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M. Ajello

Publications and source records attributed to M. Ajello.

At least 127 records · Page 7Linked to original sources
arXiv2014

Fermi-LAT Detection of Gravitational Lens Delayed Gamma-ray Flares from Blazar B0218+357

Using data from the Fermi Large Area Telescope (LAT), we report the first clear gamma-ray measurement of a delay between flares from the gravitationally lensed images of a blazar. The delay was detected in B0218+357, a known double-image lensed system, during a period of enhanced gamma-ray activity with peak fluxes consistently observed to reach >20-50 times its previous average flux. An auto-correlation function analysis identified a delay in the gamma-ray data of 11.46 +/- 0.16 days (1 sigma) that is ~1 day greater than previous radio measurements. Considering that it is beyond the capabilities of the LAT to spatially resolve the two images, we nevertheless decomposed individual sequences of superposing gamma-ray flares/delayed emissions. In three such ~8-10 day-long sequences within a ~4-month span, considering confusion due to overlapping flaring emission and flux measurement uncertainties, we found flux ratios consistent with ~1, thus systematically smaller than those from radio observations. During the first, best-defined flare, the delayed emission was detailed with a Fermi pointing, and we observed flux doubling timescales of ~3-6 hrs implying as well extremely compact gamma-ray emitting regions.

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arXiv2013

The First Pulse of the Extremely Bright GRB 130427A: A Test Lab for Synchrotron Shocks

Gamma-ray burst (GRB) 130427A is one of the most energetic GRBs ever observed. The initial pulse up to 2.5 s is possibly the brightest well-isolated pulse observed to date. A fine time resolution spectral analysis shows power-law decays of the peak energy from the onset of the pulse, consistent with models of internal synchrotron shock pulses. However, a strongly correlated power-law behavior is observed between the luminosity and the spectral peak energy that is inconsistent with curvature effects arising in the relativistic outflow. It is difficult for any of the existing models to account for all of the observed spectral and temporal behaviors simultaneously.

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arXiv2013

Diffuse $γ$-ray emission from misaligned active galactic nuclei

Active galactic nuclei (AGN) with jets seen at small viewing angles are the most luminous and abundant objects in the $γ$-ray sky. AGN with jets misaligned along the line-of-sight appear fainter in the sky, but are more numerous than the brighter blazars. We calculate the diffuse $γ$-ray emission due to the population of misaligned AGN (MAGN) unresolved by the Large Area Telescope (LAT) on the {\it Fermi} Gamma-ray Space Telescope ({\it Fermi}). A correlation between the $γ$-ray luminosity and the radio-core luminosity is established and demonstrated to be physical by statistical tests, as well as compatible with upper limits based on {\it Fermi}-LAT data for a large sample of radio-loud MAGN. We constrain the derived $γ$-ray luminosity function by means of the source count distribution of the radio galaxies (RGs) detected by the {\it Fermi}-LAT. We finally calculate the diffuse $γ$-ray flux due to the whole MAGN population. Our results demonstrate that the MAGN can contribute from 10% up to nearly the entire measured Isotropic Gamma-Ray Background (IGRB). We evaluate a theoretical uncertainty on the flux of almost an order of magnitude.

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arXiv2013

First Results from NuSTAR Observations of Mkn 421

Mkn 421 is a nearby active galactic nucleus dominated at all wavelengths by a very broad non-thermal continuum thought to arise from a relativistic jet seen at a small angle to the line of sight. Its spectral energy distribution peaks in the X-ray and TeV gamma-ray bands, where the energy output is dominated by cooling of high-energy electrons in the jet. In order to study the electron distribution and its evolution, we carried out a dedicated multi-wavelength campaign, including extensive observations by the recently launched highly sensitive hard X-ray telescope NuSTAR, between December 2012 and May 2013. Here we present some initial results based on NuSTAR data from January through March 2013, as well as calibration observations conducted in 2012. Although the observations cover some of the faintest hard X-ray flux states ever observed for Mkn 421, the sensitivity is high enough to resolve intra-day spectral variability. We find that in this low state the dominant flux variations are smooth on timescales of hours, with typical intra-hour variations of less than 5%. We do not find evidence for either a cutoff in the hard X-ray spectrum, or a rise towards a high-energy component, but rather that at low flux the spectrum assumes a power law shape with a photon index of approximately 3. The spectrum is found to harden with increasing brightness.

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arXiv2013

The Cosmic Evolution of Fermi BL Lacertae Objects

Fermi has provided the largest sample of gamma-ray selected blazars to date. In this work we use a uniformly selected set of 211 BL Lacertae (BL Lac) objects detected by it Fermi during its first year of operation. We have obtained redshift constraints for 206 out of the 211 BL Lacs in our sample making it the largest and most complete sample of BL Lacs available in the literature. We use this sample to determine the luminosity function of BL Lacs and its evolution with cosmic time. We find that for most BL Lac classes, the evolution is positive with a space density peaking at modest redshift (z~1.2). The low-luminosity, high-synchrotron peaked (HSP) BL Lacs are an exception, showing strong negative evolution, with number density increasing for z$\lesssim$0.5. Since this rise corresponds to a drop-off in the density of flat-spectrum radio quasars (FSRQs), a possible interpretation is that these HSPs represent an accretion-starved end-state of an earlier merger-driven gas-rich phase. We additionally find that the known BL Lac correlation between luminosity and photon spectral index persists after correction for the substantial observational selection effects with implications for the so called `blazar sequence'. Finally, estimating the beaming corrections to the luminosity function, we find that BL Lacs have an average Lorentz factor of $γ=6.1^{+1.1}_{-0.8}$, and that most are seen within 10$^{\circ}$ of the jet axis.

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arXiv2013

Fermi Large Area Telescope Detection of Two Very-High-Energy (E>100 GeV) Gamma-ray Photons from the z = 1.1 Blazar PKS 0426-380

We report the Fermi Large Area Telescope (LAT) detection of two very-high-energy (VHE, E>100 GeV) gamma-ray photons from the directional vicinity of the distant (redshift, z = 1.1) blazar PKS 0426-380. The null hypothesis that both the 134 and 122 GeV photons originate from unrelated sources can be rejected at the 5.5 sigma confidence level. We therefore claim that at least one of the two VHE photons is securely associated with the blazar, making PKS 0426-380 the most distant VHE emitter known to date. The results are in agreement with the most recent Fermi-LAT constraints on the Extragalactic Background Light (EBL) intensity, which imply a $z \simeq 1$ horizon for $\simeq$ 100 GeV photons. The LAT detection of the two VHE gamma-rays coincided roughly with flaring states of the source, although we did not find an exact correspondence between the VHE photon arrival times and the flux maxima at lower gamma-ray energies. Modeling the gamma-ray continuum of PKS 0426-380 with daily bins revealed a significant spectral hardening around the time of detection of the first VHE event (LAT photon index Γ $\simeq$ 1.4) but on the other hand no pronounced spectral changes near the detection time of the second one. This combination implies a rather complex variability pattern of the source in gamma rays during the flaring epochs. An additional flat component is possibly present above several tens of GeV in the EBL-corrected Fermi-LAT spectrum accumulated over the ~8-month high state.

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arXiv2013

The NuSTAR Extragalactic Survey: A First Sensitive Look at the High-Energy Cosmic X-ray Background Population

We report on the first ten identifications of sources serendipitously detected by the NuSTAR to provide the first sensitive census of the cosmic X-ray background (CXB) source population at >10 keV. We find that these NuSTAR-detected sources are ~100x fainter than those previously detected at >10 keV and have a broad range in redshift and luminosity (z=0.020-2.923 and L_10-40 keV~4x10^{41}-5x10^{45} erg/s); the median redshift and luminosity are z~0.7 and L_10-40 keV~3x10^{44} erg/s, respectively. We characterize these sources on the basis of broad-band ~0.5-32 keV spectroscopy, optical spectroscopy, and broad-band ultraviolet-to-mid-infrared SED analyzes. We find that the dominant source population is quasars with L_10-40 keV>10^{44} erg/s, of which ~50% are obscured with N_H>10^{22} cm^{-2}. However, none of the ten NuSTAR sources are Compton thick (N_H>10^{24} cm^{-2}) and we place a 90% confidence upper limit on the fraction of Compton-thick quasars (L_10-40 keV>10^{44} erg/s) selected at >10 keV of ~33% over the redshift range z=0.5-1.1. We jointly fitted the rest-frame ~10-40 keV data for all of the non-beamed sources with L_10-40 keV>10^{43} erg/s to constrain the average strength of reflection; we find R<1.4 for Γ=1.8, broadly consistent with that found for local AGNs observed at >10 keV. We also constrain the host galaxy masses and find a median stellar mass of ~10^{11} M_sun, a factor ~5 times higher than the median stellar mass of nearby high-energy selected AGNs, which may be at least partially driven by the order of magnitude higher X-ray luminosities of the NuSTAR sources. Within the low source-statistic limitations of our study, our results suggest that the overall properties of the NuSTAR sources are broadly similar to those of nearby high-energy selected AGNs but scaled up in luminosity and mass.

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arXiv2013

Constraints on the Galactic Population of TEV Pulsar Wind Nebulae Using Fermi Large Area Telescope Observations

Pulsar wind nebulae (PWNe) have been established as the most populous class of TeV gamma-ray emitters. Since launch, the Fermi Large Area Telescope (LAT)identified five high-energy (100MeV <E< 100 GeV) gamma-ray sources as PWNe, and detected a large number of PWNe candidates, all powered by young and energetic pulsars. The wealth of multi-wavelength data available and the new results provided by Fermi-LAT give us an opportunity to find new PWNe and to explore the radiative processes taking place in known ones. The TeV gamma-ray unidentifiedsources (UNIDs) are the best candidates for finding new PWNe. Using 45 months of Fermi-LAT data for energies above 10 GeV, an analysis was performed near the position of 58TeV PWNe and UNIDs within 5deg of the Galactic Plane to establish new constraints on PWNe properties and find new clues on the nature of UNIDs. Of the 58 sources, 30 were detected, and this work provides their gamma-rayfluxes for energies above 10 GeV. The spectral energy distributions (SED) andupper limits, in the multi-wavelength context, also provide new information on the source nature and can help distinguish between emission scenarios, i.e. between classification as a pulsar candidate or as a PWN candidate. Six new GeV PWNe candidates are described in detail and compared with existing models. A population study of GeV PWNe candidates as a function of the pulsar/PWN system characteristics is presented.

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arXiv2013

The AT20G view of Swift/BAT selected AGN: high-frequency radio waves meet hard X-rays

We cross-matched the 6-year Swift/Burst Alert Telescope (BAT) survey of active galactic nuclei (AGN) with the AT20G radio survey of the southern sky, which is one of the largest high-frequency radio surveys available. With these data we investigated the possible correlation between the radio and the X-ray emission at the highest radio and X-ray frequencies. We found 37 AGN with a high probability of association (>80 per cent), among which 19 are local Seyfert galaxies (with median redshift z = 0.03) and 18 blazars. We found that \approx 20 per cent of the AGN detected in hard X-rays are also bright radio sources at 20 GHz, but the apparent correlation between the radio and hard X-ray luminosity is completely driven by the different median redshifts of the two subgroups of AGN. When we consider only the local Seyfert sample we find no evidence of a correlation between their 20 GHz and 15-55 keV power. Therefore it appears that at high frequencies the radio-X connection, which had been previously observed at lower frequencies, disappears. The disappearance of the radio-X correlation at high radio and X-ray frequencies could be tested through Very Long Baseline Interferometry and the use of the Nuclear Spectroscopic Telescope Array (NuSTAR) satellite.

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arXiv2013

Long-term monitoring of PKS 0537-441 with Fermi-LAT and multiwavelength observations

We report on multiwavelength observations of the blazar PKS 0537-441 (z = 0.896) obtained from microwaves through gamma rays by SMA, REM, ATOM, Swift and Fermi during 2008 August-2010 April. Strong variability has been observed in gamma rays, with two major flaring episodes (2009 July and 2010 March) and a harder-when-brighter behaviour, quite common for FSRQs and low-synchrotron-peaked BL Lacs, in 2010 March. In the same way the SED of the source cannot be modelled by a simple synchrotron self-Compton model, as opposed to many BL Lacs, but the addition of an external Compton component of seed photons from a dust torus is needed. The 230 GHz light curve showed an increase simultaneous with the gamma-ray one, indicating co-spatiality of the mm and gamma-ray emission region likely at large distance from the central engine. The low, average, and high activity SED of the source could be fit changing only the electron distribution parameters, but two breaks in the electron distribution are necessary. The ensuing extra spectral break, located at NIR-optical frequencies, together with that in gamma rays seem to indicate a common origin, most likely due to an intrinsic feature in the underlying electron distribution. An overall correlation between the gamma-ray band with the R-band and K-band has been observed with no significant time lag. On the other hand, when inspecting the light curves on short time scales some differences are evident. In particular, flaring activity has been detected in NIR and optical bands with no evident gamma-ray counterparts in 2009 September and November. Moderate variability has been observed in X-rays with no correlation between flux and photon index. An increase of the detected X-ray flux with no counter part at the other wavelengths has been observed in 2008 October, suggesting once more a complex correlation between the emission at different energy bands.

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arXiv2013

Detection of the Characteristic Pion-Decay Signature in Supernova Remnants

Cosmic rays are particles (mostly protons) accelerated to relativistic speeds. Despite wide agreement that supernova remnants (SNRs) are the sources of galactic cosmic rays, unequivocal evidence for the acceleration of protons in these objects is still lacking. When accelerated protons encounter interstellar material, they produce neutral pions, which in turn decay into gamma rays. This offers a compelling way to detect the acceleration sites of protons. The identification of pion-decay gamma rays has been difficult because high-energy electrons also produce gamma rays via bremsstrahlung and inverse Compton scattering. We detected the characteristic pion-decay feature in the gamma-ray spectra of two SNRs, IC 443 and W44, with the Fermi Large Area Telescope. This detection provides direct evidence that cosmic-ray protons are accelerated in SNRs.

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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.

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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

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

Flux and Photon Spectral Index Distributions of Fermi-LAT Blazars And Contribution To The Extragalactic Gamma-ray Background

We present a determination of the distributions of photon spectral index and gamma-ray flux - the so called LogN-LogS relation - for the 352 blazars detected with a greater than approximately seven sigma detection threshold and located above +/- 20 degrees Galactic latitude by the Large Area Telescope of the Fermi Gamma-ray Space Telescope in its first year catalog. Because the flux detection threshold depends on the photon index, the observed raw distributions do not provide the true LogN-LogS counts or the true distribution of the photon index. We use the non-parametric methods developed by Efron and Petrosian to reconstruct the intrinsic distributions from the observed ones which account for the data truncations introduced by observational bias and includes the effects of the possible correlation between the two variables. We demonstrate the robustness of our procedures using a simulated data set of blazars and then apply these to the real data and find that for the population as a whole the intrinsic flux distribution can be represented by a broken power law with high and low indexes of -2.37 +/- 0.13 and -1.70 +/- 0.26, respectively, and the intrinsic photon index distribution can be represented by a Gaussian with mean of 2.41 +/- 0.13 and width of 0.25 +/- 0.03. We also find the intrinsic distributions for the sub-populations of BL Lac and FSRQs type blazars separately. We then calculate the contribution of Fermi blazars to the diffuse extragalactic gamma-ray background radiation. Under the assumption that the flux distribution of blazars continues to arbitrarily low fluxes, we calculate the best fit contribution of all blazars to the total extragalactic gamma-ray output to be 60%, with a large uncertainty.

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