2,325 research outputs found
The diffuse neutrino flux from FR-II radio galaxies and blazars: A source property based estimate
Water and ice Cherenkov telescopes of the present and future aim for the
detection of a neutrino signal from extraterrestrial sources at energies E>PeV.
Some of the most promising extragalactic sources are Active Galactic Nuclei
(AGN). In this paper, the neutrino flux from two kinds of AGN sources will be
estimated assuming photohadronic interactions in the jet of the AGN. The first
analyzed sample contains FR-II radio galaxies while the second AGN type
examined are blazars. The result is highly dependent on the proton's index of
the energy spectrum. To normalize the spectrum, the connection between neutrino
and disk luminosity will be used by applying the jet-disk symbiosis model from
Falcke and Biermann (1995). The maximum proton energy and thus, also the
maximum neutrino energy of the source is connected to its disk luminosity,
which was shown by Lovelace (1976) and was confirmed by Falcke et al. (1995).Comment: 24 pages, 14 figures, to be published in Astroparticle Physic
AGC 226067: A possible interacting low-mass system
We present Arecibo, GBT, VLA and WIYN/pODI observations of the ALFALFA source
AGC 226067. Originally identified as an ultra-compact high velocity cloud and
candidate Local Group galaxy, AGC 226067 is spatially and kinematically
coincident with the Virgo cluster, and the identification by multiple groups of
an optical counterpart with no resolved stars supports the interpretation that
this systems lies at the Virgo distance (D=17 Mpc). The combined observations
reveal that the system consists of multiple components: a central HI source
associated with the optical counterpart (AGC 226067), a smaller HI-only
component (AGC 229490), a second optical component (AGC 229491), and extended
low surface brightness HI. Only ~1/4 of the single-dish HI emission is
associated with AGC 226067; as a result, we find M_HI/L_g ~ 6 Msun/Lsun, which
is lower than previous work. At D=17 Mpc, AGC 226067 has an HI mass of 1.5 x
10^7 Msun and L_g = 2.4 x 10^6 Lsun, AGC 229490 (the HI-only component) has
M_HI = 3.6 x 10^6 Msun, and AGC 229491 (the second optical component) has L_g =
3.6 x 10^5 Lsun. The nature of this system of three sources is uncertain: AGC
226067 and AGC 229490 may be connected by an HI bridge, and AGC 229490 and AGC
229491 are separated by only 0.5'. The current data do not resolve the HI in
AGC 229490 and its origin is unclear. We discuss possible scenarios for this
system of objects: an interacting system of dwarf galaxies, accretion of
material onto AGC 226067, or stripping of material from AGC 226067.Comment: Accepted for publication in A&A. 6 pages, 4 figure
Detection of an Optical Counterpart to the ALFALFA Ultra-compact High Velocity Cloud AGC 249525
We report on the detection at 98% confidence of an optical counterpart to
AGC 249525, an Ultra-Compact High Velocity Cloud (UCHVC) discovered by the
ALFALFA blind neutral hydrogen survey. UCHVCs are compact, isolated HI clouds
with properties consistent with their being nearby low-mass galaxies, but
without identified counterparts in extant optical surveys. Analysis of the
resolved stellar sources in deep - and -band imaging from the WIYN pODI
camera reveals a clustering of possible Red Giant Branch stars associated with
AGC 249525 at a distance of 1.640.45 Mpc. Matching our optical detection
with the HI synthesis map of AGC 249525 from Adams et al. (2016) shows that the
stellar overdensity is exactly coincident with the highest-density HI contour
from that study. Combining our optical photometry and the HI properties of this
object yields an absolute magnitude of , a stellar
mass between and , and an HI to stellar mass ratio between 9 and 144. This object has
stellar properties within the observed range of gas-poor Ultra-Faint Dwarfs in
the Local Group, but is gas-dominated.Comment: 9 pages, 4 figures; accepted to ApJ
Are HI Supershells the Remnants of Gamma-Ray Bursts?
Gamma-Ray Bursts (GRBs) are thought to originate at cosmological distances
from the most powerful explosions in the Universe. If GRBs are not beamed then
the distribution of their number as a function of Gamma-ray flux implies that
they occur once per (0.3-40) million years per bright galaxy and that they
deposit >10^{53} ergs into their surrounding interstellar medium. The blast
wave generated by a GRB explosion would be washed out by interstellar
turbulence only after tens of millions of years when it finally slows down to a
velocity of 10 km/s. This rather long lifetime implies that there could be up
to several tens of active GRB remnants in each galaxy at any given time. For
many years, radio observations have revealed the enigmatic presence of
expanding neutral-hydrogen (HI) supershells of kpc radius in the Milky Way and
in other nearby galaxies. The properties of some supershells cannot be easily
explained in terms of conventional sources such as stellar winds or supernova
explosions. However, the inferred energy and frequency of the explosions
required to produce most of the observed supershells agree with the above GRB
parameters. More careful observations and analysis might reveal which fraction
of these supershells are GRB remnants. We show that if this link is
established, the data on HI supershells can be used to constrain the energy
output, the rate per galaxy, the beaming factor, and the environment of GRB
sources in the Universe.Comment: 8 pages, final version, ApJ Letters, in pres
Rotation in the Orion Nebula Cluster
Eighteen fields in the Orion Nebula Cluster (ONC) have been monitored for one
or more observing seasons from 1990-99 with a 0.6-m telescope at Wesleyan
University. Photometric data were obtained in Cousins I on 25-40 nights per
season. Results from the first 3 years of monitoring were analyzed by Choi &
Herbst (1996; CH). Here we provide an update based on 6 more years of
observation and the extensive optical and IR study of the ONC by Hillenbrand
(1997) and Hillenbrand et al. (1998). Rotation periods are now available for
134 ONC members. Of these, 67 were detected at multiple epochs with identical
periods by us and 15 more were confirmed by Stassun et al. (1999) in their
study of Ori OBIc/d. The bimodal period distribution for the ONC is confirmed,
but we also find a clear dependence of rotation period on mass. This can be
understood as an effect of deuterium burning, which temporarily slows the
contraction and thus spin-up of stars with M <0.25 solar masses and ages of ~1
My. Stars with M <0.25 solar masses have not had time to bridge the gap in the
period distribution at ~4 days. Excess H-K and I-K emission, as well as CaII
infrared triplet equivalent widths (Hillenbrand et al. 1998), show weak but
significant correlations with rotation period among stars with M >0.25 solar
masses. Our results provide new observational support for the importance of
disks in the early rotational evolution of low mass stars. [abridged]Comment: 18 pages of text, 17 figures, and 4 tables; accepted for publication
in The Astronomical Journa
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