical data), we detected a 2.5-millisecond pulsar, PSR J1311-3430. This unambiguously explains a formerly unidentified gamma-ray source that had been a decade-long enigma, confirming previous conjectures. The pulsar is in a circular orbit with an orbital period of only 93 minutes, the shortest of any spin-powered pulsar binary ever found.

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arXiv2012

X-Ray and Optical Observations of A 0535+26

We present recent contemporaneous X-ray and optical observations of the Be/X-ray binary system A\,0535+26 with the \textit{Fermi}/Gamma-ray Burst Monitor (GBM) and several ground-based observatories. These new observations are put into the context of the rich historical data (since $\sim$1978) and discussed in terms of the neutron star Be-disk interaction. The Be circumstellar disk was exceptionally large just before the 2009 December giant outburst, which may explain the origin of the unusual recent X-ray activity of this source. We found a peculiar evolution of the pulse profile during this giant outburst, with the two main components evolving in opposite ways with energy. A hard 30-70 mHz X-ray QPO was detected with GBM during this 2009 December giant outburst. It becomes stronger with increasing energy and disappears at energies below 25\,keV. In the long-term a strong optical/X-ray correlation was found for this system, however in the medium-term the H$_α$ EW and the V-band brightness showed an anti-correlation after $\sim$2002 Agust. Each giant X-ray outburst occurred during a decline phase of the optical brightness, while the H$_α$ showed a strong emission. In late 2010 and before the 2011 February outburst, rapid V/R variations are observed in the strength of the two peaks of the H$_α$ line. These had a period of $\sim$\,25 days and we suggest the presence of a global one-armed oscillation to explain this scenario. A general pattern might be inferred, where the disk becomes weaker and shows V/R variability beginning $\sim$\,6 months following a giant outburst.

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arXiv2012

Multi-wavelength observations of blazar AO 0235+164 in the 2008-2009 flaring state

The blazar AO 0235+164 (z = 0.94) has been one of the most active objects observed by Fermi Large Area Telescope (LAT) since its launch in Summer 2008. In addition to the continuous coverage by Fermi, contemporaneous observations were carried out from the radio to γ -ray bands between 2008 September and 2009 February. In this paper, we summarize the rich multi-wavelength data collected during the campaign (including F-GAMMA, GASP- WEBT, Kanata, OVRO, RXTE, SMARTS, Swift, and other instruments), examine the cross-correlation between the light curves measured in the different energy bands, and interpret the resulting spectral energy distributions in the context of well-known blazar emission models. We find that the γ -ray activity is well correlated with a series of near-IR/optical flares, accompanied by an increase in the optical polarization degree. On the other hand, the X-ray light curve shows a distinct 20 day high state of unusually soft spectrum, which does not match the extrapolation of the optical/UV synchrotron spectrum. We tentatively interpret this feature as the bulk Compton emission by cold electrons contained in the jet, which requires an accretion disk corona with an effective covering factor of 19% at a distance of 100 Rg . We model the broadband spectra with a leptonic model with external radiation dominated by the infrared emission from the dusty torus.

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arXiv2012

Gamma-ray observations of the Orion Molecular Clouds with the Fermi Large Area Telescope

We report on the gamma-ray observations of giant molecular clouds Orion A and B with the Large Area Telescope (LAT) on-board the Fermi Gamma-ray Space Telescope. The gamma-ray emission in the energy band between \sim100 MeV and \sim100 GeV is predicted to trace the gas mass distribution in the clouds through nuclear interactions between the Galactic cosmic rays (CRs) and interstellar gas. The gamma-ray production cross-section for the nuclear interaction is known to \sim10% precision which makes the LAT a powerful tool to measure the gas mass column density distribution of molecular clouds for a known CR intensity. We present here such distributions for Orion A and B, and correlate them with those of the velocity integrated CO intensity (WCO) at a 1° \times1° pixel level. The correlation is found to be linear over a WCO range of ~10 fold when divided in 3 regions, suggesting penetration of nuclear CRs to most of the cloud volumes. The Wco-to-mass conversion factor, Xco, is found to be \sim2.3\times10^20 cm-2(K km s-1)-1 for the high-longitude part of Orion A (l > 212°), \sim1.7 times higher than \sim1.3 \times 10^20 found for the rest of Orion A and B. We interpret the apparent high Xco in the high-longitude region of Orion A in the light of recent works proposing a non-linear relation between H2 and CO densities in the diffuse molecular gas. Wco decreases faster than the H2 column density in the region making the gas "darker" to Wco.

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arXiv2012

GeV Observations of Star-forming Galaxies with \textit{Fermi} LAT

Recent detections of the starburst galaxies M82 and NGC 253 by gamma-ray telescopes suggest that galaxies rapidly forming massive stars are more luminous at gamma-ray energies compared to their quiescent relatives. Building upon those results, we examine a sample of 69 dwarf, spiral, and luminous and ultraluminous infrared galaxies at photon energies 0.1-100 GeV using 3 years of data collected by the Large Area Telescope (LAT) on the \textit{Fermi Gamma-ray Space Telescope} (\textit{Fermi}). Measured fluxes from significantly detected sources and flux upper limits for the remaining galaxies are used to explore the physics of cosmic rays in galaxies. We find further evidence for quasi-linear scaling relations between gamma-ray luminosity and both radio continuum luminosity and total infrared luminosity which apply both to quiescent galaxies of the Local Group and low-redshift starburst galaxies (conservative $P$-values $\lesssim0.05$ accounting for statistical and systematic uncertainties). The normalizations of these scaling relations correspond to luminosity ratios of $\log(L_{0.1-100 \rm{GeV}}/L_{1.4 \rm{GHz}}) = 1.7 \pm 0.1_{\rm (statistical)} \pm 0.2_{\rm (dispersion)}$ and $\log(L_{0.1-100 \rm{GeV}}/L_{8-1000 μ\rm{m}}) = -4.3 \pm 0.1_{\rm (statistical)} \pm 0.2_{\rm (dispersion)}$ for a galaxy with a star formation rate of 1 $M_{\odot}$ yr$^{-1}$, assuming a Chabrier initial mass function. Using the relationship between infrared luminosity and gamma-ray luminosity, the collective intensity of unresolved star-forming galaxies at redshifts $0<z<2.5$ above 0.1 GeV is estimated to be 0.4-2.4 $\times 10^{-6}$ ph cm$^{-2}$ s$^{-1}$ sr$^{-1}$ (4-23% of the intensity of the isotropic diffuse component measured with the LAT). We anticipate that $\sim10$ galaxies could be detected by their cosmic-ray induced gamma-ray emission during a 10-year \textit{Fermi} mission.

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arXiv2012

Fermi LAT Search for Dark Matter in Gamma-ray Lines and the Inclusive Photon Spectrum

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.

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arXiv2012

Measurement of separate cosmic-ray electron and positron spectra with the Fermi Large Area Telescope

We measured separate cosmic-ray electron and positron spectra with the Fermi Large Area Telescope. Because the instrument does not have an onboard magnet, we distinguish the two species by exploiting the Earth's shadow, which is offset in opposite directions for opposite charges due to the Earth's magnetic field. We estimate and subtract the cosmic-ray proton background using two different methods that produce consistent results. We report the electron-only spectrum, the positron-only spectrum, and the positron fraction between 20 GeV and 200 GeV. We confirm that the fraction rises with energy in the 20-100 GeV range. The three new spectral points between 100 and 200 GeV are consistent with a fraction that is continuing to rise with energy.

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arXiv2012

SGR J1550-5418 bursts detected with the Fermi Gamma-ray Burst Monitor during its most prolific activity

We have performed detailed temporal and time-integrated spectral analysis of 286 bursts from SGR J1550-5418 detected with the Fermi Gamma-ray Burst Monitor (GBM) in January 2009, resulting in the largest uniform sample of temporal and spectral properties of SGR J1550-5418 bursts. We have used the combination of broadband and high time-resolution data provided with GBM to perform statistical studies for the source properties. We determine the durations, emission times, duty cycles and rise times for all bursts, and find that they are typical of SGR bursts. We explore various models in our spectral analysis, and conclude that the spectra of SGR J1550-5418 bursts in the 8-200 keV band are equally well described by optically thin thermal bremsstrahlung (OTTB), a power law with an exponential cutoff (Comptonized model), and two black-body functions (BB+BB). In the spectral fits with the Comptonized model we find a mean power-law index of -0.92, close to the OTTB index of -1. We show that there is an anti-correlation between the Comptonized Epeak and the burst fluence and average flux. For the BB+BB fits we find that the fluences and emission areas of the two blackbody functions are correlated. The low-temperature BB has an emission area comparable to the neutron star surface area, independent of the temperature, while the high-temperature blackbody has a much smaller area and shows an anti-correlation between emission area and temperature. We compare the properties of these bursts with bursts observed from other SGR sources during extreme activations, and discuss the implications of our results in the context of magnetar burst models.

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arXiv2012

Anisotropies in the diffuse gamma-ray background measured by the Fermi LAT

The contribution of unresolved sources to the diffuse gamma-ray background could induce anisotropies in this emission on small angular scales. We analyze the angular power spectrum of the diffuse emission measured by the Fermi LAT at Galactic latitudes |b| > 30 deg in four energy bins spanning 1 to 50 GeV. At multipoles \ell \ge 155, corresponding to angular scales \lesssim 2 deg, angular power above the photon noise level is detected at >99.99% CL in the 1-2 GeV, 2-5 GeV, and 5-10 GeV energy bins, and at >99% CL at 10-50 GeV. Within each energy bin the measured angular power takes approximately the same value at all multipoles \ell \ge 155, suggesting that it originates from the contribution of one or more unclustered source populations. The amplitude of the angular power normalized to the mean intensity in each energy bin is consistent with a constant value at all energies, C_P/ ^2 = 9.05 +/- 0.84 x 10^{-6} sr, while the energy dependence of C_P is consistent with the anisotropy arising from one or more source populations with power-law photon spectra with spectral index Γ_s = 2.40 +/- 0.07. We discuss the implications of the measured angular power for gamma-ray source populations that may provide a contribution to the diffuse gamma-ray background.

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arXiv2012

Search for Dark Matter Satellites using the FERMI-LAT

Numerical simulations based on the Lambda-CDM model of cosmology predict a large number of as yet unobserved Galactic dark matter satellites. We report the results of a Large Area Telescope (LAT) search for these satellites via the gamma-ray emission expected from the annihilation of weakly interacting massive particle (WIMP) dark matter. Some dark matter satellites are expected to have hard gamma-ray spectra, finite angular extents, and a lack of counterparts at other wavelengths. We sought to identify LAT sources with these characteristics, focusing on gamma-ray spectra consistent with WIMP annihilation through the $b \bar b$ channel. We found no viable dark matter satellite candidates using one year of data, and we present a framework for interpreting this result in the context of numerical simulations to constrain the velocity-averaged annihilation cross section for a conventional 100 GeV WIMP annihilating through the $b \bar b$ channel.

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arXiv2011

Constraining Dark Matter Models from a Combined Analysis of Milky Way Satellites with the Fermi Large Area Telescope

Satellite galaxies of the Milky Way are among the most promising targets for dark matter searches in gamma rays. We present a search for dark matter consisting of weakly interacting massive particles, applying a joint likelihood analysis to 10 satellite galaxies with 24 months of data of the Fermi Large Area Telescope. No dark matter signal is detected. Including the uncertainty in the dark matter distribution, robust upper limits are placed on dark matter annihilation cross sections. The 95% confidence level upper limits range from about 1e-26 cm^3 s^-1 at 5 GeV to about 5e-23 cm^3 s^-1 at 1 TeV, depending on the dark matter annihilation final state. For the first time, using gamma rays, we are able to rule out models with the most generic cross section (~3e-26 cm^3 s^-1 for a purely s-wave cross section), without assuming additional boost factors.

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arXiv2011

An overview of the current understanding of Gamma-ray Bursts in the Fermi era

Gamma-ray bursts are the most luminous explosions in the Universe, and their origin as well as mechanism are the focus of intense research and debate. More than three decades since their serendipitous discovery, followed by several breakthroughs from space-borne and ground-based observations, they remain one of the most interesting astrophysical phenomena yet to be completely understood. Since the launch of Fermi with its unprecedented energy band width spanning seven decades, the study of gamma-ray burst research has entered a new phase. Here we review the current theoretical understanding and observational highlights of gamma-ray burst astronomy and point out some of the potential promises of multi-wavelength observations in view of the upcoming ground based observational facilities.

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arXiv2011

Gamma-Ray and Parsec-Scale Jet Properties of a Complete Sample of Blazars From the MOJAVE Program

We investigate the Fermi LAT gamma-ray and 15 GHz VLBA radio properties of a joint gamma-ray- and radio-selected sample of AGNs obtained during the first 11 months of the Fermi mission (2008 Aug 4 - 2009 Jul 5). Our sample contains the brightest 173 AGNs in these bands above declination -30 deg. during this period, and thus probes the full range of gamma-ray loudness (gamma-ray to radio band luminosity ratio) in the bright blazar population. The latter quantity spans at least four orders of magnitude, reflecting a wide range of spectral energy distribution (SED) parameters in the bright blazar population. The BL Lac objects, however, display a linear correlation of increasing gamma-ray loudness with synchrotron SED peak frequency, suggesting a universal SED shape for objects of this class. The synchrotron self-Compton model is favored for the gamma-ray emission in these BL Lacs over external seed photon models, since the latter predict a dependence of Compton dominance on Doppler factor that would destroy any observed synchrotron SED peak - gamma-ray loudness correlation. The high-synchrotron peaked (HSP) BL Lac objects are distinguished by lower than average radio core brightness temperatures, and none display large radio modulation indices or high linear core polarization levels. No equivalent trends are seen for the flat-spectrum radio quasars (FSRQ) in our sample. Given the association of such properties with relativistic beaming, we suggest that the HSP BL Lacs have generally lower Doppler factors than the lower-synchrotron peaked BL Lacs or FSRQs in our sample.

astro-ph.CO↗

arXiv2011

The radio/gamma-ray connection in Active Galactic Nuclei in the era of the Fermi Large Area Telescope

We present a detailed statistical analysis of the correlation between radio and gamma-ray emission of the Active Galactic Nuclei (AGN) detected by Fermi during its first year of operation, with the largest datasets ever used for this purpose. We use both archival interferometric 8.4 GHz data (from the VLA and ATCA, for the full sample of 599 sources) and concurrent single-dish 15 GHz measurements from the Owens Valley Radio Observatory (OVRO, for a sub sample of 199 objects). Our unprecedentedly large sample permits us to assess with high accuracy the statistical significance of the correlation, using a surrogate-data method designed to simultaneously account for common-distance bias and the effect of a limited dynamical range in the observed quantities. We find that the statistical significance of a positive correlation between the cm radio and the broad band (E>100 MeV) gamma-ray energy flux is very high for the whole AGN sample, with a probability <1e-7 for the correlation appearing by chance. Using the OVRO data, we find that concurrent data improve the significance of the correlation from 1.6e-6 to 9.0e-8. Our large sample size allows us to study the dependence of correlation strength and significance on specific source types and gamma-ray energy band. We find that the correlation is very significant (chance probability <1e-7) for both FSRQs and BL Lacs separately; a dependence of the correlation strength on the considered gamma-ray energy band is also present, but additional data will be necessary to constrain its significance.

astro-ph.CO↗

arXiv2011

Rest-frame properties of 32 gamma-ray bursts observed by the Fermi Gamma-Ray Burst Monitor

Aims: In this paper we study the main spectral and temporal properties of gamma-ray bursts (GRBs) observed by Fermi/GBM. We investigate these key properties of GRBs in the rest-frame of the progenitor and test for possible intra-parameter correlations to better understand the intrinsic nature of these events. Methods: Our sample comprises 32 GRBs with measured redshift that were observed by GBM until August 2010. 28 of them belong to the long-duration population and 4 events were classified as short/hard bursts. For all of these events we derive, where possible, the intrinsic peak energy in the $νF_ν$ spectrum (\eprest), the duration in the rest-frame, defined as the time in which 90% of the burst fluence was observed (\tninetyrest) and the isotropic equivalent bolometric energy (\eiso). Results: The distribution of \eprest has mean and median values of 1.1 MeV and 750 keV, respectively. A log-normal fit to the sample of long bursts peaks at ~800 keV. No high-\ep population is found but the distribution is biased against low \ep values. We find the lowest possible \ep that GBM can recover to be ~ 15 keV. The \tninetyrest distribution of long GRBs peaks at ~10 s. The distribution of \eiso has mean and median values of $8.9\times 10^{52}$ erg and $8.2 \times 10^{52}$ erg, respectively. We confirm the tight correlation between \eprest and \eiso (Amati relation) and the one between \eprest and the 1-s peak luminosity ($L_p$) (Yonetoku relation). Additionally, we observe a parameter reconstruction effect, i.e. the low-energy power law index $α$ gets softer when \ep is located at the lower end of the detector energy range. Moreover, we do not find any significant cosmic evolution of neither \eprest nor \tninetyrest.

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arXiv2011

Fermi/GBM observations of the ultra-long GRB 091024: A burst with an optical flash

In this paper we examine gamma-ray and optical data of GRB 091024, a gamma-ray burst (GRB) with an extremely long duration of T90~1020 s, as observed with the Fermi Gamma-Ray Burst Monitor (GBM). We present spectral analysis of all three distinct emission episodes using data from Fermi/GBM. Because of the long nature of this event, many ground-based optical telescopes slewed to its location within a few minutes and thus were able to observe the GRB during its active period. We compare the optical and gamma-ray light curves. Furthermore, we estimate a lower limit on the bulk Lorentz factor from the variability and spectrum of the GBM light curve and compare it with that obtained from the peak time of the forward shock of the optical afterglow. From the spectral analysis we note that, despite its unusually long duration, this burst is similar to other long GRBs, i.e. there is spectral evolution (both the peak energy and the spectral index vary with time) and spectral lags are measured. We find that the optical light curve is highly anti-correlated to the prompt gamma-ray emission, with the optical emission reaching the maximum during an epoch of quiescence in the prompt emission. We interpret this behavior as the reverse shock (optical flash), expected in the internal-external shock model of GRB emission but observed only in a handful of GRBs so far. The lower limit on the initial Lorentz factor deduced from the variability time scale ($Γ_{min}=195_{-110}^+{90}$)is consistent within the error to the one obtained using the peak time of the forward shock ($Γ_0=120$) and is also consistent with Lorentz factors of other long GRBs.

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