1,275 research outputs found
The Near-IR-Optical-UV Emission of BL Lacertae Objects
Near--infrared, optical and ultraviolet quasi--simultaneous observations of
11 BL Lacertae objects are reported. For all but one source the dereddened
spectral flux distribution in the Hz frequency
range can be described by a single power law f with
average spectral index = 0.88 0.42 (standard deviation) plus,
where relevant, the contribution of the host galaxy. In most cases the non
simultaneous soft X--ray fluxes obtained by the {\it Einstein Observatory} lie
on or below the extrapolation of the power law. The results are compared with
the average spectral properties of other samples of BL Lacs studied separately
in the IR--optical and in the UV bands. The implications for existing models of
the objects are shortly discussed.Comment: 23 pages, latex file, 2 figures available as postscript files
appended at the end of the latex text file, Ref. S.I.S.S.A. 31/94/
Abundance stratification in Type Ia supernovae - V. SN 1986G bridging the gap between normal and subluminous SNe Ia
A detailed spectroscopic analysis of SN 1986G has been performed. SN 1986G
`bridges the gap' between normal and sub luminous type Ia supernova (SNe Ia).
The abundance tomography technique is used to determine the abundance
distribution of the elements in the ejecta. SN 1986G was found to be a low
energy Chandrasekhar mass explosion. Its kinetic energy was 70% of the standard
W7 model (0.9x10erg). Oxygen dominates the ejecta from the outermost
layers down to 9000kms , intermediate mass elements (IME) dominate
from 9000kms to 3500kms with Ni and Fe dominating
the inner layers 3500kms. The final masses of the main elements
in the ejecta were found to be, O=0.33M, IME=0.69M, stable NSE=0.21M,
Ni=0.14M. An upper limit of the carbon mass is set at C=0.02M. The
spectra of SN1986G consist of almost exclusively singly ionised species.
SN1986G can be thought of as a low luminosity extension of the main population
of SN Ia, with a large deflagration phase that produced more IMEs than a
standard SN Ia.Comment: Accepted for publication in MNRAS, update
Optical and infrared photometry of the blazar PKS0537-441
We present a large collection of photometric data on the Blazar PKS 0537-441
in the VRIJHK bands taken in 2004-2009. At least three flare-like episodes with
months duration, and >3 mag amplitude are apparent. The spectral energy
distribution is consistent with a power law, and no indication of a thermal
component is found. We searched for short time scale variability, and an
interesting event was identified in the J band, with a duration of ~25 minutes.Comment: 10 pages, 3 figures, in press in ApJ
A Rapid X-ray Flare from Markarian 501
We present X-ray observations of the BL Lacertae (BL Lac) object Markarian
501 (Mrk 501), taken with the Rossi X-ray Timing Explorer in 1998 May as part
of a multi-wavelength campaign. The X-ray light curve shows a very rapid flare
in which the 2-10 keV flux increased by ~60% in <200 seconds. This rapid rise
is followed by a drop-off in the 2-10 keV flux of ~40% in <600 seconds. The
10-15 keV variation in this flare is roughly a factor of two on similar
time-scales. During the rise of the flare, the 3-15 keV spectral index hardened
from 2.02 +/- 0.03 to 1.87 +/- 0.04, where it remained during the decay of the
flare. This is the fastest variation ever seen in X-rays from Mrk 501 and among
the fastest seen at any wavelength for this object. The shift in the energy at
which the spectral power peaks (from 30 keV during
the flare) is also among the most rapid shifts seen from this object. This
flare occurs during an emission state (2-10 keV flux approximately 1.2e-10 erg
cm^-2 s^-1) that is approximately 25% of the peak flux observed in 1997 April
from this object but which is still high compared to its historical average
X-ray flux. The variations in the hardness ratio are consistent with the low
energy variations leading those at high energies during the development and
decay of the flare. This pattern is rare among high frequency peaked BL Lac
objects like Mrk 501, but has been seen recently in two other TeV emitting BL
Lacs, Mrk 421 and PKS 2155-304. The hard lag is consistent with a flare
dominated by the acceleration time-scale for a simple relativistic shock model
of flaring.Comment: 11 pages, 3 figures, accepted for publication in Astrophys. J.
Letter
An XMM-Newton look at the strongly variable radio-weak BL Lac Fermi J1544-0639
Fermi J1544-0639/ASASSN-17gs/AT2017egv was identified as a gamma-ray/optical
transient on May 15, 2017. Subsequent multiwavelength observations suggest that
this source may belong to the new class of radio-weak BL Lacs. We studied the
X-ray spectral properties and short-term variability of Fermi J1544-0639 to
constrain the X-ray continuum emission mechanism of this peculiar source. We
present the analysis of an XMM-Newton observation, 56 ks in length, performed
on February 21, 2018. The source exhibits strong X-ray variability, both in
flux and spectral shape, on timescales of ~10 ks, with a harder-when-brighter
behaviour typical of BL Lacs. The X-ray spectrum is nicely described by a
variable broken power law, with a break energy of around 2.7 keV consistent
with radiative cooling due to Comptonization of broad-line region photons. We
find evidence for a `soft excess', nicely described by a blackbody with a
temperature of ~0.2 keV, consistent with being produced by bulk Comptonization
along the jet.Comment: 11 pages, 12 figures. Accepted for publication in Astronomy &
Astrophysic
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