1,582 research outputs found
The X-ray light curve of Gamma-ray bursts: clues to the central engine
We present the analysis of a large sample of gamma-ray burst (GRB) X-ray
light curves in the rest frame to characterise their intrinsic properties in
the context of different theoretical scenarios. We determine the morphology,
time scales, and energetics of 64 long GRBs observed by \emph{Swift}/XRT
\emph{without} flaring activity. We furthermore provide a one-to-one comparison
to the properties of GRBs \emph{with} X-ray flares. We find that the steep
decay morphology and its connection with X-ray flares favour a scenario in
which a central engine origin. We show that this scenario can also account for
the shallow decay phase, provided that the GRB progenitor star has a
self-similar structure with a constant envelope-to-core mass ratio . However, difficulties arise for very long duration
( s) shallow phases. Alternatively, a spinning-down magnetar
whose emitted power refreshes the forward shock can quantitatively account for
the shallow decay properties. In particular we demonstrate that this model can
account for the plateau luminosity vs. end time anticorrelation.Comment: 12 pages, 8 figures, accepted for publication in A&
Gamma-Ray Burst long lasting X-ray flaring activity
In this paper we shed light on late time (i.e. with peak time t_{pk} \gtrsim
1000 s) flaring activity. We address the morphology and energetic of flares in
the window \sim 10^3-10^6 s to put constraints on the temporal evolution of the
flare properties and to identify possible differences in the mechanism
producing the early and late time flaring emission, if any. This requires the
complete understanding of the observational biases affecting the detection of
X-ray flares superimposed on a fading continuum at t > 1000 s. We consider all
the Swift GRBs that exhibit late time flares. Our sample consists of 36 flares,
14 with redshift measurements. We inherit the strategy of data analysis from
Chincarini et al. (2010) in order to make a direct comparison with the early
time flare properties. The morphology of the flare light curve is the same for
both early time and late time flares, while they differ energetically. The
width of late time flares increases with time similarly to the early time
flares. Simulations confirmed that the increase of the width with time is not
due to the decaying statistics, at least up to 10^4 s. The energy output of
late time flares is one order of magnitude lower than the early time flare one,
being \sim 1% E_{prompt}. The evolution of the peak luminosity as well as the
distribution of the peak flux-to-continuum ratio for late time flares indicate
that the flaring emission is decoupled from the underlying continuum,
differently from early time flares/steep decay. A sizable fraction of late time
flares are compatible with afterglow variability. The internal shock origin
seems the most promising explanation for flares. However, some differences that
emerge between late and early time flares suggest that there could be no unique
explanation about the nature of late time flares.Comment: 8 pages, 6 figures, accepted for publication in Astronomy and
Astrophysic
PS16dtm: A Tidal Disruption Event in a Narrow-line Seyfert 1 Galaxy
[Abridged] We present observations of PS16dtm, a luminous transient that
occurred at the nucleus of a known Narrow-line Seyfert 1 galaxy hosting a
10 M black hole. The transient was previously claimed to be a Type
IIn SLSN due to its luminosity and hydrogen emission lines. The light curve
shows that PS16dtm brightened by about two magnitudes in ~50 days relative to
the archival host brightness and then exhibited a plateau phase for about 100
days followed by the onset of fading in the UV. During the plateau PS16dtm
showed no color evolution, maintained a blackbody temperature of 1.7 x 10
K, and radiated at approximately of the SMBH. The spectra exhibit
multi-component hydrogen emission lines and strong FeII emission, show little
evolution with time, and closely resemble the spectra of NLS1s while being
distinct from those of Type IIn SNe. Moreover, PS16dtm is undetected in the
X-rays to a limit an order of magnitude below an archival X-ray detection of
its host galaxy. These observations strongly link PS16dtm to activity
associated with the SMBH and are difficult to reconcile with a SN origin or any
known form of AGN variability, and therefore we argue that it is a TDE in which
the accretion of the stellar debris powers the rise in the continuum and
excitation of the pre-existing broad line region, while providing material that
obscures the X-ray emitting region of the pre-existing AGN accretion disk. A
detailed TDE model fit to the light curve indicates that PS16dtm will remain
bright for several years; we further predict that the X-ray emission will
reappear on a similar timescale as the accretion rate declines. Finally, we
place PS16dtm in the context of other TDEs and find that TDEs in AGN galaxies
are an order of magnitude more efficient and reach Eddington luminosities,
likely due to interaction of the stellar debris with the pre-existing accretion
disk.Comment: 19 pages, 17 figures, Submitted to Ap
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 Unusually Luminous Extragalactic Nova SN 2010U
We present observations of the unusual optical transient SN 2010U, including
spectra taken 1.03 days to 15.3 days after maximum light that identify it as a
fast and luminous Fe II type nova. Our multi-band light curve traces the fast
decline (t_2 = 3.5 days) from maximum light (M_V = -10.2 mag), placing SN 2010U
in the top 0.5% of the most luminous novae ever observed. We find typical
ejecta velocities of approximately 1100 km/s and that SN 2010U shares many
spectral and photometric characteristics with two other fast and luminous Fe II
type novae, including Nova LMC 1991 and M31N-2007-11d. For the extreme
luminosity of this nova, the maximum magnitude vs. rate of decline relationship
indicates a massive white dwarf progenitor with a low pre-outburst accretion
rate. However, this prediction is in conflict with emerging theories of nova
populations, which predict that luminous novae from massive white dwarfs should
preferentially exhibit an alternate spectral type (He/N) near maximum light.Comment: 16 pages, 16 figures. Submitted to the Astrophysical Journa
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