280 research outputs found
Optical spectroscopic observations of gamma-ray blazar candidates II. The 2013 KPNO campaign in the Northern Hemisphere
We recently started a systematic search of low-energy counterparts of the
unidentified gamma-ray sources (UGSs) listed in the Fermi-Large Area Telescope
(LAT) First Source Catalog (1FGL) and the Fermi-LAT 2-Year Source Catalog
(2FGL).} The main goal of our investigation is to find active galaxies
belonging to the blazar class that lie within the positional uncertainty region
of the UGSs and thus could be their potential low-energy counterparts. To
achieve our aims, we first adopted several procedures based on the peculiar
observational properties of blazars in the radio and in the IR. Then we carried
out a follow-up spectroscopic campaign in the optical band to verify the nature
of the candidates selected as potential counterparts of the UGSs. Here we
present the results of the observations carried out in 2013 in the Northern
Hemisphere at Kitt Peak National Observatory (KPNO). Optical spectroscopy is
crucial to confirm the nature of the sources and can be used to estimate their
redshifts; it will also allow us to test the robustness of our methods when the
whole campaign is completed. Here we present the optical spectroscopic
observations of 39 sources. Within our sample we found that 6 sources are
blazars, candidates to be low-energy counterparts of the UGSs listed in the
2FGL. We confirm that an additional 8 sources, previously classified as active
galaxies of uncertain type and associated in the 2FGL, are also all BL Lac
objects. Moreover, we also present 20 new spectra for known blazars listed in
the Multi-frequency Catalogue of Blazars as having an uncertain redshift and/or
being classified as BL Lac candidates. We conclude that our methods for
selecting gamma-ray blazar candidates allows us to discover new blazars and
increase the list of potential low-energy counterparts for the Fermi UGSs.Comment: 27 pages, 39 figures, 1 table, A&A accepted for publication
(pre-proof version
Dust in the wind: the role of recent mass loss in long gamma-ray bursts
We study the late-time (t>0.5 days) X-ray afterglows of nearby (z<0.5) long
Gamma-Ray Bursts (GRB) with Swift and identify a population of explosions with
slowly decaying, super-soft (photon index Gamma_x>3) X-ray emission that is
inconsistent with forward shock synchrotron radiation associated with the
afterglow. These explosions also show larger-than-average intrinsic absorption
(NH_x,i >6d21 cm-2) and prompt gamma-ray emission with extremely long duration
(T_90>1000 s). Chance association of these three rare properties (i.e. large
NH_x,i, super-soft Gamma_x and extreme duration) in the same class of
explosions is statistically unlikely. We associate these properties with the
turbulent mass-loss history of the progenitor star that enriched and shaped the
circum-burst medium. We identify a natural connection between NH_x,i Gamma_x
and T_90 in these sources by suggesting that the late-time super-soft X-rays
originate from radiation reprocessed by material lost to the environment by the
stellar progenitor before exploding, (either in the form of a dust echo or as
reprocessed radiation from a long-lived GRB remnant), and that the interaction
of the explosion's shock/jet with the complex medium is the source of the
extremely long prompt emission. However, current observations do not allow us
to exclude the possibility that super-soft X-ray emitters originate from
peculiar stellar progenitors with large radii that only form in very dusty
environments.Comment: 6 pages, Submitted to Ap
The First VLBI Image of the Young, Oxygen-Rich Supernova Remnant in NGC 4449
We report on sensitive 1.4-GHz VLBI radio observations of the unusually luminous supernova remnant SNR 4449-1 in the galaxy NGC 4449, which gave us the first well-resolved image of this object. The remnant\u27s radio morphology consists of two approximately parallel bright ridges, suggesting similarities to the barrel shape seen for many older Galactic supernova remnants or possibly to SN 1987A. The angular extent of the remnant is 65 × 40 mas, corresponding to (3.7 × 2.3) × 1018 (D/3.8 Mpc) cm. We also present a new, high signal-to-noise ratio optical spectrum. By comparing the remnant\u27s linear size to the maximum velocities measured from optical lines, as well as using constraints from historical images, we conclude that the supernova explosion occurred between ∼1905 and 1961, likely around 1940. The age of the remnant is therefore likely ∼70 yr. We find that SNR 4449-1\u27s shock wave is likely still interacting with the circumstellar rather than interstellar medium
Optical archival spectra of blazar candidates of uncertain type in the 3 Fermi Large Area Telescope Catalog
Despite the fact that blazars constitute the rarest class among active
galactic nuclei (AGNs) they are the largest known population of associated
-ray sources. Many of the -ray objects listed in the
Fermi-Large Area Telescope Third Source catalog (3FGL) are classified as blazar
candidates of uncertain type (BCUs), either because they show multifrequency
behaviour similar to blazars but lacking optical spectra in the literature, or
because the quality of such spectra is too low to confirm their nature. Here we
select, out of 585 BCUs in the 3FGL, 42 BCUs which we identify as probable
blazars by their WISE infrared colors and which also have optical spectra that
are available in the Sloan Digital Sky Survey (SDSS) and/or Six-Degree Field
Galaxy Survey Database (6dFGS). We confirm the blazar nature of all of the
sources. We furthermore conclude that 28 of them are BL Lacs, 8 are radio-loud
quasars with flat radio spectrum and 6 are BL Lac whose emission is dominated
by their host galaxy
Relativistic supernovae have shorter-lived central engines or more extended progenitors: the case of SN\,2012ap
Deep late-time X-ray observations of the relativistic, engine-driven, type Ic
SN2012ap allow us to probe the nearby environment of the explosion and reveal
the unique properties of relativistic SNe. We find that on a local scale of
~0.01 pc the environment was shaped directly by the evolution of the progenitor
star with a pre-explosion mass-loss rate <5x10^-6 Msun yr-1 in line with GRBs
and the other relativistic SN2009bb. Like sub-energetic GRBs, SN2012ap is
characterized by a bright radio emission and evidence for mildly relativistic
ejecta. However, its late time (t~20 days) X-ray emission is ~100 times fainter
than the faintest sub-energetic GRB at the same epoch, with no evidence for
late-time central engine activity. These results support theoretical proposals
that link relativistic SNe like 2009bb and 2012ap with the weakest observed
engine-driven explosions, where the jet barely fails to breakout. Furthermore,
our observations demonstrate that the difference between relativistic SNe and
sub-energetic GRBs is intrinsic and not due to line-of-sight effects. This
phenomenology can either be due to an intrinsically shorter-lived engine or to
a more extended progenitor in relativistic SNe.Comment: Version accepted to ApJ. Significantly broadened discussio
The Gamma-ray Blazar Quest: new optical spectra, state of art and future perspectives
We recently developed a procedure to recognize gamma-ray blazar candidates
within the positional uncertainty regions of the unidentified/unassociated
gamma-ray sources (UGSs). Such procedure was based on the discovery that Fermi
blazars show peculiar infrared colors. However, to confirm the real nature of
the selected candidates, optical spectroscopic data are necessary. Thus, we
performed an extensive archival search for spectra available in the literature
in parallel with an optical spectroscopic campaign aimed to reveal and confirm
the nature of the selected gamma-ray blazar candidates. Here, we first search
for optical spectra of a selected sample of gamma-ray blazar candidates that
can be potential counterparts of UGSs using the Sloan Digital Sky Survey (SDSS
DR12). This search enables us to update the archival search carried out to
date. We also describe the state-of-art and the future perspectives of our
campaign to discover previously unknown gamma-ray blazars.Comment: 11 pages, 2 figures, 5 tables, pre-proof version, accepted for
publication of Astrophysics and Space Scienc
The First VLBI Image of the Young, Oxygen-Rich Supernova Remnant in NGC 4449
We report on sensitive 1.4-GHz VLBI radio observations of the unusually
luminous supernova remnant SNR 4449-1 in the galaxy NGC 4449, which gave us the
first well-resolved image of this object. The remnant's radio morphology
consists of two approximately parallel bright ridges, suggesting similarities
to the barrel shape seen for many older Galactic supernova remnants or possibly
to SN 1987A. The angular extent of the remnant is 65 x 40 mas, corresponding to
(3.7 x 2.3) x 10^{18} (D/3.8 Mpc) cm. We also present a new, high
signal-to-noise optical spectrum. By comparing the remnant's linear size to the
maximum velocities measured from optical lines, as well as using constraints
from historical images, we conclude that the supernova explosion occurred
between ~1905 and 1961, likely around 1940. The age of the remnant is therefore
likely ~70 yr. We find that SNR 4449-1's shock wave is likely still interacting
with the circumstellar rather than interstellar medium.Comment: 7 pages, Accepted for publication in MNRA
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