Abstract: We present the results of two years of radio and X-ray monitoring of the
magnetar XTE J1810$-$197 since the radio re-activation in late 2018. Single
pulse analysis of radio observations from the Lovell and MkII telescopes at
1564 MHz and the Effelsberg telescope at 6 GHz has resulted in the detection of
a total of 91 giant pulses (GPs) between MJDs 58858 and 59117. These GPs appear
to be confined to two specific phase ranges (0.473 <= \phi <= 0.502$ and 0.541
<= \phi <= 0.567). We also observe that the first detection of GP emission
corresponds to a minimum in the spin-down rate. Simultaneous radio and X-ray
observations were performed on MJDs 59009 and 59096. The 0.5-10 keV X-ray
spectrum from NICER is well characterised by a two component blackbody model
that can be interpreted as two hot spots on the polar cap of the neutron star.
The blackbody temperature decreases with time, consistent with the previous
outburst, while the change in the pulsed fraction does not follow the same
trend as was seen in the previous outburst. The radio and X-ray flux of XTE
J1810-197 are correlated during the initial phase of the outburst (MJD 58450 -
MJD 58550) and an increase in the radio flux is observed later that may be
correlated to the onset of GPs. We argue that the disparity in the evolution of
the current outburst compared to the previous one can be attributed to a change
in geometry of the neutron star.
Abstract: We present a model for the broadband radio-to-$\gamma$-ray spectral energy
distribution of the compact radio source, PKS 1718-649. Because of its young
age (100 years) and proximity ($z=0.014$), PKS 1718-649 offers a unique
opportunity to study nuclear conditions and the jet/host galaxy feedback
process at the time of an initial radio jet expansion. PKS 1718-649 is one of a
handful of young radio jets with $\gamma$-ray emission confirmed with the
Fermi/LAT detector. We show that this $\gamma$-ray emission can be successfully
explained by Inverse Compton scattering of the ultraviolet photons, presumably
from an accretion flow, off non-thermal electrons in the expanding radio lobes.
The origin of the X-ray emission in PKS 1718-649 is more elusive. While Inverse
Compton scattering of the infrared photons emitted by a cold gas in the
vicinity of the expanding radio lobes contributes significantly to the X-ray
band, the data require that an additional X-ray emission mechanism is at work,
e.g. a weak X-ray corona or a radiatively inefficient accretion flow, expected
from a LINER type nucleus such as that of PKS 1718-649. We find that the jet in
PKS 1718-649 has low power, $L_j \simeq 2.2 \times 10^{42}$ erg s$^{-1}$, and
expands in an environment with density $n_0 \simeq 20$ cm$^{-3}$. The inferred
mass accretion rate and gas mass reservoir within 50-100 pc are consistent with
estimates from the literature obtained by tracing molecular gas in the
innermost region of the host galaxy with SINFONI and ALMA.
Abstract: We model the multiwavelength properties of binaries accreting at
super-critical rates with the aim to better understand the observational
properties of Ultra-luminous X-ray Sources (ULXs). We calculate an extended
grid of binary systems which evolve through Roche Lobe Overflow and undergo
case A mass transfer from massive donors (up to 25 Msol) onto massive Black
Holes (BH) (up to 100 Msol). Angular momentum loss with the ejection of mass
through an outflow is incorporated. We apply our super-Eddington accretion
model to these systems, computing their evolutionary tracks on the
color-magnitude diagram (CMD) for the Johnson and HST photometric systems. We
found that the tracks occupy specific positions on the CMD depending on the
evolutionary stage of the donor and of the binary. Moreover, their shapes are
similar, regardless the BH mass. More massive BHs lead to more luminous tracks.
We additionally compute their optical-through-X-ray Spectral Energy
Distribution (SED) considering the effects of a Comptonizing corona which
surrounds the innermost regions of the disc. We apply our model to four ULXs:
NGC4559 X-7, NGC 5204 X-1, Holmberg II X-1 and NGC 5907 ULX-2. We found that
accretion onto BHs with mass in the range 35-55 Msol is consistent with to the
observational properties of these sources. We finally explore and discuss the
possibility to extend our model also to ULXs powered by accreting Pulsars
(PULXs).
Comments:
20 pages, 14 figures, Accepted for publication in MNRAS
Abstract: The orbital International Gamma-Ray Astrophysics Laboratory (INTEGRAL),
launched in 2002, continues its successful work in observing the sky at
energies above 20 keV. The growing INTEGRAL data archive allows one to conduct
a hard X-ray all-sky survey including a number of deep extragalactic fields and
the deepest ever hard X-ray survey of the Galaxy. Taking advantage of the data
gathered over 17 years with the IBIS coded-mask telescope on board INTEGRAL, we
conducted a survey of hard X-ray sources in the 17-60 keV band, providing flux
information in different energy bands up to 290 keV. The catalog of sources
includes 929 objects, 890 of which exceed a detection threshold of 4.5 sigma
and the rest are detected at 4.0-4.5 sigma and belong to known cataloged
INTEGRAL sources and sources from the on-going all-sky survey by the BAT
telescope of the Neil Gehrels Swift Observatory. Among the identified sources
with known or suspected nature, 376 are associated with Galaxy and Magellanic
clouds, including 145 low-mass X-ray binaries, 115 high-mass X-ray binaries, 79
cataclysmic variables, and 37 of other types; and 440 are extragalactic,
including 429 active galactic nuclei (AGNs), 2 ultra-luminous sources (ULXs),
supernova remnant AT2018cow and 8 galaxy clusters. 113 sources from the catalog
remain unclassified. 46 objects are detected in the hard X-ray band for the
first time. The cumulative LogN-LogS distribution of non-blazar AGNs, based on
356 sources detected at S/N>4.5 sigma, is measured down to flux 2E-12
erg/s/cm^2 and can be described by a power law with a slope of 1.44 +/- 0.09
and a normalization of 8E-3 sources per deg^2 at fluxes >1E-11 erg/s/cm^2. The
LogN-LogS distribution of unclassified sources indicates that the majority of
them are of extragalactic origin.
Comments:
5 pages, 2 figures, 7 tables (1 long table). Submitted to MNRAS. Comments are welcome before publication
Abstract: Recent observations have shown that pulsars are surrounded by extended
regions which emit TeV-scale gamma rays through the inverse Compton scattering
of very high energy electrons and positrons. Such TeV halos are responsible for
a large fraction of the Milky Way's TeV-scale gamma-ray emission. In this
paper, we calculate the gamma-ray spectrum from the population of TeV halos
located within the Andromeda Galaxy, predicting a signal that is expected to be
detectable by the Cherenkov Telescope Array (CTA). We also calculate the
contribution from TeV halos to the isotropic gamma-ray background (IGRB),
finding that these sources should contribute significantly to this flux at the
highest measured energies, constituting up to $\sim 20\%$ of the signal
observed above $\sim 0.1 \, {\rm TeV}$.
Abstract: High velocity clouds moving toward the disk will reach the Galactic plane and
will inevitably collide with the disk. In these collisions a system of two
shocks is produced, one propagating through the disk and the other develops
within the cloud. The shocks produced within the clouds in these interactions
have velocities of hundreds of kilometers per second. When these shocks are
radiative they may be inefficient in accelerating fresh particles, however they
can reaccelerate and compress Galactic cosmic rays from the background. In this
work we investigate the interactions of Galactic cosmic rays within a shocked
high velocity cloud, when the shock is induced by the collision with the disk.
This study is focused in the case of radiative shocks. We aim to establish
under which conditions these interactions lead to significant nonthermal
emission, especially gamma rays. We model the interaction of cosmic ray protons
and electrons reaccelerated and further energized by compression in shocks
within the clouds, under very general assumptions. We also consider secondary
electron-positron pairs produced by the cosmic ray protons when colliding with
the material of the cloud. We conclude that nearby clouds reaccelerating
Galactic cosmic rays in local shocks can produce high-energy radiation that
might be detectable with existing and future gamma-ray detectors. The emission
produced by electrons and secondary pairs is important at radio wavelengths,
and in some cases it may be relevant at hard X-rays. Concerning higher
energies, the leptonic contribution to the spectral energy distribution is
significant at soft gamma rays.
Abstract: To reduce and analyze astronomical images, astronomers can rely on a wide
range of libraries providing low-level implementations of legacy algorithms.
However, combining these routines into robust and functional pipelines requires
a major effort which often ends up in instrument-specific and poorly
maintainable tools, yielding products that suffer from a low-level of
reproducibility and portability. In this context, we present prose, a Python
framework to build modular and maintainable image processing pipelines. Built
for astronomy, it is instrument-agnostic and allows the construction of
pipelines using a wide range of building blocks, pre-implemented or
user-defined. With this architecture, our package provides basic tools to deal
with common tasks such as automatic reduction and photometric extraction. To
demonstrate its potential, we use its default photometric pipeline to process
26 TESS candidates follow-up observations and compare their products to the
ones obtained with AstroImageJ, the reference software for such endeavors. We
show that prose produces light curves with lower white and red noise while
requiring less user interactions and offering richer functionalities for
reporting.
Abstract: The third Gravitational-wave Transient Catalog (GWTC-3) describes signals
detected with Advanced LIGO and Advanced Virgo up to the end of their third
observing run. Updating the previous GWTC-2.1, we present candidate
gravitational waves from compact binary coalescences during the second half of
the third observing run (O3b) between 1 November 2019, 15:00 UTC and 27 March
2020, 17:00 UTC. There are 35 compact binary coalescence candidates identified
by at least one of our search algorithms with a probability of astrophysical
origin $p_\mathrm{astro} > 0.5$. Of these, 18 were previously reported as
low-latency public alerts, and 17 are reported here for the first time. Based
upon estimates for the component masses, our O3b candidates with
$p_\mathrm{astro} > 0.5$ are consistent with gravitational-wave signals from
binary black holes or neutron star-black hole binaries, and we identify none
from binary neutron stars. However, from the gravitational-wave data alone, we
are not able to measure matter effects that distinguish whether the binary
components are neutron stars or black holes. The range of inferred component
masses is similar to that found with previous catalogs, but the O3b candidates
include the first confident observations of neutron star-black hole binaries.
Including the 35 candidates from O3b in addition to those from GWTC-2.1, GWTC-3
contains 90 candidates found by our analysis with $p_\mathrm{astro} > 0.5$
across the first three observing runs. These observations of compact binary
coalescences present an unprecedented view of the properties of black holes and
neutron stars.
Comments:
82 pages (10 pages author list, 30 pages main text, 22 pages appendices, 20 pages bibliography), 16 figures, 14 tables. Data products available from this https URL
Abstract: We use 47 gravitational-wave sources from the Third LIGO-Virgo-KAGRA
Gravitational-Wave Transient Catalog (GWTC-3) to estimate the Hubble parameter
$H(z)$, including its current value, the Hubble constant $H_0$. Each
gravitational-wave (GW) signal provides the luminosity distance to the source
and we estimate the corresponding redshift using two methods: the redshifted
masses and a galaxy catalog. Using the binary black hole (BBH) redshifted
masses, we simultaneously infer the source mass distribution and $H(z)$. The
source mass distribution displays a peak around $34\, {\rm M_\odot}$, followed
by a drop-off. Assuming this mass scale does not evolve with redshift results
in a $H(z)$ measurement, yielding $H_0=68^{+13}_{-7} {\rm
km\,s^{-1}\,Mpc^{-1}}$ ($68\%$ credible interval) when combined with the $H_0$
measurement from GW170817 and its electromagnetic counterpart. This represents
an improvement of 13% with respect to the $H_0$ estimate from GWTC-1. The
second method associates each GW event with its probable host galaxy in the
catalog GLADE+, statistically marginalizing over the redshifts of each event's
potential hosts. Assuming a fixed BBH population, we estimate a value of
$H_0=68^{+8}_{-6} {\rm km\,s^{-1}\,Mpc^{-1}}$ with the galaxy catalog method,
an improvement of 41% with respect to our GWTC-1 result and 20% with respect to
recent $H_0$ studies using GWTC-2 events. However, we show that this result is
strongly impacted by assumptions about the BBH source mass distribution; the
only event which is not strongly impacted by such assumptions (and is thus
informative about $H_0$) is the well-localized event GW190814.
Abstract: It is commonly believed that blazar jets are relativistic magnetized plasma
outflows from supermassive black holes. One key question is how the jets
dissipate magnetic energy to accelerate particles and drive powerful
multi-wavelength flares. Relativistic magnetic reconnection has been proposed
as the primary plasma physical process in the blazar emission region. Recent
numerical simulations have shown strong acceleration of nonthermal particles
that may lead to multi-wavelength flares. Nevertheless, previous works have not
directly evaluated $\gamma$-ray signatures from first-principle simulations. In
this paper, we employ combined particle-in-cell and polarized radiation
transfer simulations to study multi-wavelength radiation and optical
polarization signatures under the leptonic scenario from relativistic magnetic
reconnection. We find harder-when-brighter trends in optical and {\it
Fermi-LAT} $\gamma$-ray bands as well as closely correlated optical and
$\gamma$-ray flares. The optical polarization angle swings are also accompanied
by $\gamma$-ray flares with trivial time delays. Intriguingly, we find highly
variable synchrotron self Compton signatures due to inhomogeneous particle
distributions during plasmoid mergers. This feature may result in fast
$\gamma$-ray flares or orphan $\gamma$-ray flares under the leptonic scenario,
complementary to the frequently considered mini-jet scenario. It may also infer
neutrino emission with low secondary synchrotron flux under the hadronic
scenario, if plasmoid mergers can accelerate protons to very high energy.
Abstract: Very-long-baseline interferometry (VLBI) observations of active galactic
nuclei at millimeter wavelengths have the power to reveal the launching and
initial collimation region of extragalactic radio jets, down to $10-100$
gravitational radii ($r_g=GM/c^2$) scales in nearby sources. Centaurus A is the
closest radio-loud source to Earth. It bridges the gap in mass and accretion
rate between the supermassive black holes (SMBHs) in Messier 87 and our
galactic center. A large southern declination of $-43^{\circ}$ has however
prevented VLBI imaging of Centaurus A below ${\lambda}1$cm thus far. Here, we
show the millimeter VLBI image of the source, which we obtained with the Event
Horizon Telescope at $228$GHz. Compared to previous observations, we image
Centaurus A's jet at a tenfold higher frequency and sixteen times sharper
resolution and thereby probe sub-lightday structures. We reveal a
highly-collimated, asymmetrically edge-brightened jet as well as the fainter
counterjet. We find that Centaurus A's source structure resembles the jet in
Messier 87 on ${\sim}500r_g$ scales remarkably well. Furthermore, we identify
the location of Centaurus A's SMBH with respect to its resolved jet core at
${\lambda}1.3$mm and conclude that the source's event horizon shadow should be
visible at THz frequencies. This location further supports the universal scale
invariance of black holes over a wide range of masses.
Comments:
27 pages, 9 figures. This is a post-peer-review, pre-copyedit version of an article published in Nature Astronomy. The final authenticated version is available online at: this https URL
Abstract: We search for gravitational-wave signals associated with gamma-ray bursts
detected by the Fermi and Swift satellites during the second half of the third
observing run of Advanced LIGO and Advanced Virgo (1 November 2019 15:00 UTC-27
March 2020 17:00 UTC).We conduct two independent searches: a generic
gravitational-wave transients search to analyze 86 gamma-ray bursts and an
analysis to target binary mergers with at least one neutron star as short
gamma-ray burst progenitors for 17 events. We find no significant evidence for
gravitational-wave signals associated with any of these gamma-ray bursts. A
weighted binomial test of the combined results finds no evidence for
sub-threshold gravitational wave signals associated with this GRB ensemble
either. We use several source types and signal morphologies during the
searches, resulting in lower bounds on the estimated distance to each gamma-ray
burst. Finally, we constrain the population of low luminosity short gamma-ray
bursts using results from the first to the third observing runs of Advanced
LIGO and Advanced Virgo. The resulting population is in accordance with the
local binary neutron star merger rate.
Abstract: A peculiar aspect of Cherenkov telescopes is that they are designed to detect
atmospheric light flashes on the time scale of nanoseconds, being almost blind
to stellar sources. As a consequence, the pointing calibration of these
instruments cannot be done in general exploiting the standard astrometry of the
focal plane. In this paper we validate a procedure to overcome this problem for
the case of the innovative ASTRI telescope, developed by INAF, exploiting sky
images produced as an ancillary output by its novel Cherenkov camera. In fact,
this instrument implements a statistical technique called "Variance method"
(VAR) owning the potentiality to image the star field (angular resolution $\sim
11'$). We demonstrate here that VAR images can be exploited to assess the
alignment of the Cherenkov camera with the optical axis of the telescope down
to $\sim 1''$. To this end, we evaluate the position of the stars with
sub-pixel precision thanks to a deep investigation of the convolution between
the point spread function and the pixel distribution of the camera, resulting
in a transformation matrix that we validated with simulations. After that, we
considered the rotation of the field of view during long observing runs,
obtaining light arcs that we exploited to investigate the alignment of the
Cherenkov camera with high precision, in a procedure that we have already
tested on real data. The strategy we have adopted, inherited from optical
astronomy, has never been performed on Variance images from a Cherenkov
telescope until now, and it can be crucial to optimize the scientific accuracy
of the incoming MiniArray of ASTRI telescopes.
Comments:
Pre-print version, 15 pages, 9 figures, accepted and published on Experimental Astronomy
Abstract: This study aims to determine the main physical parameters (N(H3) hydrogen
column density and Td dust temperature) of the Interstellar medium, and their
distribution in the extended star-forming region, which includes IRAS
05156+3643, 05162+3639, 05168+3634, 05177+3636, and 05184+3635 sources. We also
provide a comparative analysis of the properties of the Interstellar medium and
young stellar objects. Analysis of the results revealed that Interstellar
medium forms relativity dense condensations around IRAS sources, which are
interconnected by a filament structure. In general, in sub-regions Td varies
from 11 to 24 K, and N(H3) - from 1.0 to 4.0 x 10^23 cm^(-2). The masses of the
ISM vary from 1.7 x 10^4 to 2.1 x 10^5 Msol. All BGPSv2 objects identified in
this star-forming region are located at the N(H3) maximum. The direction of the
outflows, which were found in two sub-regions, IRAS 05168+3634 and 05184+3635,
correlates well with the isodenses direction. The sub-regions with the highest
N(H3) and Interstellar medium mass have the largest percentage of young stellar
objects with Class I evolutionary stage. The wide spread of the evolutionary
ages of stars in all sub-regions (from 10^5 to 10^7 years) suggests that the
process of star formation in the considered region is sequential. In those
sub-regions where the mass of the initial, parent molecular cloud is larger,
this process is likely to proceed more actively. On the Gaia EDR3 database, it
can be assumed that all sub-regions are embedded in the single molecular cloud
and belong to the same star-forming region, which is located at a distance of
about 1.9 kpc.
Abstract: We present first results of the evolution of cold cosmic gas obtained through
a set of state-of-the-art numerical simulations (ColdSIM). We model
time-dependent atomic and molecular non-equilibrium chemistry coupled to HI and
H$_2$ self-shielding, various UV backgrounds as suggested by the recent
literature, H$_2$ dust grain catalysis, photoelectric heating, cosmic-ray
heating, as well as hydrodynamics, star formation and feedback effects. By
means of such non-equilibrium calculations we are finally able to reproduce the
latest HI and H$_2$ observational data. The neutral-gas mass density parameter
results around $\Omega_{\rm neutral} \!\sim\! 10^{-3}$ and increases from lower
to higher redshift ($z$). The molecular-gas mass density parameter shows peak
values of $ \Omega_{\rm H_2} \! \sim \! 10^{-4}$, while expected H$_2$
fractions can be as high as 50% of the cold gas mass at $ z\!\sim$4-8, in line
with the latest high-$z$ measurements. Both observed HI and H$_2$ trends are
well reproduced by our non-equilibrium H$_2$-based star formation modelling.
H$_2$ depletion times remain below the Hubble time and comparable to the
dynamical time at all epochs. These findings suggest that, besides HI,
non-equilibrium H$_2$ analyses are key probes for assessing the cold gas and
the role of UV background radiation. Abridged.
Comments:
To appear in the Proceedings of the International Conference entitled "mm Universe $@$ NIKA2", Rome (Italy), June 2021, EPJ Web of conferences
Abstract: Very long baseline interferometric (VLBI) localisations of repeating fast
radio bursts (FRBs) have demonstrated a diversity of local environments: from
nearby star-forming regions to globular clusters. Here we report the VLBI
localisation of FRB 20201124A using an ad-hoc array of dishes that also
participate in the European VLBI Network (EVN). In our campaign, we detected 18
total bursts from FRB 20201124A at two separate epochs. By combining the
visibilities from both observing epochs, we were able to localise FRB 20201124A
with a 1-$\sigma$ error of 4.5 milliarcseconds (mas). We use the relatively
large burst sample to investigate astrometric accuracy, and find that for
$\gtrsim20$ baselines ($\gtrsim7$ dishes) that we can robustly reach
milliarcsecond precision even using single-burst data sets. Sub-arcsecond
precision is still possible for single bursts, even when only $\sim$ six
baselines (four dishes) are available. We explore two methods for determining
the individual burst positions: the peaks of the dirty maps and a Gaussian fit
to the cross fringe pattern on the dirty maps. We found the latter to be more
reliable due to the lower mean and standard deviation in the offsets from the
FRB position. Our VLBI work places FRB 20201124A 705$\pm$26 mas (1-$\sigma$
errors) from the optical centre of the host galaxy, and consistent with
originating from within the recently-discovered extended radio structure
associated with star-formation in the host galaxy. Future high-resolution
optical observations, e.g. with Hubble Space Telescope, can determine the
proximity of our FRB 20201124A VLBI position to nearby knots of star formation.
Abstract: We present our third set of results from our mid-infrared imaging survey of
Milky Way Giant HII (GHII) regions with our detailed analysis of W49A, one of
the most distant, yet most luminous, GHII regions in the Galaxy. We used the
FORCAST instrument on the Stratospheric Observatory For Infrared Astronomy
(SOFIA) to obtain 20 and 37$\mu$m images of the entire ~5.0' x 3.5'
infrared-emitting area of W49A at a spatial resolution of ~3". Utilizing these
SOFIA data in conjunction with previous multi-wavelength observations from the
near-infrared to radio, including Spitzer-IRAC and Herschel-PACS archival data,
we investigate the physical nature of individual infrared sources and
sub-components within W49A. For individual compact sources we used the
multi-wavelength photometry data to construct spectral energy distributions
(SEDs) and fit them with massive young stellar object (MYSO) SED models, and
find 22 sources that are likely to be MYSOs. Ten new sources are identified for
the first time in this work. Even at 37$\mu$m we are unable to detect infrared
emission from the sources on the western side of the extremely extinguished
ring of compact radio emission sources known as the Welch Ring. Utilizing
multi-wavelength data, we derived luminosity-to-mass ratio and virial
parameters of the extended radio sub-regions of W49A to estimate their relative
ages and find that overall the sub-components of W49A have a very small spread
in evolutionary state compared to our previously studied GHII regions.
Comments:
35 pages, 15 figures, 10 tables, accepted for publication in ApJ
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