Abstract
Extreme high-frequency-peaked BL Lac objects (EHBLs) are blazars that exhibit extremely energetic synchrotron emission. They also feature nonthermal gamma-ray emission whose peak lies in the ...very high-energy (VHE,
E
> 100 GeV) range, and in some sources exceeds 1 TeV: this is the case for hard-TeV EHBLs such as 1ES 0229+200. With the aim of increasing the EHBL population, 10 targets were observed with the MAGIC telescopes from 2010 to 2017, for a total of 265 hr of good-quality data. The data were complemented by coordinated
Swift
observations. The X-ray data analysis confirms that all but two sources are EHBLs. The sources show only a modest variability and a harder-when-brighter behavior, typical for this class of objects. At VHE gamma-rays, three new sources were detected and a hint of a signal was found for another new source. In each case, the intrinsic spectrum is compatible with the hypothesis of a hard-TeV nature of these EHBLs. The broadband spectral energy distributions (SEDs) of all sources are built and modeled in the framework of a single-zone, purely leptonic model. The VHE gamma-ray-detected sources were also interpreted with a spine–layer model and a proton synchrotron model. The three models provide a good description of the SEDs. However, the resulting parameters differ substantially in the three scenarios, in particular the magnetization parameter. This work presents the first mini catalog of VHE gamma-ray and multiwavelength observations of EHBLs.
ABSTRACT
M 87 is one of the closest (z = 0.004 36) extragalactic sources emitting at very high energies (VHE, E > 100 GeV). The aim of this work is to locate the region of the VHE gamma-ray emission ...and to describe the observed broad-band spectral energy distribution (SED) during the low VHE gamma-ray state. The data from M 87 collected between 2012 and 2015 as part of a MAGIC monitoring programme are analysed and combined with multiwavelength data from Fermi-LAT, Chandra, HST, EVN, VLBA, and the Liverpool Telescope. The averaged VHE gamma-ray spectrum can be fitted from ∼100 GeV to ∼10 TeV with a simple power law with a photon index of (−2.41 ± 0.07), while the integral flux above 300 GeV is $(1.44\pm 0.13)\times 10^{-12}\, \mathrm{cm}^{-2}\, \mathrm{s}^{-1}$. During the campaign between 2012 and 2015, M 87 is generally found in a low-emission state at all observed wavelengths. The VHE gamma-ray flux from the present 2012–2015M 87 campaign is consistent with a constant flux with some hint of variability ($\sim 3\, \sigma$) on a daily time-scale in 2013. The low-state gamma-ray emission likely originates from the same region as the flare-state emission. Given the broad-band SED, both a leptonic synchrotron self-Compton and a hybrid photohadronic model reproduce the available data well, even if the latter is preferred. We note, however, that the energy stored in the magnetic field in the leptonic scenario is very low, suggesting a matter-dominated emission region.
Context.
Markarian 501 (Mrk 501) is a very high-energy (VHE) gamma-ray blazar located at
z
= 0.034, which is regularly monitored by a wide range of multi-wavelength instruments, from radio to VHE ...gamma rays. During a period of almost two weeks in July 2014, the highest X-ray activity of Mrk 501 was observed in ∼14 years of operation of the
Neil Gehrels Swift
Gamma-ray Burst Observatory.
Aims.
We characterize the broadband variability of Mrk 501 from radio to VHE gamma rays during the most extreme X-ray activity measured in the last 14 years, and evaluate whether it can be interpreted within theoretical scenarios widely used to explain the broadband emission from blazars.
Methods.
The emission of Mrk 501 was measured at radio with Metsähovi, at optical–UV with KVA and
Swift
/UVOT, at X-ray with
Swift
/XRT and
Swift
/BAT, at gamma ray with
Fermi
-LAT, and at VHE gamma rays with the FACT and MAGIC telescopes. The multi-band variability and correlations were quantified, and the broadband spectral energy distributions (SEDs) were compared with predictions from theoretical models.
Results.
The VHE emission of Mrk 501 was found to be elevated during the X-ray outburst, with a gamma-ray flux above 0.15 TeV varying from ∼0.5 to ∼2 times the Crab nebula flux. The X-ray and VHE emission both varied on timescales of 1 day and were found to be correlated. We measured a general increase in the fractional variability with energy, with the VHE variability being twice as large as the X-ray variability. The temporal evolution of the most prominent and variable segments of the SED, characterized on a day-by-day basis from 2014 July 16 to 2014 July 31, is described with a one-zone synchrotron self-Compton model with variations in the break energy of the electron energy distribution (EED), and with some adjustments in the magnetic field strength and spectral shape of the EED. These results suggest that the main flux variations during this extreme X-ray outburst are produced by the acceleration and the cooling of the high-energy electrons. A narrow feature at ∼3 TeV was observed in the VHE spectrum measured on 2014 July 19 (MJD 56857.98), which is the day with the highest X-ray flux (>0.3 keV) measured during the entire
Swift
mission. This feature is inconsistent with the classical analytic functions to describe the measured VHE spectra (power law, log-parabola, and log-parabola with exponential cutoff) at more than 3
σ
. A fit with a log-parabola plus a narrow component is preferred over the fit with a single log-parabola at more than 4
σ
, and a dedicated Monte Carlo simulation estimated the significance of this extra component to be larger than 3
σ
. Under the assumption that this VHE spectral feature is real, we show that it can be reproduced with three distinct theoretical scenarios: (a) a pileup in the EED due to stochastic acceleration; (b) a structured jet with two-SSC emitting regions, with one region dominated by an extremely narrow EED; and (c) an emission from an IC pair cascade induced by electrons accelerated in a magnetospheric vacuum gap, in addition to the SSC emission from a more conventional region along the jet of Mrk 501.
Fast radio bursts (FRBs) are bright flashes observed typically at GHz frequencies with millisecond duration, whose origin is likely extragalactic. Their nature remains mysterious, motivating searches ...for counterparts at other wavelengths. FRB 121102 is so far the only source known to repeatedly emit FRBs and is associated with a host galaxy at redshift z ≃ 0.193.We conducted simultaneous observations of FRB 121102 with the Arecibo and MAGIC telescopes during several epochs in 2016-2017. This allowed searches for millisecond time-scale burst emission in very-high-energy (VHE) gamma-rays as well as the optical band. While a total of five FRBs were detected during these observations, no VHE emission was detected, neither of a persistent nature nor burst-like associated with the FRBs. The average integral flux upper limits above 100 GeV at 95 per cent confidence level are 6.6 × 10 -12 photons cm -2 s -1 (corresponding to luminosity LVHE ≲ 10 45 erg s -1 ) over the entire observation period, and 1.2 × 10 -7 photons cm -2 s -1 (LVHE ≳ 10 49 erg s -1 ) over the total duration of the five FRBs. We constrain the optical U-band flux to be below 8.6 mJy at 5σ level for 1-ms intervals around the FRB arrival times. A bright burst with U-band flux 29 mJy and duration ~12 ms was detected 4.3 s before the arrival of one FRB. However, the probability of spuriously detecting such a signal within the sampled time space is 1.5 per cent (2.2, post-trial), i.e. consistent with the expected background. We discuss the implications of the obtained upper limits for constraining FRB models.
We report the detection of pulsed gamma-ray emission from the Geminga pulsar (PSR J0633+1746) between 15 GeV and 75 GeV. This is the first time a middle-aged pulsar has been detected up to these ...energies. Observations were carried out with the MAGIC telescopes between 2017 and 2019 using the low-energy threshold Sum-Trigger-II system. After quality selection cuts, ∼80 h of observational data were used for this analysis. To compare with the emission at lower energies below the sensitivity range of MAGIC, 11 years of
Fermi
-LAT data above 100 MeV were also analysed. From the two pulses per rotation seen by
Fermi
-LAT, only the second one,
P
2, is detected in the MAGIC energy range, with a significance of 6.3
σ
. The spectrum measured by MAGIC is well-represented by a simple power law of spectral index Γ = 5.62 ± 0.54, which smoothly extends the
Fermi
-LAT spectrum. A joint fit to MAGIC and
Fermi
-LAT data rules out the existence of a sub-exponential cut-off in the combined energy range at the 3.6
σ
significance level. The power-law tail emission detected by MAGIC is interpreted as the transition from curvature radiation to Inverse Compton Scattering of particles accelerated in the northern outer gap.
Aims.
We measure the Crab Nebula
γ
-ray spectral energy distribution in the ~100 TeV energy domain and test the validity of existing leptonic emission models at these high energies.
Methods.
We used ...the novel very large zenith angle observations with the MAGIC telescope system to increase the collection area above 10 TeV. We also developed an auxiliary procedure of monitoring atmospheric transmission in order to assure proper calibration of the accumulated data. This employs recording optical images of the stellar field next to the source position, which provides a better than 10% accuracy for the transmission measurements.
Results.
We demonstrate that MAGIC very large zenith angle observations yield a collection area larger than a square kilometer. In only ~ 56 h of observations, we detect the
γ
-ray emission from the Crab Nebula up to 100 TeV, thus providing the highest energy measurement of this source to date with Imaging Atmospheric Cherenkov Telescopes. Comparing accumulated and archival MAGIC and
Fermi
/LAT data with some of the existing emission models, we find that none of them provides an accurate description of the 1 GeV to 100 TeV
γ
-ray signal.
Bound-state double-beta decay Babič, A; Štefánik, D; Krivoruchenko, M I ...
Journal of physics. Conference series,
07/2018, Letnik:
1056, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We study a new mode of the neutrinoless and two-neutrino double-beta decays in which a single electron is emitted from the atom. The other electron is assumed to occupy one of the vacant s1/2 or p1/2 ...subshells of the daughter ion. Such process could manifest itself through an additional background signal in the single-electron spectra, which will be accessible in the next-generation experiment SuperNEMO. We calculate the phase-space factors in terms of relativistic electron wave functions obtained as the solutions to the Dirac equation and evaluated at the nuclear radius, while taking into account the shielding effect of nuclear charge via the multiconfiguration Dirac-Hartree-Fock package Grasp2K. Half-lives are estimated for the most relevant double-beta-decay isotopes and experimental significance is discussed.
We report on observations of the pulsar/Be star binary system PSR J2032+4127/MT91 213 in the energy range between and with the Very Energetic Radiation Imaging Telescope Array and Major Atmospheric ...Gamma Imaging Cherenkov telescope arrays. The binary orbit has a period of approximately 50 years, with the most recent periastron occurring on 2017 November 13. Our observations span from 18 months prior to periastron to one month after. A new point-like gamma-ray source is detected, coincident with the location of PSR J2032+4127/MT91 213. The gamma-ray light curve and spectrum are well characterized over the periastron passage. The flux is variable over at least an order of magnitude, peaking at periastron, thus providing a firm association of the TeV source with the pulsar/Be star system. Observations prior to periastron show a cutoff in the spectrum at an energy around . This result adds a new member to the small population of known TeV binaries, and it identifies only the second source of this class in which the nature and properties of the compact object are firmly established. We compare the gamma-ray results with the light curve measured with the X-ray Telescope on board the Neil Gehrels Swift Observatory and with the predictions of recent theoretical models of the system. We conclude that significant revision of the models is required to explain the details of the emission that we have observed, and we discuss the relationship between the binary system and the overlapping steady extended source, TeV J2032+4130.
The measurement of the absolute rate of cerebral metabolic oxygen consumption (CMRO2) is likely to offer a valuable biomarker in many brain diseases and could prove to be important in our ...understanding of neural function. As such there is significant interest in developing robust MRI techniques that can quantify CMRO2 non-invasively. One potential MRI method for the measurement of CMRO2 is via the combination of fMRI and cerebral blood flow (CBF) data acquired during periods of hypercapnic and hyperoxic challenges. This method is based on the combination of two, previously independent, signal calibration techniques. As such analysis of the data has been approached in a stepwise manner, feeding the results of one calibration experiment into the next. Analysing the data in this manner can result in unstable estimates of the output parameter (CMRO2), due to the propagation of errors along the analysis pipeline. Here we present a forward modelling approach that estimates all the model parameters in a one-step solution. The method is implemented using a regularized non-linear least squares approach to provide a robust and computationally efficient solution. The proposed framework is compared with previous analytical approaches using modelling studies and in vivo acquisitions in healthy volunteers (n=10). The stability of parameter estimates is demonstrated to be superior to previous methods (both in vivo and in simulation). In vivo estimates made with the proposed framework also show better agreement with expected physiological variation, demonstrating a strong negative correlation between baseline CBF and oxygen extraction fraction. It is anticipated that the proposed analysis framework will increase the reliability of absolute CMRO2 measurements made with calibrated BOLD.
•We propose a framework for the analysis of calibrated fMRI to measure baseline OEF.•We examine the framework with modelling studies and in vivo acquisitions.•The new approach is shown to be more robust than previous analytical methods.
Context. PKS 1510–089 is a flat spectrum radio quasar strongly variable in the optical and GeV range. To date, very high-energy (VHE, > 100 GeV) emission has been observed from this source either ...during long high states of optical and GeV activity or during short flares. Aims. We search for low-state VHE gamma-ray emission from PKS 1510–089. We characterize and model the source in a broadband context, which would provide a baseline over which high states and flares could be better understood. Methods. PKS 1510–089 has been monitored by the MAGIC telescopes since 2012. We use daily binned Fermi-LAT flux measurements of PKS 1510–089 to characterize the GeV emission and select the observation periods of MAGIC during low state of activity. For the selected times we compute the average radio, IR, optical, UV, X-ray, and gamma-ray emission to construct a low-state spectral energy distribution of the source. The broadband emission is modeled within an external Compton scenario with a stationary emission region through which plasma and magnetic fields are flowing. We also perform the emission-model-independent calculations of the maximum absorption in the broad line region (BLR) using two different models. Results. The MAGIC telescopes collected 75 hr of data during times when the Fermi-LAT flux measured above 1 GeV was below 3 × 10−8 cm−2 s−1, which is the threshold adopted for the definition of a low gamma-ray activity state. The data show a strongly significant (9.5σ) VHE gamma-ray emission at the level of (4.27 ± 0.61stat) × 10−12 cm−2 s−1 above 150 GeV, a factor of 80 lower than the highest flare observed so far from this object. Despite the lower flux, the spectral shape is consistent with earlier detections in the VHE band. The broadband emission is compatible with the external Compton scenario assuming a large emission region located beyond the BLR. For the first time the gamma-ray data allow us to place a limit on the location of the emission region during a low gamma-ray state of a FSRQ. For the used model of the BLR, the 95% confidence level on the location of the emission region allows us to place it at a distance > 74% of the outer radius of the BLR.
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