The nature of the hard X-ray source XSSJ12270-4859 is still unclear though it was claimed to be a magnetic Cataclysmic Variable. We here present a broad-band X-ray and gamma ray study based on a recent XMM-Newton observation and archival INTEGRAL and RXTE data. From the Fermi/LAT 1-year point source catalogue, we tentatively associate XSSJ12270-4859 with 1FGLJ1227.9-4852, a source of high energy gamma rays with emission up to 10GeV. We complement the study with UV photometry from XMM-Newton and ground-based optical and near-IR photometry. The X-ray emission is highly variable showing flares and intensity dips. The X-ray flares consist of flare-dip pairs. Flares are also detected in the UV range but not the dips. Aperiodic dipping behaviour is also observed during X-ray quiescence but not in the UV. The 0.2-100keV spectrum is featureless and described by a power law model with Gamma=1.7. The 100MeV-10GeV spectrum is instead represented by a power law index of 2.45. The luminosity ratio between 0.1-100GeV and 0.2--100keV is ~0.8, hence the GeV emission is a significant component of the total energy output. Furthermore, the X-ray spectrum does not greatly change during flares, quiescence and the dips seen in quiescence but it hardens during the post-flare dips. Optical photometry reveals a period of 4.32hr likely related to the binary orbit. Near-IR, possibly ellipsoidal, variations are detected. Large amplitude variability on shorter (tens mins) timescales are found to be non-periodic. The observed variability at all wavelengths and the spectral characteristics strongly favour a low-mass atypical low-luminosity X-ray binary and are against a Cataclysmic Variable nature. The association with a Fermi/LAT high energy gamma ray source further strengths this interpretation.
We report on gamma-ray analysis of the region containing the bright TeV source HESS J1640-465 and the close-by TeV source HESS J1641-463 using 64 months of observations with the Fermi Large Area Telescope (LAT). Previously only one GeV source was reported in this region and was associated with HESS J1640-465. With an increased dataset and the improved sensitivity afforded by the reprocessed data (P7REP) of the LAT, we now report the detection, morphological study and spectral analysis of two distinct sources above 100 MeV. The softest emission in this region comes from the TeV source HESS J164…
We present detailed broadband timing and spectral analysis of the persistent, low luminosity and slowly spinning pulsar X Persei using a deep Suzaku observation of the source. The spectrum is unusually hard with a cyclotron resonance scattering feature (CRSF), the presence of which has been debated. By comparing the spectral models relevant for accretion powered pulsars, we have obtained the best constraint on the broadband spectral model of X Persei obtained so far. The CRSF is not confirmed in the average spectrum. We have also identified, the presence of different intensity levels in the so…
A growing number of Be and Oe stars, named the gamma Cas stars, are known for their unusually hard and intense X-ray emission. This emission could either trace accretion by a compact companion or magnetic interaction between the star and its decretion disk. To test these scenarios, we carried out a detailed optical monitoring of HD45314, the hottest member of the class of gamma Cas stars, along with dedicated X-ray observations on specific dates. High-resolution optical spectra were taken to monitor the emission lines formed in the disk, while X-ray spectroscopy was obtained at epochs when the…
We present preliminary results obtained by a novel difference method for the study of the nature of the knee in the energy spectrum of the primary cosmic radiation. We have applied this method to data from the GAMMA experiment in Armenia. The analysis provides evidence for the possible existence of a nearby source of primary cosmic rays in the Southern hemisphere.
For a sample of Swift and Fermi GRBs, we show that the minimum variability timescale and the spectral lag of the prompt emission is related to the bulk Lorentz factor in a complex manner: For small $Γ$'s, the variability timescale exhibits a shallow (plateau) region. For large $Γ$'s, the variability timescale declines steeply as a function of $Γ$ ($δT\proptoΓ^{-4.05\pm0.64}$). Evidence is also presented for an intriguing correlation between the peak times, t$_p$, of the afterglow emission and the prompt emission variability timescale.
One of the main scientific objectives of the recent Fermi mission is investigating the origin of the unidentified gamma-ray sources (UGSs). Despite the large improvements of Fermi in the gamma-ray source localization with respect to the past gamma-ray missions, about 1/3 of the gamma-ray objects detected do not have yet an assigned counterpart a low energies. We recently developed a new association method to identify if there is a gamma-ray blazar candidate within the positional uncertainty region of a generic gamma-ray source. This method is entirely based on the discovery that blazars, the l…
About one third of the gamma-ray sources listed in the second Fermi LAT catalog (2FGL) have no firmly established counterpart at lower energies so being classified as unidentified gamma-ray sources (UGSs). Here we propose a new approach to find candidate counterparts for the UGSs based on the 325 MHz radio survey performed with Westerbork Synthesis Radio Telescope (WSRT) in the northern hemisphere. First we investigate the low-frequency radio properties of blazars, the largest known population of gamma-ray sources; then we search for sources with similar radio properties combining the informat…
Despite the large number of discoveries made recently by Fermi, the origins of the so called unidentified gamma-ray sources remain unknown. The large number of these sources suggests that among them there could be a population that significantly contributes to the isotropic gamma-ray background and is therefore crucial to understand their nature. The first step toward a complete comprehension of the unidentified gamma-ray source population is to identify those that can be associated with blazars, the most numerous class of extragalactic sources in the gamma-ray sky. Recently, we discovered tha…
Host galaxies are an excellent means of probing the natal environments that generate gamma-ray bursts (GRBs). Recent work on the host galaxies of short-duration GRBs has offered new insights into the parent stellar populations and ages of their enigmatic progenitors. Similarly, surveys of long-duration GRB (LGRB) host environments and their ISM properties have produced intriguing new results with important implications for long GRB progenitor models. These host studies are also critical in evaluating the utility of LGRBs as potential tracers of star formation and metallicity at high redshifts.…
According to the second Fermi LAT Catalog (2FGL), about one third of the gamma-ray sources listed have no assigned counterparts at lower energies. Many statistical methods have been developed to find proper counterparts for these sources. We explore the sky area covered at low radio frequency by Westerbork in the Southern Hemisphere (WISH) survey to search for blazar-like associations among the unidentified gamma-ray sources listed in the 2FGL (UGSs). Searching the WISH and NRAO VLA Sky Survey (NVSS) radio surveys within the positional uncertainty regions of the 2FGL UGSs, we select as gamma-r…
We report on gamma-ray orbital modulation of the transitioning millisecond pulsar binary XSS J12270-4859 detected in the Fermi Large Area Telescope (LAT) data. We use long-term optical data taken with the XMM-Newton Optical Monitor and the Swift UltraViolet Optical Telescope to inspect radio timing solutions that are limited to relatively short time intervals, and find that extrapolation of the solutions aligns well the phasing of the optical data over 15 years. The Fermi-LAT data folded on the timing solutions exhibit significant modulation (p = 5x10^-6) with a gamma-ray minimum at the inferi…
We report hints for the correlation between the X-ray and gamma-ray emission in the nearby galaxy NGC 4945, which harbors both an active galactic nucleus and a nuclear starburst region. We have divided the Fermi/LAT observations of NGC 4945 into two datasets, comprising events detected during the low (L) and high (H) level of X-ray emission from the active nucleus of this galaxy, determined using the Swift/BAT light curve. Both datasets contain an equal amount of 3.8 years of LAT data and NGC 4945 is detected with a similar statistical significance of ~15 $σ$ in L and 14 $σ$ in H. However, the…
A significant fraction (~30%) of the high-energy gamma-ray sources listed in the second Fermi LAT catalog (2FGL) are still of unknown origin, being not yet associated with counterparts at low energies. We recently developed a new association method to identify if there is a gamma-ray blazar candidate within the positional uncertainty region of a generic 2FGL source. This method is entirely based on the discovery that blazars have distinct infrared colors with respect to other extragalactic sources found thanks, to the Wide-field Infrared Survey Explorer (WISE) all-sky observations. Several imp…
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 M…
The nature of the first unidentified VHE gamma-ray source with significant angular offset from the Galactic plane of 3.5 degrees, HESS J1507-622, is explored. Fermi-LAT data in the high-energy (HE, 100 MeV < E < 100 GeV) gamma-ray range collected over 34 month are used to describe the spectral energy distribution (SED) of the source. HESS J1507-622 is detected in the Fermi energy range and its spectrum is best described by a power law in energy with Gamma=1.7 +/- 0.1 stat +/- 0.2_sys and integral flux between (0.3-300) GeV of F = (2.0 +/-0.5_stat +/- 1.0_sys) x 10^-9 cm^-2 s^-1. With the avail…
We present a new method for identifying blazar candidates by examining the locus, i.e. the region occupied by the Fermi gamma-ray blazars in the three-dimensional color space defined by the WISE infrared colors. This method is a refinement of our previous approach that made use of the two-dimensional projection of the distribution of WISE gamma-ray emitting blazars (the Strip) in the three WISE color-color planes (Massaro et al. 2012a). In this paper, we define the three-dimensional locus by means of a Principal Component (PCs) analysis of the colors distribution of a large sample of blazars c…
The shock wave of supernova remnants (SNRs) and the wind termination shock in pulsar wind nebula (PWNe) are considered as prime candidates to accelerate the bulk of Galactic cosmic ray (CR) ions and electrons. The SNRs hosting a PWN (known as composite SNRs) provide excellent laboratories to test these hypotheses. The SNR G327.1-1.1 belongs to this category and exhibits a shell and a bright central PWN, both seen in radio and X-rays. Interestingly, the radio observations of the PWN show an extended blob of emission and a curious narrow finger structure pointing towards the offset compact X-ray…
Before the launch of the Fermi satellite only two classes of Active Galactic Nuclei (AGN) were known to generate relativistic jets and thus to emit up to the gamma-ray energy range: blazars and radio galaxies, both hosted in giant elliptical galaxies. The first four years of observations by the Large Area Telescope (LAT) on board Fermi confirmed that these two populations represent the most numerous identified sources in the extragalactic gamma-ray sky, but the discovery of variable gamma-ray emission from 5 radio-loud Narrow-Line Seyfert 1 (NLSy1) galaxies revealed the presence of a possible …
Among the different types of sources shining in the high energy sky, gamma-ray binaries are rapidly becoming the subject of major interest. In fact, in the last few years a number of High Mass X-ray Binaries (HMXBs) have been firmly detected from MeV to TeV energies, providing secure evidences that particles can be efficiently accelerated up to very high energies in such galactic systems. Similarly to this general and emerging class of gamma-ray binaries, in principle Supergiant Fast X-ray Transients (SFXTs) have all the "ingredients" to be transient high energy emitters. In this context, the …
Blazar emission models are usually divided into two big families, leptonic and hadronic, according to the particles which are responsible for the gamma-ray component of their spectral energy distributions. Even though leptonic models have been successful in explaining the gamma-ray emission from most blazars, hadronic models still present an intriguing alternative. They have the unique advantage to link the emission of photons, neutrinos, and cosmic rays from the astrophysical source. We have developed a stationary one-zone lepto-hadronic code to model the emission from blazars in both leptoni…
We explore the potential of INTEGRAL to improve our understanding of the low fluence regime for explosive transients, such as GRBs. We probe the nature of the so-called "WEAK" INTEGRAL triggers, when the gamma-ray instruments record intensity spikes that are below the usual STRONG significance thresholds. In a targeted Swift follow-up campaign, we observed 15 WEAK triggers. We find six of these can be classified as GRBs. This includes GRB150305A, a GRB discovered from our campaign alone. We also identified a source coincident with one trigger, IGRW151019, as a candidate AGN. We show that real …
Very-high-energy (VHE, E > 100 GeV) gamma radiation has already been detected from several supernova remnants (SNRs). These objects, which are well-studied in radio, optical and X-ray wavelengths, constitute one of the most intriguing source classes in VHE astronomy. H.E.S.S., an array of four imaging atmospheric Cherenkov telescopes in Namibia, has recorded an extensive dataset of VHE gamma-ray observations covering the central region of the Milky Way, both from pointed observations as well as from the Galactic Plane Survey conducted in the inner region of the Galaxy. From radio observations,…
The Yale/SMARTS optical-near-IR monitoring program has followed the variations in emission of the Fermi-LAT monitored blazars in the southern sky with closely spaced observations since 2008. We report the discovery of an optical-near-IR (OIR) outburst with no accompanying gamma-rays in the blazar PKS 0208-512, one of the targets of this program. While the source undergoes three outbursts of 1 mag or more at OIR wavelengths lasting for longer than 3 months during 2008-2011, only interval 1 and 3 have corresponding bright phases in GeV energies lasting longer than 1 month. The OIR outburst durin…
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 th…
We discuss a very peculiar subgroup of gamma-ray bursts among the BATSE sources. These bursts are very short ($T_{90} \le $0.1 s), hard, and came predominantly from a restricted direction of the sky (close to the Galactic anti-center). We analyze their arrival times and possible correlations, as well as the profiles of individual bursts. We find no peculiarities in the arrival times of Very Short Bursts (VSBs) despite their highly non-uniform spatial distribution. There is no dependence in the burst shapes on location. Bursts coming both from the burst-enhancement Galactic Anticenter region an…
About 20 years ago we published the data concerning some peculiarities of the behavior of cosmic radiation EAS hadron component spectra. The results pointed to the possible existence in the interstellar space of the background of weakly interacting objects of the mass (the energy of the resonance oscillations) of the order of 37 eV. The absence of such background particle creation in accelerator experiments may mean that it is not the elementary particle but the object of some different nature. On the other hand, the experimental data of the last years are pointing to the existence of cosmic g…
In this talk I present the main spectral and temporal properties of Fermi/GBM gamma-ray bursts (GRBs) with known redshift. Key properties of these GRBs in the rest-frame of the progenitor are investigated to better understand the intrinsic nature of these events. The sample comprises 47 GRBs with measured redshift that were observed by GBM until May 2012. 39 sources belong to the long-duration population and 8 events were classified as short bursts. For all of these events we derive, where possible, the intrinsic peak energy in the νFν spectrum (Ep,rest), the duration in the rest-frame, define…
Nearly one-third of the gamma-ray sources detected by Fermi are still unidentified, despite significant recent progress in this effort. On the other hand, all the gamma-ray extragalactic sources associated in the second Fermi-LAT catalog have a radio counterpart. Motivated by this observational evidence we investigate all the radio sources of the major radio surveys that lie within the positional uncertainty region of the unidentified gamma-ray sources (UGSs) at 95% level of confidence. First we search for their infrared counterparts in the all-sky survey performed by the Wide-field Infrared S…
Until now the existence of extended emission is an intriguing property of short bursts. It is not clear what is the nature of the extended emission. It might be a rising x-ray afterglow, or it could be a manifestation of the prolonged activity of a central engine. We consider short duration gamma-ray bursts, emphasizing the common properties of short bursts and short burst with extended emission. Assuming that the extended emission with broad dynamic range is a common property of short bursts, we propose a two jet model which can describe both short main episode of hard spectra emission, speci…
We present Fermi-LAT and multi-frequency, multi-epoch VLBA data for the TeV blazar Mrk421. We collected the data during a long and intensive multi-frequency campaign in 2011. We study the gamma-ray light curve, the photon index evolution and their connection to the radio data on sub-parsec scales, including total intensity, polarized flux density, polarization angle, spectral index, and rotation measure both in the core and the jet region. The VLBA data were obtained at 15 and 24 GHz for 12 epochs and at 43 GHz for 23 epochs, thus providing the best temporal and spatial coverage in the radio b…
Context: Thirteen years after the discovery of the first afterglows, the nature of dark gamma-ray bursts (GRB) still eludes explanation: while each ng-duration GRB typically has an X-ray afterglow, optical/NIR emission is only seen for 40-60% of them. Aim: Here we use the afterglow detection statistics of the systematic follow-up observations performed with GROND since mid-2007 in order to derive the fraction of "dark bursts" according to different methods, and to distinguish between various scenarios for "dark bursts". Method: Observations were performed with the 7-channel "Gamma-Ray Optical …
Variable gamma-ray emission has been discovered in five Radio-loud Narrow Line Seyfert 1 (NLSy1) galaxies by the Large Area Telescope (LAT) onboard the Fermi Gamma-Ray Space Telescope. This has clearly demonstrated that these NLSy1 galaxies do have relativistic jets similar to two other cases of gamma-ray emitting Active Galactic Nuclei (AGN), namely blazars and radio galaxies. We present here our results on the multi-band analysis of two gamma-ray emitting NLSy1 galaxies namely PKS 1502+036 (z = 0.409) and PKS 2004-447 (z = 0.240) using archival data. We generate multi-band long term light cu…
The black hole transient GRS 1716-249 was monitored from the radio to the gamma-ray band during its 2016-2017 outburst. This paper focuses on the Spectral Energy Distribution (SED) obtained in 2017 February-March, when GRS 1716-249 was in a bright hard spectral state. The soft gamma-ray data collected with the INTEGRAL/SPI telescope show the presence of a spectral component which is in excess of the thermal Comptonisation emission. This component is usually interpreted as inverse Compton emission from a tiny fraction of non-thermal electrons in the X-ray corona. We find that hybrid thermal/non…
The Italian gamma-ray satellite AGILE has recently reported the detection of some variable high-energy sources likely of galactic origin. These sources do not have any obvious counterpart at lower energies. We propose that these sources are produced in proton-dominated jets of galactic microquasars. We develop a model for microquasar jets that takes into account both primary leptons and protons and all relevant radiative processes, including secondary particle emission and gamma-ray attenuation due to pair creation in the jet. We obtain spectral energy distributions that correspond to what is …
Host galaxies are an excellent means of probing the natal environments that generate gamma-ray bursts (GRBs). Surveys of long-duration GRB (LGRB) host environments and their ISM properties have produced intriguing new results with important implications for LGRB progenitor models. These host studies are also critical in evaluating the utility of LGRBs as potential tracers of star formation and metallicity at high redshifts, particularly when considering the implications for properties of host galaxies above z ~ 6. I will summarize our group's latest research on LGRB host galaxies, and discuss …
One of the most intriguing and challenging quests of current astrophysics is to understand the physical conditions and processes responsible for production of high-energy particles, and emission of γ-rays. A combination of high-resolution Very Long Baseline Interferometry (VLBI) images with broadband flux variability measurements is a unique way to probe the emission mechanisms at the bases of jets. Our analysis of γ-ray flux variability observed by the Fermi-LAT (Large Area Telescope) along with the parsec-scale jet kinematics suggests that the $γ$-ray emission in blazar S5 0716+714 has a sig…
A significant fraction ($\sim 30$ %) of the high-energy gamma-ray sources listed in the second $\textit{Fermi}$ LAT (2FGL) catalog are still of unknown origin, being not yet associated with counterparts at lower energies. In order to investigate the nature of these enigmatic sources, we present here an extensive search of X-ray sources lying in the positional uncertainty region of a selected sample of these Unidentified Gamma-ray Sources (UGSs) that makes use of all available observations performed by the $\textit{Swift}$ X-ray Telescope before March 31, 2013, available for 205 UGSs. To detect…
We have discovered remarkable jet- and arc-like molecular features toward the rich and young stellar cluster Westerlund2. The jet has a length of ~100 pc and a width of ~10 pc, while the arc shows a crescent shape with a radius of ~30 pc. These molecular features each have masses of ~10000 solar mass and show spatial correlations with the surrounding lower density HI gas. The jet also shows an intriguing positional alignment with the core of the TeV gamma ray source HESS J1023-575 and with the MeV/GeV gamma-ray source recently reported by the Fermi collaboration. We argue that the jet and arc …
We report the results of our exploratory program carried out with the Southern Astrophysical Research (SOAR) telescope aimed at associating counterparts and establishing the nature of the Fermi Unidentified gamma-ray Sources (UGS). We selected the optical counterparts of 6 UGSs from the Fermi catalog on the basis of our recently discovered tight connection between infrared and gamma-ray emission found for the gamma-ray blazars detected by the Wide-Field Infrared Survey Explorer (WISE) in its the all-sky survey. We perform for the first time a spectroscopic study of the low-energy counterparts …
We report on the results of deep X-ray follow-up observations of four unidentified Fermi/LAT gamma-ray sources at high Galactic latitudes using Suzaku. The studied objects were detected with high significance during the first 3 months of Fermi/LAT operation, and subsequently better localized in the Fermi/LAT 1 year catalog (1FGL). Possible associations with pulsars and active galaxies have subsequently been discussed, and our observations provide an important contribution to this debate. In particular, an X-ray point source was found within the 95% confidence error circle of 1FGL J1231.1-1410.…
PKS 0521-36 is an Active Galactic Nucleus (AGN) with uncertain classification. We investigate the properties of this source from radio to gamma rays. The broad emission lines in the optical and UV bands and steep radio spectrum indicate a possible classification as an intermediate object between broad-line radio galaxies (BLRG) and steep spectrum radio quasars (SSRQ). On pc-scales PKS 0521-36 shows a knotty structure similar to misaligned AGN. The core dominance and the gamma-ray properties are similar to those estimated for other SSRQ and BLRG detected in gamma rays, suggesting an intermediat…
Here we report our first spectral observations of Be/X-ray and gamma-ray binaries obtained with the new Echelle spectrograph of the National Astronomical Observatory Rozhen. For four objects (LSI+61303, gamma Cas, MWC 148, 4U 2206+54), we report the parameters and estimate the sizes of their circumstellar discs using different emission lines (H-alpha, H-beta, H-gamma, HeI and FeII). For MWC 148, we find that the compact object goes deeply through the disc. The flank inflections of H-alpha can be connected with inner ring formed at the periastron passage or radiation transfer effects. We point …
Motivated by the recent indications for a 750 GeV resonance in the di-photon final state at the LHC, in this work we analyse the compatibility of the excess with the broad photon excess detected at the Galactic Centre. Intriguingly, by analysing the parameter space of an effective models where a 750 GeV pseudoscalar particles mediates the interaction between the Standard Model and a scalar dark sector, we prove the compatibility of the two signals. We show, however, that the LHC mono-jet searches and the Fermi LAT measurements strongly limit the viable parameter space. We comment on the possib…
We review the multiwavelength properties of the few known gamma-ray binaries, focusing on extended emission recently resolved with Chandra. We discuss the implications of these findings for the nature of compact objects and for physical processes operating in these systems.
Relativistic jets are one of the most powerful manifestations of the release of energy produced around supermassive black holes at the centre of active galactic nuclei (AGN). Their emission is observed across the entire electromagnetic spectrum, from the radio band to gamma rays. Despite decades of efforts, many aspects of the physics of relativistic jets remain elusive. In particular, the location and the mechanisms responsible for the high-energy emission and the connection of the variability at different wavelengths are among the greatest challenges in the study of AGN. From the comparison …
The Fermi Large Area Telescope gamma-ray source 3FGL J2039.6$-$5618 contains a periodic optical and X-ray source that was predicted to be a "redback" millisecond pulsar (MSP) binary system. However, the conclusive identification required the detection of pulsations from the putative MSP. To better constrain the orbital parameters for a directed search for gamma-ray pulsations, we obtained new optical light curves in 2017 and 2018, which revealed long-term variability from the companion star. The resulting orbital parameter constraints were used to perform a targeted gamma-ray pulsation search …
SGR J1550-5418 exhibited three active bursting episodes in 2008 October and in 2009 January and March, emitting hundreds of typical Soft Gamma Repeater (SGR) bursts in soft gamma-rays. The second episode was especially intense, and our untriggered burst search on Fermi/GBM data (8-1000 keV) revealed ~450 bursts emitted over 24 hours during the peak of this activity. Using the GBM data, we identified a ~150-s-long enhanced persistent emission during 2009 January 22 that exhibited intriguing timing and spectral properties: (i) clear pulsations up to ~110 keV at the spin period of the neutron sta…
We recently developed a procedure to recognize gamma-ray blazar candidates within the positional uncertainty regions of the unidentified/unassociated gamma-ray sources (UGSs). Such procedure was based on the discovery that Fermi blazars show peculiar infrared colors. However, to confirm the real nature of the selected candidates, optical spectroscopic data are necessary. Thus, we performed an extensive archival search for spectra available in the literature in parallel with an optical spectroscopic campaign aimed to reveal and confirm the nature of the selected gamma-ray blazar candidates. Her…
Non-blazar Active Galactic Nuclei (AGN) have emerged as a new gamma-ray emitting source class on the extragalactic sky and started to deepen our understanding of the physical processes and the nature of AGN in general. The detection of Narrow Line Seyfert 1 galaxies in the Fermi-LAT energy regime, for example, offers important information for our understanding of jet formation and radio-loudness. Radio galaxies, on the other hand, have become particularly interesting at high (HE) and very high (VHE) gamma-ray energies. With their jets not directly pointing towards us (i.e. misaligned), they of…
The nature of the recently detected HE and VHE gamma-ray emission of Tycho's supernova remnant (SNR) is studied. A nonlinear kinetic theory of cosmic ray (CR) acceleration in supernova remnants (SNRs) is employed to investigate the properties of Tycho's SNR and their correspondence to the existing experimental data, taking into account that the ambient interstellar medium (ISM) is expected to be clumpy. It is demonstrated that the overall steep gamma-ray spectrum observed can be interpreted as the superposition of two spectra produced by the CR proton component in two different ISM phases: The…
We present a broadband study of gamma-ray burst (GRB) 091024A within the context of other ultra-long-duration GRBs. An unusually long burst detected by Konus-Wind, Swift, and Fermi, GRB 091024A has prompt emission episodes covering ~1300 s, accompanied by bright and highly structured optical emission captured by various rapid-response facilities, including the 2-m autonomous robotic Faulkes North and Liverpool Telescopes, KAIT, S-LOTIS, and SRO. We also observed the burst with 8- and 10-m class telescopes and determine the redshift to be z = 1.0924 \pm 0.0004. We find no correlation between th…
The Fermi bubbles are one of the most remarkable features in the gamma-ray sky revealed by the Fermi Large Area Telescope (LAT). The nature of the gamma-ray emission and the origin of the bubbles are still open questions. In this note, we will review some basic features of leptonic and hadronic modes of gamma-ray production. At the moment, gamma rays are our best method to study the bubbles, but in order to resolve the origin of the bubbles multi-wavelength and multi-messenger observations will be crucial.
New generation TeV gamma-ray telescopes have discovered many new sources, including several enigmatic unidentified TeV objects. HESS J0632+057 is a particularly interesting unidentified TeV source since: it is a point source, it has a possible hard-spectrum X-ray counterpart and a positionally consistent Be star, it has evidence of long-term VHE flux variability, and it is postulated to be a newly detected TeV/X-ray binary. We have obtained Swift X-ray telescope observations of this source from MJD 54857 to 54965, in an attempt to ascertain its nature and to investigate the hypothesis that it'…
We present an analysis of the optical spectra available in the Sloan Digital Sky survey data release nine (SDSS DR9) for the blazars listed in the ROMA-BZCAT and for the gamma-ray blazar candidates selected according to their IR colors. First, we adopt a statistical approach based on MonteCarlo simulations to find the optical counterparts of the blazarslisted in the ROMA-BZCAT catalog. Then we crossmatched the SDSS spectroscopic catalog with our selected samples of blazars and gamma-ray blazar candidates searching for those with optical spectra available to classify our blazar-like sources and…
