50 research outputs found
Photometry of comet 9P/Tempel 1 during the 2004/2005 approach and the Deep Impact module impact
The results of the 9P/Tempel 1 CARA (Cometary Archive for Amateur
Astronomers) observing campaign is presented. The main goal was to perform an
extended survey of the comet as a support to the Deep Impact (DI) Mission. CCD
R, I and narrowband aperture photometries were used to monitor the
quantity. The observed behaviour showed a peak of 310 cm 83 days before
perihelion, but we argue that it could be distorted by the phase effect, too.
The phase effect is roughly estimated around 0.0275 mag/degree, but we had no
chance for direct determination because of the very similar geometry of the
observed apparitions. The log-slope of was around -0.5 between about
180--100 days before the impact but evolved near the steady-state like 0 value
by the impact time. The DI module impact caused an about 60%{} increase in the
value of and a cloud feature in the coma profile which was observed
just after the event. The expansion of the ejecta cloud was consistent with a
fountain model with initial projected velocity of 0.2 km/s and =0.73.
Referring to a 25~000 km radius area centered on the nucleus, the total cross
section of the ejected dust was 8.2/ km 0.06 days after the impact, and
1.2/ km 1.93 days after the impact ( is the dust albedo). 5 days
after the event no signs of the impact were detected nor deviations from the
expected activity referring both to the average pre-impact behaviour and to the
previous apparitions ones.Comment: 25 pages (including cover pages), 9 figures, 1 table, accepted by
Icarus DI Special Issu
Multifrequency monitoring of the blazar 0716+714 during the GASP-WEBT-AGILE campaign of 2007
Since the CGRO operation in 1991-2000, one of the primary unresolved
questions about the blazar gamma-ray emission has been its possible correlation
with the low-energy (in particular optical) emission. To help answer this
problem, the Whole Earth Blazar Telescope (WEBT) consortium has organized the
GLAST-AGILE Support Program (GASP) to provide the optical-to-radio monitoring
data to be compared with the gamma-ray detections by the AGILE and GLAST
satellites. This new WEBT project started in early September 2007, just before
a strong gamma-ray detection of 0716+714 by AGILE. We present the GASP-WEBT
optical and radio light curves of this blazar obtained in July-November 2007,
about various AGILE pointings at the source. We construct NIR-to-UV spectral
energy distributions (SEDs), by assembling GASP-WEBT data together with UV data
from the Swift ToO observations of late October. We observe a contemporaneous
optical-radio outburst, which is a rare and interesting phenomenon in blazars.
The shape of the SEDs during the outburst appears peculiarly wavy because of an
optical excess and a UV drop-and-rise. The optical light curve is well sampled
during the AGILE pointings, showing prominent and sharp flares. A future
cross-correlation analysis of the optical and AGILE data will shed light on the
expected relationship between these flares and the gamma-ray events.Comment: 5 pages, 5 figures, to be published in A&A (Letters); revised to
match the final version (changes in Fig. 5 and related text
Another look at the BL Lacertae flux and spectral variability
The GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope
(WEBT) monitored BL Lacertae in 2008-2009 at radio, near-IR, and optical
frequencies. During this period, high-energy observations were performed by
XMM-Newton, Swift, and Fermi. We analyse these data with particular attention
to the calibration of Swift UV data, and apply a helical jet model to interpret
the source broad-band variability. The GASP-WEBT observations show an optical
flare in 2008 February-March, and oscillations of several tenths of mag on a
few-day time scale afterwards. The radio flux is only mildly variable. The UV
data from both XMM-Newton and Swift seem to confirm a UV excess that is likely
caused by thermal emission from the accretion disc. The X-ray data from
XMM-Newton indicate a strongly concave spectrum, as well as moderate flux
variability on an hour time scale. The Swift X-ray data reveal fast (interday)
flux changes, not correlated with those observed at lower energies. We compare
the spectral energy distribution (SED) corresponding to the 2008 low-brightness
state, which was characterised by a synchrotron dominance, to the 1997 outburst
state, where the inverse-Compton emission was prevailing. A fit with an
inhomogeneous helical jet model suggests that two synchrotron components are at
work with their self inverse-Compton emission. Most likely, they represent the
radiation from two distinct emitting regions in the jet. We show that the
difference between the source SEDs in 2008 and 1997 can be explained in terms
of pure geometrical variations. The outburst state occurred when the
jet-emitting regions were better aligned with the line of sight, producing an
increase of the Doppler beaming factor. Our analysis demonstrates that the jet
geometry can play an extremely important role in the BL Lacertae flux and
spectral variability.Comment: 12 pages, 10 figures, accepted for publication in A&
Dissecting the long-term emission behaviour of the BL Lac object Mrk 421
We report on long-term multiwavelengthmonitoring of blazar Mrk 421 by the GLAST-AGILE
Support Program of the Whole Earth Blazar Telescope (GASP-WEBT) collaboration and
Steward Observatory, and by the Swift and Fermi satellites. We study the source behaviour in
the period 2007â2015, characterized by several extreme flares. The ratio between the optical,
X-ray and Îł -ray fluxes is very variable. The Îł -ray flux variations show a fair correlation with
the optical ones starting from 2012.We analyse spectropolarimetric data and find wavelengthdependence
of the polarization degree (P), which is compatible with the presence of the
host galaxy, and no wavelength dependence of the electric vector polarization angle (EVPA).
Optical polarimetry shows a lack of simple correlation between P and flux and wide rotations of
the EVPA.We build broad-band spectral energy distributions with simultaneous near-infrared
and optical data from the GASP-WEBT and ultraviolet and X-ray data from the Swift satellite.
They show strong variability in both flux and X-ray spectral shape and suggest a shift of
the synchrotron peak up to a factor of âŒ50 in frequency. The interpretation of the flux and
spectral variability is compatible with jet models including at least two emitting regions that
can change their orientation with respect to the line of sight.http://10.0.4.69/mnras/stx2185Accepted manuscrip
WEBT multiwavelength monitoring and XMM-Newton observations of BL Lacertae in 2007-2008. Unveiling different emission components
In 2007-2008 we carried out a new multiwavelength campaign of the Whole Earth
Blazar Telescope (WEBT) on BL Lacertae, involving three pointings by the
XMM-Newton satellite, to study its emission properties. The source was
monitored in the optical-to-radio bands by 37 telescopes. The brightness level
was relatively low. Some episodes of very fast variability were detected in the
optical bands. The X-ray spectra are well fitted by a power law with photon
index of about 2 and photoelectric absorption exceeding the Galactic value.
However, when taking into account the presence of a molecular cloud on the line
of sight, the data are best fitted by a double power law, implying a concave
X-ray spectrum. The spectral energy distributions (SEDs) built with
simultaneous radio-to-X-ray data at the epochs of the XMM-Newton observations
suggest that the peak of the synchrotron emission lies in the near-IR band, and
show a prominent UV excess, besides a slight soft-X-ray excess. A comparison
with the SEDs corresponding to previous observations with X-ray satellites
shows that the X-ray spectrum is extremely variable. We ascribe the UV excess
to thermal emission from the accretion disc, and the other broad-band spectral
features to the presence of two synchrotron components, with their related SSC
emission. We fit the thermal emission with a black body law and the non-thermal
components by means of a helical jet model. The fit indicates a disc
temperature greater than 20000 K and a luminosity greater than 6 x 10^44 erg/s.Comment: 11 pages, 7 figures, accepted for publication in A&
Long-term multi-wavelength variability and correlation study of Markarian 421 from 2007 to 2009
We study the multi-band variability and correlations of the TeV blazar Mrk
421 on year time scales, which can bring additional insight on the processes
responsible for its broadband emission. We observed Mrk 421 in the very high
energy (VHE) gamma-ray range with the Cherenkov telescope MAGIC-I from March
2007 to June 2009 for a total of 96 hours of effective time after quality cuts.
The VHE flux variability is quantified with several methods, including the
Bayesian Block algorithm, which is applied to data from Cherenkov telescopes
for the first time. The 2.3 year long MAGIC light curve is complemented with
data from the Swift/BAT and RXTE/ASM satellites and the KVA, GASP-WEBT, OVRO,
and Mets\"ahovi telescopes from February 2007 to July 2009, allowing for an
excellent characterisation of the multi-band variability and correlations over
year time scales. Mrk 421 was found in different gamma-ray emission states
during the 2.3 year long observation period. Flares and different levels of
variability in the gamma-ray light curve could be identified with the Bayesian
Block algorithm. The same behaviour of a quiet and active emission was found in
the X-ray light curves measured by Swift/BAT and the RXTE/ASM, with a direct
correlation in time. The behaviour of the optical light curve of GASP-WEBT and
the radio light curves by OVRO and Mets\"ahovi are different as they show no
coincident features with the higher energetic light curves and a less variable
emission. The fractional variability is overall increasing with energy. The
comparable variability in the X-ray and VHE bands and their direct correlation
during both high- and low-activity periods spanning many months show that the
electron populations radiating the X-ray and gamma-ray photons are either the
same, as expected in the Synchrotron-Self-Compton mechanism, or at least
strongly correlated, as expected in electromagnetic cascades.Comment: Corresponding authors: Ann-Kristin Overkemping
([email protected]), Marina Manganaro
([email protected]), Diego Tescaro ([email protected]), To be published
in Astronomy&Astrophysics (A&A), 12 pages, 9 figure
Multiwavelength observations of 3C 454.3. III. Eighteen months of AGILE monitoring of the "Crazy Diamond"
We report on 18 months of multiwavelength observations of the blazar 3C 454.3
(Crazy Diamond) carried out in July 2007-January 2009. We show the results of
the AGILE campaigns which took place on May-June 2008, July-August 2008, and
October 2008-January 2009. During the May 2008-January 2009 period, the source
average flux was highly variable, from an average gamma-ray flux F(E>100MeV) >
200E-8 ph/cm2/s in May-June 2008, to F(E>100MeV)~80E-8 ph/cm2/s in October
2008-January 2009. The average gamma-ray spectrum between 100 MeV and 1 GeV can
be fit by a simple power law (Gamma_GRID ~ 2.0 to 2.2). Only 3-sigma upper
limits can be derived in the 20-60 keV energy band with Super-AGILE. During
July-August 2007 and May-June 2008, RXTE measured a flux of F(3-20 keV)=
8.4E-11 erg/cm2/s, and F(3-20 keV)=4.5E-11 erg/cm2/s, respectively and a
constant photon index Gamma_PCA=1.65. Swift/XRT observations were carried out
during all AGILE campaigns, obtaining a F(2-10 keV)=(0.9-7.5)E-11 erg/cm2/s and
a photon index Gamma_XRT=1.33-2.04. BAT measured an average flux of ~5 mCrab.
GASP-WEBT monitored 3C 454.3 during the whole 2007-2008 period from the radio
to the optical. A correlation analysis between the optical and the gamma-ray
fluxes shows a time lag of tau=-0.4 days. An analysis of 15 GHz and 43 GHz VLBI
core radio flux observations shows an increasing trend of the core radio flux,
anti- correlated with the higher frequency data. The modeling SEDs, and the
behavior of the long-term light curves in different energy bands, allow us to
compare the jet properties during different emission states, and to study the
geometrical properties of the jet on a time-span longer than one year.Comment: Accepted for publication in ApJ. Adapted Abstract. 17 pages, 19
Figures, 5 Table
The GASP-WEBT monitoring of 3C 454.3 during the 2008 optical-to-radio and gamma-ray outburst
Since 2001, the radio quasar 3C 454.3 has undergone a period of high optical
activity, culminating in the brightest optical state ever observed, during the
2004-2005 outburst. The Whole Earth Blazar Telescope (WEBT) consortium has
carried out several multifrequency campaigns to follow the source behaviour.
The GLAST-AGILE Support Program (GASP) was born from the WEBT to provide
long-term continuous optical-to-radio monitoring of a sample of gamma-loud
blazars, during the operation of the AGILE and GLAST (now known as Fermi GST)
gamma-ray satellites. The main aim is to shed light on the mechanisms producing
the high-energy radiation, through correlation analysis with the low-energy
emission. Thus, since 2008 the monitoring task on 3C 454.3 passed from the WEBT
to the GASP, while both AGILE and Fermi detected strong gamma-ray emission from
the source. We present the main results obtained by the GASP at optical, mm,
and radio frequencies in the 2008-2009 season, and compare them with the WEBT
results from previous years. An optical outburst was observed to peak in mid
July 2008, when Fermi detected the brightest gamma-ray levels. A
contemporaneous mm outburst maintained its brightness for a longer time, until
the cm emission also reached the maximum levels. The behaviour compared in the
three bands suggests that the variable relative brightness of the
different-frequency outbursts may be due to the changing orientation of a
curved inhomogeneous jet. The optical light curve is very well sampled during
the entire season, which is also well covered by the various AGILE and Fermi
observing periods. The relevant cross-correlation studies will be very
important in constraining high-energy emission models.Comment: 5 pages, 5 figures, to be published in A&A (Letters
Dust Environment Model of the Interstellar Comet 2I/Borisov
2I/Borisov is the first interstellar comet discovered on 2019 August 30, and it soon showed a coma and a dust tail. This study reports the results of images obtained at the Telescopio Nazionale Galileo telescope, on La Palma - Canary Islands, in 2019 November and December. The images have been obtained with the R filter in order to apply our dust tail model. The model has been applied to the comet 67P/Churyumov-Gerasimenko and compared to the Rosetta dust measurements showing a very good agreement. It has been applied to the comet 2I/Borisov, using almost the same parameters, obtaining a dust environment similar to that of 67P/Churyumov-Gerasimenko, suggesting that the activity may be very similar. The dust tail analysis provided a dust-loss rate Qd â 35 kg s-1 in 2019 November and Qd â 30 kg s-1 in 2019 December
AGILE detection of intense gamma-ray emission from the blazar PKS 1510-089
We report the detection by the AGILE (Astro-rivelatore Gamma a Immagini
LEggero) satellite of an intense gamma-ray flare from the source AGL
J1511-0909, associated with the powerful quasar PKS 1510-089, during ten days
of observations from 23 August to 1 September 2007. During the observation
period, the source was in optical decrease following a flaring event monitored
by the GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope
(WEBT). The simultaneous gamma-ray, optical, and radio coverage allows us to
study the spectral energy distribution and the theoretical models based on the
synchrotron and inverse Compton (IC) emission mechanisms. AGILE observed the
source with its two co-aligned imagers, the Gamma-Ray Imaging Detector and the
hard X-ray imager Super-AGILE sensitive in the 30 MeV - 50 GeV and 18 - 60 keV
bands, respectively. Between 23 and 27 August 2007, AGILE detected gamma-ray
emission from PKS 1510-089 when this source was located about 50 degrees
off-axis, with an average flux of (270 +/- 65) x 10^{-8} photons cm^{-2} s^{-1}
for photon energy above 100 MeV. In the following period, 28 August - 1
September, after a satellite re-pointing, AGILE detected the source at about 35
degrees off-axis, with an average flux (E > 100 MeV) of (195 +/- 30) x 10^{-8}
photons cm^{-2} s^{-1}. No emission was detected by Super-AGILE, with a 3-sigma
upper limit of 45 mCrab in 200 ksec. The spectral energy distribution is
modelled with a homogeneous one-zone synchrotron self Compton (SSC) emission
plus contributions by external photons: the SSC emission contributes primarily
to the X-ray band, whereas the contribution of the IC from the external disc
and the broad line region match the hard gamma-ray spectrum observed.Comment: 5 pages, 5 figures, accepted for publication in A&