Science
Multimessenger observations of a flaring blazar coincident with high-energy neutrino IceCube-170922A
The IceCube Collaboration, Fermi-LAT, MAGIC, AGILE, ASAS-SN, HAWC, H.E.S.S., INTEGRAL, et al
Previous detections of individual astrophysical sources of neutrinos are limited to the Sun and the supernova 1987A, whereas the origins of the diffuse flux of high-energy cosmic neutrinos remain unidentified. On 22 September 2017, detected a high-energy neutrino, IceCube-170922A, with an energy of ~290 tera-electronvolts. Its arrival direction was consistent with the location of a known gamma-ray blazar, TXS 0506+056, observed to be in a flaring state. An extensive multi wavelength campaign followed, ranging from radio frequencies to gamma-rays. These observations characterize the variability and energetics of the blazar and include the detection of TXS 0506+056 in very-high-energy gamma-rays. This observation of a neutrino in spatial coincidence with a gamma-ray-emitting blazar during an active phase suggests that blazers may be a source of high-energy neutrinos.
1807.04275
Interpretation of the coincident observation of a high energy neutrino and a bright flare
Gao
On 22 Sept 2017, the IceCube Neutrino Observatory reported a muon track from a neutrino with a very good positional accuracy. The alert triggered a number of astronomical follow-up campaigns, and the Fermi gamma-ray telescope found as counterpart an object named TS50506+056 in a very bright, flaring state; this observation may be the first direct evidence for an extragalactic source of very high-energy cosmic rays. While this and subsequent observations provide the observational picture across the EM spectrum, answering where in the spectrum signatures of CRs arise and what the source properties must be, given the observational constraints, requires a self-consistent description of the processes at work. Here, perform a detailed time-dependent modeling of these relevant processes and study a set of self-consistent models for the source. Find a slow but over-proportional increase of the neutrino flux during the flare compared to the production enhancement of energetic CRs. Also demonstrate that interactions of energetic CR ions result in predominantly hard X-ray emission, strongly constraining the expected number of neutrinos, and to a lesser degree in TeV gamma rays. Optical photons and GeV-scale gamma rays are predominantly radiated by electrons. The results indicate that especially future X-ray and TeV-scale gamma-ray observations of nearby objects can be used to identify more such events.
1807.04277 (https://www.nature.com/articles/s41550-018-0508-y)
Gravitational lensing detection of an extremely dense environment around a galaxy cluster
Sereno, Giocoli, et al
Galaxy clusters form at the highest density nodes of the cosmic web. The clustering of massive haloes is enhanced relative to the general mass distribution and matter beyond the viral region is strongly correlated to the halo mass (halo bias). Clustering can be further enhanced depending on halo properties other than mass (secondary bias). The questions of how much and why the regions surrounding rich clusters are over-dense are still unanswered. Here, report the analysis of the environment bias in a sample of very massive clusters, selected through the Sunyaev-Zel'dovich effect by the Planck mission. Present the first detection of the correlated DM associated to a single cluster, PSZ2 G099.86+58.45. The system is extremely rare in the current paradigm of structure formation. The gravitational lensing signal was traced up to 30 Mpc with high S/N ratio ~3.4. The measured shear is very large and points at environment matter density in notable excess of the cosmological mean. The boosting of the correlated DM density around high mass haloes can be very effective. Together with ensemble studies of the large scale structure, lensing surveys can picture the surroundings of single haloes.
1807.04300
The blazar TXS 0506_056 associated with a high-energy neutrino: insights into extragalactic jets and cosmic ray acceleration
Ahnen, et al
A neutrino with energy of ~290 eV, IceCube-170922A, was detected in coincidence with the BL Lac object TXS 0506_056 during enhanced gamma-ray activity, with chance coincidence being rejected at ~3 sigma level. Monitored the object in the VHE band with the MAGIC telescopes for ~41 hours from 1.3 to 40.4 days after the neutrino detection. Day-timescale variability is clearly resolved. Interpret the quasi-simultaneous neutrino and broadband EM observations with a novel open-zone left-hadronic model, based on interactions of e- and p+ co-accelerated in the jet with external photons originating from a slow-moving plasma sheath surrounding the faster jet spine. Can reproduce the multi wavelength spectra of TXS 0506+056 with neutrino rate and energy compatible with IceCube-170922A, and with plausible values for the jet power of ~1e45 - 4e46 erg/s. The steep spectrum observed by MAGIC is concordant with internal gamma-gamma absorption above a few tens of GeV entailed by photohadronic production of a ~290 TeV neutrino, corroborating a genuine connection between the multi-messenger signals. In contrast to previous predictions of predominantly hadronic emission from neutrino sources, the gamma-rays can be mostly ascribed to inverse Compton up-scattering of external photons by accelerated electrons. The X-ray and VHE bands provide crucial constraints on the emission from both accelerated electrons and protons. Infer that the maximum energy of protons in the jet co-moving frame can be in the range ~1e14 to 1e18 eV.
1807.04461
Dissecting the region around IceCube-170922: the blazar TXS 0506+056 as the first cosmic neutrino source
Padovani, et al
Present the dissection in space, time and energy of the region around the IceCube-170922A neutrino alert. This study is motivated by: (1) the first association between a neutrino alert and a blazar in a flaring state, TXS 0506+056; (2) the evidence of a neutrino flaring activity during 2014-2015 from the same direction; (3) the lack of an accompanying simultaneous gamma-ray enhancement from the same counterpart; (4) the contrasting flaring activity of a neighboring bright gamma-ray source, the blazar PKS 0502+049, during 2014-2015. The study makes use of multi-wavelength archival data accessed through Open Universe tools and includes a new analysis of Fermi-LAT data. Find that PKS 0502+049 contaminates the gamma-ray emission region at low energies but TXS 0506+056 dominates the sky above a few GeV. TXS 0506+056, which is a very strong (top percent) radio and gamma-ray source, is in a high gamma-ray state during the neutrino alert but in a low through hard gamma-ray state in coincidence with the neutrino flare. Both states can be reconciled with the energy associated with the neutrino emission and, in particular during the low/hard state, there is evidence that TXS 0506+056 has undergone a hadronic flare with very important implications for blazar modeling. All multi-messenger diagnostics reported here support a single coherent picture in which TXS 0506+056, a very high gamma-ray blazar, is the only counterpart of all the neutrino emissions in the region and therefore the most plausible first non-stellar neutrino and, hence, cosmic ray source.
1807.04468
On measuring the Galactic dark matter halo with hypervelocity stars
Contigiani, Rossi, Marchetti
HVSs travel from the Galactic Centre to the DM halo of the MW, where they are observed with velocities in excess of the Galactic escape speed. Their trajectories make them a unique probe of the still poorly constrained DM component of the Galaxy. In this paper, present a new method to constrain the Galactic potential with HVSs. The likelihood is constructed by efficiently calculating the local HVS density at any point of the Galaxy by back-propagating the phase space position and quantifying the ejection probability along the orbit. This method is particularly suited to the data from the ESA mission Gaia. Therefore, to showcase the method, apply it to a simulated Gaia sample of ~200 stars in Galactic potentials with 3 different dark matter components, parameterized by a spheroidal NFW profile. Find that individual HVSs exhibit a degeneracy in the scale mass-scale radius plane (M_s-r_s) and are able to measure only the combination alpha=M_s/r_s^2, likewise a degeneracy is also present between alpha and the spheroidal axis-ratio q. When the whole sample is considered, both parameters are nailed down with sub-percentage precision (~1% and 0.1% for alpha and q respectively) and no systematic bias is observed. This remarkable power to constrain deviations from a symmetric halo is a consequence of the Galactocentric origin of HVSs. To compare the results with other probes, break the degeneracy in the scale parameters and impose a mass concentration relation. The result is a competitive precision on the viral mass M200 of about 10%.
1807.04493
Different stellar rotation in the two main sequences of the young globular cluster NGC1818: first direct spectroscopic evidence
Marino, et al
Present a spectroscopic analysis of MS stars in the young globular cluster NGC1818 (age~40 Myrs) in the LMC. The photometric survey on MC clusters has revealed that NGC1818, similarly to the other young objects with age 600 Myrs, displays not only an extended MS Turn-Off (eMSTO), as observed in intermediate-age clusters (age~1-2 Gyrs), but also a split MS. The most straightforward interpretation of the double MS is the presence of two stellar populations; a sequence of slowly-rotating stars lying on the blue-MS and a sequence of fast rotators, with rotation close to the breaking speed, defining a red-MS. Report the first direct spectroscopic measurements of projected rotational velocities vsini for the double MS, eMSTO and Be stars of a young cluster. The analysis of line profiles includes non-LTE effects, required for correctly deriving v sini values. The results suggest that: (i) the mean rotation for blue- and red-MS stars is vsini=71±10 km/s (sigma=37 km/s) and vsini=202±23 km/s (sigma=91 km/s), respectively; (ii) eMSTO stars have different vsini, which are generally lower than those inferred for red-MS stars, and (iii) as expected, Be stars display the highest vsini values. This analysis supports the idea that distinct rotational velocities play an important role in the appearance of multiple stellar populations in the color-magnitude diagrams of young clusters, and poses new constraints to the current scenarios.
1807.04565
The escape speed curve of the Galaxy obtained from Gaia DR2 implies a heavy Milky Way
Monari, et al
Measure the escape speed curve of the MW based on the analysis of the velocity distribution of ~2850 counter-rotating halo stars from Gaia DR2. The distances were estimated through the StarHorse code, and only stars with distance errors smaller than 10% were used in the study. The escape speed curve is measured at Galactocentric radii ranging from ~5 kpc to~10.5 kpc. The local Galactic escape at the Sun's position is estimated to be v_e (r_sun)=580 ± 63 km/s, and it rises towards the Galactic center. Defined as the minimum speed required to reach three viral radii, the estimate of the escape speed as a function of radius implies, for a NFW profile and local circular velocity of 240 km/s, a dark matter mass M_200=1.28+0.68-0.50 e12 Msun and a high concentration c_200=11.09+2.94-1.79. Assuming the mass-concentration relation of LCDM, get M_200=1.55-0.51+0.64 e12 Msun, c_200=7.93-0.27+0.33, for a local circular velocity of 228 km/s.
1807.04607
VERITAS observations of the BL Lac object TXS 0506+056
Abeysekara, et al
On 2017 Sept. 22, the IceCube Neutrino Observatory reported the detection of the high-energy neutrino event /icnu, of potential astrophysical origin. It was soon determined that the neutrino direction was consistent with the location of the gamma-ray blazar txs, which was in an elevated gamma-ray emission state as measured by the Fermi satellite. VERITAS observations of the neutrino/blazar region started on 2017 Sept 23 in response to the neutrino alert and continued through 2018 February 6. While no significant very-high-energy (VHE; E>100 GeV) emission was observed for the blazar by VERITAS in the 2 week period immediately following the IceCube alert, TXS 0506+056 was detected by VERITAS with a significance of 5.8 sigma in the full 35-hour data set. The average photon flux of the source during this period was (8.9±1.6)e-12 /cm^2/s, or 1.6% of the CrabNebular flux, above an energy threshold of 100 GeV, with a soft spectral index of 4.8±1.3.
1807.04672
Bias due to neutrinos must not uncorrect'd go
Vagnozzi, et al
In cosmologies with massive neutrinos, the galaxy bias defined with respect to the total matter field (CDM, baryons, and non-relativistic neutrinos) depends on the sum of the neutrino masses M_nu, and becomes scale-dependent even on large scales. This effect has been usually neglected given the sensitivity of current surveys, but becomes a severe systematic for future surveys aiming to provide the first detection of non-zero M_nu. The effect can be corrected for by defining the bias with respect to the density field of CDM and baryons instead of the total matter field. In this work, provide a simple prescription for correctly mitigating the neutrino-induced scale-dependent bias effect in a practical way. Clarify a number of subtleties regarding how to properly implement this correction in the presence of z-space distortions and non-linear evolution of perturbations. Perform a MCMC analysis on simulated galaxy clustering data that match the expected sensitivity of the Euclid survey. Find that the neutrino-induced scale-dependent bias can lead to important shifts in both the inferred mean value of M_nu, as well as its uncertainty. Show how these shifts propagate to other cosmological parameters correlated with M_nu, such as the CDM physical density Omega_cdm h^2 and the scalar spectral index n_s. In conclusion, find that correctly accounting for the neutrino-induced scale-dependent bias will be of crucial importance for future galaxy clustering analyses. Encourage the cosmology community to correctly account for this effect using the simple prescription presented in this work. The tools necessary to easily correct for the neutrino-induced scale-dependent bias will be made publicly available in an upcoming release of the Boltzmann solver CLASS.
1807.04748
Blazar flares as an origin of high-energy cosmic neutrinos?
Murase, Oikonomou, Petropoulou
Consider implications of high-energy neutrino emission from blazar flares, including the recent event IceCube-170922A and the 2014-2015 neutrino flare that could originate from TXS 0506+056. First, discuss their contribution to the diffuse neutrino intensity taking into account various observational constraints, and show that blazar flares like those of TXS 0506+056 can make <(1-10)% of the total neutrino intensity. Even though blazars are likely to be subdominant in the diffuse neutrino intensity at sub-PeV energies, energetic flares can still be detected with a rate of <1 per year. Also argue that the neutrino output of blazers can be dominated by flares in the standard leptonic scenario for their gamma-ray emission. Second, consider multi-messenger constraints on the source modeling. Show that luminous neutrino flares should be accompanied by luminous broadband cascade emission, emerging also in X rays and gamma rays. This implies that not only gamma-ray telescopes like Fermi but also X-ray sky monitors such as Swift and MAXI are critical to test the canonical picture based on the single-zone modeling. Also suggest 2-zone models that can avid the X-ray constraints.
1807.05180
Predictably missing satellites: sub halo abundance in Milky Way-like haloes
Fielder, Mao, Newman, Zentner, Licquia
On small scales there have been a number of claims of discrepancies between the standard CDM model and observations. The 'missing satellites problem' infamously describes the overabundance of sub haloes from CDM simulations compared to the number of satellites observed in the MW. A variety of solutions to this discrepancy have been proposed; however, the impact of the specific properties of the MW halo relative to the typical halo of its mass have yet to be explored. Motivated by recent studies that identified ways in which the MW is atypical (e.g., Licquia+0215), investigate how the properties of DM haloes with mass comparable to the Galaxy's --- including concentration, spin, shape and scale factor of the last major merger --- correlate with the sub halo abundance. Using zoom-in simulations of MW-like haloes, build 2 models of sub halo abundance as functions of host halo properties and conclude that the MW should be expected to have 22-44% fewer sub haloes with low maximum rotation velocities (V_max^sat) ~ 10 km/s at the 95% confidence level and up to 72% fewer than average sub haloes with high rotation velocities (V_max^sat >~ 30 km/s, comparable to the Magellatnic Clouds) than would be expected for a typical halo of the MW's mass. Concentration is the most informative single parameter for predicting sub halo abundance. The results imply that models tuned to explain the missing satellites problem assuming typical sub halo abundances for the Galaxy will be overcorrecting.
1807.04877
Gravitational waves from ultrashort period exoplanets
Cunha, Silva, Lima
In the last two decades, thousands of extrasolar planets were discovered based on different observational techniques, and their number must increase substantially in virtue of the ongoing and near-future approved missions and facilities. It is shown that interesting signatures of binary systems from nearby exoplanets and their parent stars can also be obtained measuring the pattern of gravitational waves that will be made available by the new generation of detectors including the space-based LISA observatory. As an example, a subset of exoplanets with extremely short periods (less than 80 min) is discussed. All of them have gravitational luminosity, L_GW ~1e30 erg/s, strain h~1e-22, frequencies f_gw>1e-4 Hz, and as such as within the standard sensitivity curve of LISA. The analysis suggests that the emitted gravitational wave pattern may also provide an efficient tool to discover ultrashort period exoplanets.
1807.05909
Hypervelocity stars in the Gaia era: runaway B stars beyond the speed limit of classical ejection mechanisms
Irrgang, Kreuzer, Heber
Young massive stars in the halo are supposed to be runaway stars from the Galactic disk. Possible ejection scenarios are binary supernovaejections (BSE) or dynamical ejections from star clusters (DE). Hypervelocity stars (HVSs) are extreme runaway stars that are potentially unbound to the Galaxy. Powerful acceleration mechanisms like the tidal disruption of a binary by a supermassive black hole are required. Hence, HVSs are believed to originate in the Galactic center (GC). Gaia DR2 offers the opportunity to study HVSs in a n unprecedented manner. Revisit the most interesting HVSs, i.e., 15 stars for which proper motions with the HST were obtained in the pre-Gaia era, to unravel their origin. By carrying out kinematic analyses in 3 different Galactic mass models, kinetic properties are obtained that help to constrain the spatial origins of the HVSs. While HVSs previously considered unbound remain unbound in 2 Galactic potential, most stars become bound in the most massive Galactic model. For 9 stars (including 5 unbound candidates), the GC can be rules out as spatial origin at 2 sigma confidence. Using optical and infrared photometry to determine its spectrophotometric distance, confirm that HVS3 originates in the LMC. The results suggest that a large fraction of the HVSs are actually disk runaway stars launched close to or beyond Galactic escape velocities. Population synthesis models predict that only a small fraction of the HVSs stems from BSE. Furthermore, a maximum ejection velocity of 540 km/s is predicted for BSE and a similar limit has been found for DE. The ejection velocities of 5 of the non-GC HVSs are close to or above this limit, calling for the existence of another dynamical ejection mechanism (e.g., massive perturbs such as intermediate mass BHs) besides the classical scenarios mentioned above.
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