arXiv subjects

M. Böttcher

Publications and source records attributed to M. Böttcher.

At least 73 records · Page 4Linked to original sources

arXiv2021

Multi-messenger and transient astrophysics with the Cherenkov Telescope Array

The discovery of gravitational waves, high-energy neutrinos or the very-high-energy counterpart of gamma-ray bursts has revolutionized the high-energy and transient astrophysics community. The development of new instruments and analysis techniques will allow the discovery and/or follow-up of new transient sources. We describe the prospects for the Cherenkov Telescope Array (CTA), the next-generation ground-based gamma-ray observatory, for multi-messenger and transient astrophysics in the decade ahead. CTA will explore the most extreme environments via very-high-energy observations of compact objects, stellar collapse events, mergers and cosmic-ray accelerators.

astro-ph.HE↗

arXiv2021

Probing extreme environments with the Cherenkov Telescope Array

The physics of the non-thermal Universe provides information on the acceleration mechanisms in extreme environments, such as black holes and relativistic jets, neutron stars, supernovae or clusters of galaxies. In the presence of magnetic fields, particles can be accelerated towards relativistic energies. As a consequence, radiation along the entire electromagnetic spectrum can be observed, and extreme environments are also the most likely sources of multi-messenger emission. The most energetic part of the electromagnetic spectrum corresponds to the very-high-energy (VHE, E>100 GeV) gamma-ray regime, which can be extensively studied with ground based Imaging Atmospheric Cherenkov Telescopes (IACTs). The results obtained by the current generation of IACTs, such as H.E.S.S., MAGIC, and VERITAS, demonstrate the crucial importance of the VHE band in understanding the non-thermal emission of extreme environments in our Universe. In some objects, the energy output in gamma rays can even outshine the rest of the broadband spectrum. The Cherenkov Telescope Array (CTA) is the next generation of IACTs, which, with cutting edge technology and a strategic configuration of ~100 telescopes distributed in two observing sites, in the northern and southern hemispheres, will reach better sensitivity, angular and energy resolution, and broader energy coverage than currently operational IACTs. With CTA we can probe the most extreme environments and considerably boost our knowledge of the non-thermal Universe.

astro-ph.IM↗

arXiv2021

First direct neutrino-mass measurement with sub-eV sensitivity

We report the results of the second measurement campaign of the Karlsruhe Tritium Neutrino (KATRIN) experiment. KATRIN probes the effective electron anti-neutrino mass, $m_ν$, via a high-precision measurement of the tritium $β$-decay spectrum close to its endpoint at $18.6\,\mathrm{keV}$. In the second physics run presented here, the source activity was increased by a factor of 3.8 and the background was reduced by $25\,\%$ with respect to the first campaign. A sensitivity on $m_ν$ of $0.7\,\mathrm{eV/c^2}$ at $90\,\%$ confidence level (CL) was reached. This is the first sub-eV sensitivity from a direct neutrino-mass experiment. The best fit to the spectral data yields $m_ν^2 = (0.26\pm0.34)\,\mathrm{eV^4/c^4}$, resulting in an upper limit of $m_ν<0.9\,\mathrm{eV/c^2}$ ($90\,\%$ CL). By combining this result with the first neutrino mass campaign, we find an upper limit of $m_ν<0.8\,\mathrm{eV/c^2}$ ($90\,\%$ CL).

hep-ex↗

arXiv2021

Precision measurement of the electron energy-loss function in tritium and deuterium gas for the KATRIN experiment

The KATRIN experiment is designed for a direct and model-independent determination of the effective electron anti-neutrino mass via a high-precision measurement of the tritium $β$-decay endpoint region with a sensitivity on $m_ν$ of 0.2$\,$eV/c$^2$ (90% CL). For this purpose, the $β$-electrons from a high-luminosity windowless gaseous tritium source traversing an electrostatic retarding spectrometer are counted to obtain an integral spectrum around the endpoint energy of 18.6$\,$keV. A dominant systematic effect of the response of the experimental setup is the energy loss of $β$-electrons from elastic and inelastic scattering off tritium molecules within the source. We determined the \linebreak energy-loss function in-situ with a pulsed angular-selective and monoenergetic photoelectron source at various tritium-source densities. The data was recorded in integral and differential modes; the latter was achieved by using a novel time-of-flight technique. We developed a semi-empirical parametrization for the energy-loss function for the scattering of 18.6-keV electrons from hydrogen isotopologs. This model was fit to measurement data with a 95% T$_2$ gas mixture at 30$\,$K, as used in the first KATRIN neutrino mass analyses, as well as a D$_2$ gas mixture of 96% purity used in KATRIN commissioning runs. The achieved precision on the energy-loss function has abated the corresponding uncertainty of $σ(m_ν^2)<10^{-2}\,\mathrm{eV}^2$ [arXiv:2101.05253] in the KATRIN neutrino-mass measurement to a subdominant level.

physics.ins-det↗

arXiv2021

Analysis methods for the first KATRIN neutrino-mass measurement

We report on the data set, data handling, and detailed analysis techniques of the first neutrino-mass measurement by the Karlsruhe Tritium Neutrino (KATRIN) experiment, which probes the absolute neutrino-mass scale via the $β$-decay kinematics of molecular tritium. The source is highly pure, cryogenic T$_2$ gas. The $β$ electrons are guided along magnetic field lines toward a high-resolution, integrating spectrometer for energy analysis. A silicon detector counts $β$ electrons above the energy threshold of the spectrometer, so that a scan of the thresholds produces a precise measurement of the high-energy spectral tail. After detailed theoretical studies, simulations, and commissioning measurements, extending from the molecular final-state distribution to inelastic scattering in the source to subtleties of the electromagnetic fields, our independent, blind analyses allow us to set an upper limit of 1.1 eV on the neutrino-mass scale at a 90\% confidence level. This first result, based on a few weeks of running at a reduced source intensity and dominated by statistical uncertainty, improves on prior limits by nearly a factor of two. This result establishes an analysis framework for future KATRIN measurements, and provides important input to both particle theory and cosmology.

hep-ex↗

arXiv2021

Search for dark matter annihilation in the dwarf irregular galaxy WLM with H.E.S.S

We search for an indirect signal of dark matter through very high-energy gamma rays from the Wolf-Lundmark-Melotte (WLM) dwarf irregular galaxy. The pair annihilation of dark matter particles would produce Standard Model particles in the final state such as gamma rays, which might be detected by ground-based Cherenkov telescopes. Dwarf irregular galaxies represent promising targets as they are dark matter dominated objects with well measured kinematics and small uncertainties on their dark matter distribution profiles. In 2018, the H.E.S.S. five-telescope array observed the dwarf irregular galaxy WLM for 18 hours. We present the first analysis based on data obtained from an imaging atmospheric Cherenkov telescope for this subclass of dwarf galaxy. As we do not observe any significant excess in the direction of WLM, we interpret the result in terms of constraints on the velocity-weighted cross section for dark matter pair annihilation as a function of the dark matter particle mass for various continuum channels as well as the prompt gamma-gamma emission. For the $τ^+τ^-$ channel the limits reach a $\langle σv \rangle$ value of about $4\times 10^{-22}$ cm3s-1 for a dark matter particle mass of 1 TeV. For the prompt gamma-gamma channel, the upper limit reaches a $\langle σv \rangle$ value of about $5 \times10^{-24}$ cm3s-1 for a mass of 370 GeV. These limits represent an improvement of up to a factor 200 with respect to previous results for the dwarf irregular galaxies for TeV dark matter search.

astro-ph.HE↗

arXiv2021

Sensitivity of the Cherenkov Telescope Array for probing cosmology and fundamental physics with gamma-ray propagation

The Cherenkov Telescope Array (CTA), the new-generation ground-based observatory for $γ$-ray astronomy, provides unique capabilities to address significant open questions in astrophysics, cosmology, and fundamental physics. We study some of the salient areas of $γ$-ray cosmology that can be explored as part of the Key Science Projects of CTA, through simulated observations of active galactic nuclei (AGN) and of their relativistic jets. Observations of AGN with CTA will enable a measurement of $γ$-ray absorption on the extragalactic background light with a statistical uncertainty below 15% up to a redshift $z=2$ and to constrain or detect $γ$-ray halos up to intergalactic-magnetic-field strengths of at least 0.3pG. Extragalactic observations with CTA also show promising potential to probe physics beyond the Standard Model. The best limits on Lorentz invariance violation from $γ$-ray astronomy will be improved by a factor of at least two to three. CTA will also probe the parameter space in which axion-like particles could constitute a significant fraction, if not all, of dark matter. We conclude on the synergies between CTA and other upcoming facilities that will foster the growth of $γ$-ray cosmology.

astro-ph.HE↗

arXiv2020

Observation of a sudden cessation of a very-high-energy gamma-ray flare in PKS 1510-089 with H.E.S.S. and MAGIC in May 2016

The flat spectrum radio quasar (FSRQ) PKS 1510-089 is known for its complex multiwavelength behavior, and is one of only a few FSRQs detected at very high energy (VHE, $E>100\,$GeV) $γ$-rays. VHE $γ$-ray observations with H.E.S.S. and MAGIC during late May and early June 2016 resulted in the detection of an unprecedented flare, which reveals for the first time VHE $γ$-ray intranight variability in this source. While a common variability timescale of $1.5\,$hr is found, there is a significant deviation near the end of the flare with a timescale of $\sim 20\,$min marking the cessation of the event. The peak flux is nearly two orders of magnitude above the low-level emission. For the first time, curvature is detected in the VHE $γ$-ray spectrum of PKS 1510-089, which is fully explained through absorption by the extragalactic background light. Optical R-band observations with ATOM reveal a counterpart of the $γ$-ray flare, even though the detailed flux evolution differs from the VHE ightcurve. Interestingly, a steep flux decrease is observed at the same time as the cessation of the VHE flare. In the high energy (HE, $E>100\,$MeV) $γ$-ray band only a moderate flux increase is observed with Fermi-LAT, while the HE $γ$-ray spectrum significantly hardens up to a photon index of 1.6. A search for broad-line region (BLR) absorption features in the $γ$-ray spectrum indicates that the emission region is located outside of the BLR. Radio VLBI observations reveal a fast moving knot interacting with a standing jet feature around the time of the flare. As the standing feature is located $\sim 50\,$pc from the black hole, the emission region of the flare may have been located at a significant distance from the black hole. If this correlation is indeed true, VHE $γ$ rays have been produced far down the jet where turbulent plasma crosses a standing shock.

astro-ph.HE↗

arXiv2020

An extreme particle accelerator in the Galactic plane: HESS J1826$-$130

The unidentified very-high-energy (VHE; E $>$ 0.1 TeV) $γ$-ray source, HESS J1826$-$130, was discovered with the High Energy Stereoscopic System (HESS) in the Galactic plane. The analysis of 215 h of HESS data has revealed a steady $γ$-ray flux from HESS J1826$-$130, which appears extended with a half-width of 0.21$^{\circ}$ $\pm$ 0.02$^{\circ}_{\text{stat}}$ $\pm$ 0.05$^{\circ}_{\text{sys}}$. The source spectrum is best fit with either a power-law function with a spectral index $Γ$ = 1.78 $\pm$ 0.10$_{\text{stat}}$ $\pm$ 0.20$_{\text{sys}}$ and an exponential cut-off at 15.2$^{+5.5}_{-3.2}$ TeV, or a broken power-law with $Γ_{1}$ = 1.96 $\pm$ 0.06$_{\text{stat}}$ $\pm$ 0.20$_{\text{sys}}$, $Γ_{2}$ = 3.59 $\pm$ 0.69$_{\text{stat}}$ $\pm$ 0.20$_{\text{sys}}$ for energies below and above $E_{\rm{br}}$ = 11.2 $\pm$ 2.7 TeV, respectively. The VHE flux from HESS J1826$-$130 is contaminated by the extended emission of the bright, nearby pulsar wind nebula (PWN), HESS J1825$-$137, particularly at the low end of the energy spectrum. Leptonic scenarios for the origin of HESS J1826$-$130 VHE emission related to PSR J1826$-$1256 are confronted by our spectral and morphological analysis. In a hadronic framework, taking into account the properties of dense gas regions surrounding HESS J1826$-$130, the source spectrum would imply an astrophysical object capable of accelerating the parent particle population up to $\gtrsim$200 TeV. Our results are also discussed in a multiwavelength context, accounting for both the presence of nearby supernova remnants (SNRs), molecular clouds, and counterparts detected in radio, X-rays, and TeV energies.

astro-ph.HE↗

arXiv2020

Multi-Frequency Observations of the Candidate Neutrino Emitting Blazar BZB J0955+3551

The recent spatial and temporal coincidence of the blazar TXS 0506+056 with the IceCube detected neutrino event IC-170922A has opened up a realm of multi-messenger astronomy with blazar jets as a plausible site of cosmic-ray acceleration. After TXS 0506+056, a second blazar, BZB J0955+3551, has recently been found to be spatially coincident with the IceCube detected neutrino event IC-200107A and undergoing its brightest X-ray flare measured so far. Here, we present the results of our multi-frequency campaign to study this peculiar event that includes observations with the NuSTAR, Swift, NICER, and 10.4 m Gran Telescopio Canarias (GTC). The optical spectroscopic observation from GTC secured its redshift as $z=0.55703^{+0.00033}_{-0.00021}$ and the central black hole mass as 10$^{8.90\pm0.16}~M_{\odot}$. Both NuSTAR and NICER data reveal a rapid flux variability albeit at low-significance ($\lesssim3.5σ$). We explore the origin of the target photon field needed for the photo-pion production using analytical calculations and considering the observed optical-to-X-ray flux level. We conclude that seed photons may originate from outside the jet, similar to that reported for TXS 0506+056, although a scenario invoking a co-moving target photon field (e.g., electron-synchrotron) can not be ruled out. The electromagnetic output from the neutrino-producing photo-hadronic processes are likely to make only a sub-dominant contribution to the observed spectral energy distribution suggesting that the X-ray flaring event may not be directly connected with IC-200107A.

astro-ph.HE↗

arXiv2020

Search for high-redshift blazars with Fermi/LAT

High-$z$ blazars (z $\geq 2.5$) are the most powerful class of persistent $γ$-ray sources in the Universe. These objects possess the highest jet powers and luminosities and have black hole masses often in excess of $10^9$ solar masses. In addition, high-$z$ blazars are important cosmological probes and serve as test objects for blazar evolution models. Due to their large distance, their high-energy emission typically peaks below the GeV range, which makes them difficult to study with Fermi/LAT. Therefore, only the very brightest objects are detectable and, to date, only a small number of high-z blazars have been detected with Fermi/LAT. In this work, we studied the monthly binned long-term $γ$-ray emission of a sample of 176 radio and optically detected blazars that have not been reported as known $γ$-ray sources in the 3FGL catalog. In order to account for false-positive detections, we calculated monthly Fermi/LAT light curves for a large sample of blank sky positions and derived the number of random fluctuations that we expect at various test statistic (TS) levels. For a given blazar, a detection of TS > 9 in at least one month is expected $\sim 15\%$ of the time. Although this rate is too high to secure detection of an individual source, half of our sample shows such single-month $γ$-ray activity, indicating a population of high-energy blazars at distances of up to z=5.2. Multiple TS > 9 monthly detections are unlikely to happen by chance, and we have detected several individual new sources in this way, including the most distant $γ$-ray blazar, BZQ J1430+4204 (z = 4.72). Finally, two new $γ$-ray blazars at redshifts of z = 3.63 and z = 3.11 are unambiguously detected via very significant (TS > 25) flares in individual monthly time bins.

astro-ph.HE↗

arXiv2020

Search for dark matter signals towards a selection of recently-detected DES dwarf galaxy satellites of the Milky Way with H.E.S.S

Dwarf spheroidal galaxy satellites of the Milky Way are prime targets for indirect detection of dark matter with gamma rays due to their proximity, high dark matter content and absence of non-thermal emission processes. Recently, the Dark Energy Survey (DES) revealed the existence of new ultra-faint dwarf spheroidal galaxies in the southern-hemisphere sky, therefore ideally located for ground-based observations with the imaging atmospheric Cherenkov telescope array H.E.S.S. We present a search for very-high-energy ($E\gtrsim100$ GeV) gamma-ray emission using H.E.S.S. observations carried out recently towards Reticulum II, Tucana II, Tucana III, Tucana IV and Grus II satellites. No significant very-high-energy gamma-ray excess is found from the observations on any individual object nor in the combined analysis of all the datasets. Using the most recent modeling of the dark matter distribution in the dwarf galaxy halo, we compute for the first time on DES satellites individual and combined constraints from Cherenkov telescope observations on the annihilation cross section of dark matter particles in the form of Weakly Interacting Massive Particles. The combined 95% C.L. observed upper limits reach $\langle σv \rangle \simeq 1 \times 10^{-23}$ cm$^3$s$^{-1}$ in the $W^+W^-$ channel and $4 \times 10^{-26}$ cm$^3$s$^{-1}$ in the $γγ$ channels for a dark matter mass of 1.5 TeV. The H.E.S.S. constraints well complement the results from Fermi-LAT, HAWC, MAGIC and VERITAS and are currently the most stringent in the $γγ$ channels in the multi-GeV/multi-TeV mass range.

astro-ph.HE↗

arXiv2020

On the Detection Potential of Blazar Flares for Current Neutrino Telescopes

Blazar jets are extreme environments, in which relativistic proton interactions with an ultraviolet photon field could give rise to photopion production. High-confidence associations of individual high-energy neutrinos with blazar flares could be achieved via spatially and temporally coincident detections. In 2017, the track-like, extremely high-energy neutrino event IC 170922A was found to coincide with increased $γ$-ray emission from the blazar TXS 0506+056, leading to the identification of the most promising neutrino point source candidate so far. We calculate the expected number of neutrino events that can be detected with IceCube, based on a broadband parametrization of bright short-term blazar flares that were observed in the first 6.5 years of \textit{Fermi}/LAT observations. We find that the integrated keV-to-GeV fluence of most individual blazar flares is far too small to yield a substantial Poisson probability for the detection of one or more neutrinos with IceCube. We show that the sample of potentially detectable high-energy neutrinos from individual blazar flares is rather small. We further show that the blazars 3C 279 and PKS 1510$-$089 dominate the all-sky neutrino prediction from bright and short-term blazar flares. In the end, we discuss strategies to search for more significant associations in future data unblindings of IceCube and KM3NeT.

astro-ph.HE↗

arXiv2020

Resolving acceleration to very high energies along the Jet of Centaurus A

The nearby radio galaxy Centaurus A belongs to a class of Active Galaxies that are very luminous at radio wavelengths. The majority of these galaxies show collimated relativistic outflows known as jets, that extend over hundreds of thousands of parsecs for the most powerful sources. Accretion of matter onto the central super-massive black hole is believed to fuel these jets and power their emission, with the radio emission being related to the synchrotron radiation of relativistic electrons in magnetic fields. The origin of the extended X-ray emission seen in the kiloparsec-scale jets from these sources is still a matter of debate, although Cen A's X-ray emission has been suggested to originate in electron synchrotron processes. The other possible explanation is Inverse Compton (IC) scattering with CMB soft photons. Synchrotron radiation needs ultra-relativistic electrons ($\sim50$ TeV), and given their short cooling times, requires some continuous re-acceleration mechanism to be active. IC scattering, on the other hand, does not require very energetic electrons, but requires jets that stay highly relativistic on large scales ($\geq$1 Mpc) and that remain well-aligned with the line of sight. Some recent evidence disfavours inverse Compton-CMB models, although other evidence seems to be compatible with them. In principle, the detection of extended gamma-ray emission, directly probing the presence of ultra-relativistic electrons, could distinguish between these options, but instruments have hitherto been unable to resolve the relevant structures. At GeV energies there is also an unusual spectral hardening in Cen A, whose explanation is unclear. Here we report observations of Cen A at TeV energies that resolve its large-scale jet. We interpret the data as evidence for the acceleration of ultra-relativistic electrons in the jet, and favour the synchrotron explanation for the X-rays.

astro-ph.HE↗

arXiv2020

Optical Observations Reveal Strong Evidence for High Energy Neutrino Progenitor

We present the earliest astronomical observation of a high energy neutrino error box in which its variability was discovered after high-energy neutrinos detection. The one robotic telescope of the MASTER global international network (Lipunov et al. 2010) automatically imaged the error box of the very high-energy neutrino event IceCube-170922A. Observations were carried out in minute after the IceCube-170922A neutrino event was detected by the IceCube observatory at the South Pole. MASTER found the blazar TXS 0506+056 to be in the off-state after one minute and then switched to the on-state no later than two hours after the event. The effect is observed at a 50-sigma significance level. Also we present own unique 16-years light curve of blazar TXS 0506+056 (518 data set).

astro-ph.HE↗

arXiv2020

Probing the magnetic field in the GW170817 outflow using H.E.S.S. observations

The detection of the first electromagnetic counterpart to the binary neutron star (BNS) merger remnant GW170817 established the connection between short $γ$-ray bursts and BNS mergers. It also confirmed the forging of heavy elements in the ejecta (a so-called kilonova) via the r-process nucleosynthesis. The appearance of non-thermal radio and X-ray emission, as well as the brightening, which lasted more than 100 days, were somewhat unexpected. Current theoretical models attempt to explain this temporal behavior as either originating from a relativistic off-axis jet or a kilonova-like outflow. In either scenario, there is some ambiguity regarding how much energy is transported in the non-thermal electrons versus the magnetic field of the emission region. Combining the VLA (radio) and Chandra (X-ray) measurements with observations in the GeV-TeV domain can help break this ambiguity, almost independently of the assumed origin of the emission. Here we report for the first time on deep H.E.S.S. observations of GW170817 / GRB 170817A between 124 and 272 days after the BNS merger with the full H.E.S.S. array of telescopes, as well as on an updated analysis of the prompt (<5 days) observations with the upgraded H.E.S.S. phase-I telescopes. We discuss implications of the H.E.S.S. measurement for the magnetic field in the context of different source scenarios.

astro-ph.HE↗

arXiv2020

Very high energy $γ$-ray emission from two blazars of unknown redshift and upper limits on their distance

We report on the detection of very-high-energy (VHE; $E > 100$ GeV) $γ$-ray emission from the BL Lac objects KUV 00311-1938 and PKS 1440-389 with the High Energy Stereoscopic System (H.E.S.S.). H.E.S.S. observations were accompanied or preceded by multi-wavelength observations with Fermi/LAT, XRT and UVOT on board the Swift satellite, and ATOM. Based on an extrapolation of the Fermi/LAT spectrum towards the VHE $γ$-ray regime, we deduce a 95% confidence level upper limit on the unknown redshift of KUV 00311-1938 of z < 0.98, and of PKS 1440-389 of z < 0.53. When combined with previous spectroscopy results the redshift of KUV 00311-1938 is constrained to $0.51 \leq z < 0.98$ and for PKS 1440-389 to $0.14 \lessapprox z < 0.53$.

astro-ph.HE↗

arXiv2020

Prospects for gamma-ray observations of narrow-line Seyfert 1 galaxies with the Cherenkov Telescope Array II. Gamma-gamma absorption in the broad-line region radiation fields

Gamma-ray emitting narrow-line Seyfert 1 ($γ$-NLS1) galaxies possibly harbour relatively low-mass black holes (10$^6$-10$^8$ M$_{\odot}$) accreting close to the Eddington limit, and share many characteristics with their sibling sources, flat-spectrum radio quasars. Although they have been detected in the MeV--GeV band with Fermi-LAT, they have never been seen in the very high energy band with current imaging atmospheric Cherenkov telescopes (IACTs). Thus, they are key targets for the next-generation IACT, the Cherenkov Telescope Array (CTA). In a previous work we selected, by means of extensive simulations, the best candidates for a prospective CTA detection (SBS 0846$+$513, PMN J0948$+$0022, and PKS 1502$+$036) taking into account the effects of both the intrinsic absorption (approximated with a cut-off at 30 GeV), and the extra-galactic background light on the propagation of $γ$-rays. In this work we simulate the spectra of these three sources by adopting more realistic broad-line region (BLR) absorption models. In particular, we consider the detailed treatment of $γ$-$γ$ absorption in the radiation fields of the BLR as a function of the location of the $γ$-ray emission region with parameters inferred from observational constraints. We find that, due to the energy range extent and its sensitivity, CTA is particularly well suited to locate the $γ$-ray emitting region in $γ$-NLS1. In particular CTA will be able not only to distinguish whether the $γ$-ray emitting region is located inside or outside the BLR, but also where inside the BLR it may be.

astro-ph.HE↗