Detection of persistent VHE gamma-ray emission from PKS 1510-089 by the MAGIC telescopes during low states between 2012 and 2017

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 gammaray 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 emissio-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 x10(-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 (9.50-sigma ) VHE gamma-ray emission at the level of (4.27 +/- 0.61(stat)) x 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

Periastron Observations of TeV Gamma-Ray Emission from a Binary System with a 50-year Period

We report on observations of the pulsar/Be star binary system PSR J2032+4127/MT91 213 in the energy range between 100 GeV and 20 TeV 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 0.5 TeV. 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

Study of the variable broadband emission of Markarian 501 during the most extreme Swift X-ray activity

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 similar to 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 Metsahovi, 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 similar to 0.5 to similar 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 similar to 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 sigma. A fit with a log-parabola plus a narrow component is preferred over the fit with a single log-parabola at more than 4 sigma, and a dedicated Monte Carlo simulation estimated the significance of this extra component to be larger than 3 sigma. 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.</div