930 research outputs found
The Ultraluminous X-ray Sources near the Center of M82
We report the identification of a recurrent ultraluminous X-ray source (ULX),
a highly absorbed X-ray source (possibly a background AGN), and a young
supernova remnant near the center of the starburst galaxy M82. From a series of
Chandra observations taken from 1999 to 2005, we found that the transient ULX
first appeared in 1999 October. The source turned off in 2000 January, but
later reappeared and has been active since then. The X-ray luminosity of this
source varies from below the detection level (~2.5e38 erg/s) to its active
state in between ~7e39 erg/s and 1.3e40 erg/s (in the 0.5-10 keV energy band)
and shows unusual spectral changes. The X-ray spectra of some Chandra
observations are best fitted with an absorbed power-law model with photon index
ranging from 1.3 to 1.7. These spectra are similar to those of Galactic black
hole binary candidates seen in the low/hard state except that a very hard
spectrum was seen in one of the observations. By comparing with near infrared
images taken with the Hubble Space Telescope, the ULX is found to be located
within a young star cluster. Radio imaging indicates that it is associated with
a H II region. We suggest that the ULX is likely to be a > 100 solar mass
intermediate-mass black hole in the low/hard state. In addition to the
transient ULX, we also found a highly absorbed hard X-ray source which is
likely to be an AGN and an ultraluminous X-ray emitting young supernova remnant
which may be related to a 100-year old gamma-ray burst event, within 2 arcsec
of the transient ULX.Comment: 9 pages, 8 figures. Accepted for publication in Ap
Observing Supernova 1987A with the Refurbished Hubble Space Telescope
Observations with the Hubble Space Telescope (HST), conducted since 1990, now
offer an unprecedented glimpse into fast astrophysical shocks in the young
remnant of supernova 1987A. Comparing observations taken in 2010 using the
refurbished instruments on HST with data taken in 2004, just before the Space
Telescope Imaging Spectrograph failed, we find that the Ly-a and H-a lines from
shock emission continue to brighten, while their maximum velocities continue to
decrease. We observe broad blueshifted Ly-a, which we attribute to resonant
scattering of photons emitted from hotspots on the equatorial ring. We also
detect NV~\lambda\lambda 1239,1243 A line emission, but only to the red of
Ly-A. The profiles of the NV lines differ markedly from that of H-a, suggesting
that the N^{4+} ions are scattered and accelerated by turbulent electromagnetic
fields that isotropize the ions in the collisionless shock.Comment: Science, accepted. Science Express, 02 Sept 2010. 5 figures.
Supporting online material can be found at
http://www.sciencemag.org/cgi/content/full/sci;science.1192134/DC
Observations of the Crab Nebula and its pulsar in the far-ultraviolet and in the optical
We present HST/STIS far-UV observations of the Crab nebula and its pulsar.
Broad, blueshifted absorption arising in the nebula is seen in C IV 1550,
reaching about 2500 km/s. This can be interpreted as evidence for a fast outer
shell, and we adopt a spherically symmetric model to constrain the properties
of this. We find that the density appears to decrease outward in the shell. A
lower limit to the mass is 0.3 solar masses with an accompanying kinetic energy
of 1.5EE{49} ergs. A massive 10^{51} erg shell cannot be excluded, but is less
likely if the density profile is much steeper than R^{-4} and the velocity is
<6000 km/s. The observations cover the region 1140-1720 A. With the time-tag
mode of the spectrograph we obtain the pulse profile. It is similar to that in
the near-UV, although the primary peak is marginally narrower. Together with
the near-UV data, and new optical data from NOT, our spectrum of the pulsar
covers the entire region from 1140-9250 A. Dereddening the spectrum gives a
flat spectrum for E(B-V)=0.52, R=3.1. This dereddened spectrum of the Crab
pulsar can be fitted by a power law with spectral index alpha_{\nu} = 0.11 +/-
0.04. The main uncertainty is the amount and characteristics of the interstel-
lar reddening, and we have investigated the dependence of \alpha_{\nu} on
E(B-V) and R. In the extended emission covered by our 25" x 0.5" slit in the
far-UV, we detect C IV 1550 and He II 1640 emission lines from the Crab nebula.
Several interstellar absorption lines are detected toward the pulsar. The Ly
alpha absorption indicates a column density of 3.0+/-0.5\EE{21} cm^{-2} of
neutral hydrogen, which agrees well with our estimate of E(B-V)=0.52 mag. Other
lines show no evidence of severe depletion of metals in atomic gas.Comment: 18 pages emulateapj style, including 10 figures. ApJ, accepte
Measurement of Cosmic-ray Muons and Muon-induced Neutrons in the Aberdeen Tunnel Underground Laboratory
We have measured the muon flux and production rate of muon-induced neutrons
at a depth of 611 m water equivalent. Our apparatus comprises three layers of
crossed plastic scintillator hodoscopes for tracking the incident cosmic-ray
muons and 760 L of gadolinium-doped liquid scintillator for producing and
detecting neutrons. The vertical muon intensity was measured to be cmssr. The yield of
muon-induced neutrons in the liquid scintillator was determined to be
neutrons/(gcm). A fit to the recently measured neutron
yields at different depths gave a mean muon energy dependence of for liquid-scintillator targets.Comment: 14 pages, 17 figures, 3 table
Circumstellar Na I and Ca II lines in type IIP supernovae and SN 1998S
We study a possibility of detection of circumstellar absorption lines of Na I
D and Ca II H,K in spectra of type IIP supernovae at the photospheric
epoch. The modelling shows that the circumstellar lines of Na I doublet will
not be seen in type IIP supernovae for moderate wind density, e.g.,
characteristic of SN 1999em, whereas rather pronounced Ca II lines with P Cygni
profile should be detectable. A similar model is used to describe Na I and Ca
II circumstellar lines seen in SN 1998S, type IIL with a dense wind. We show
that line intensities in this supernova are reproduced, if one assumes an
ultraviolet excess, which is caused primarily by the comptonization of
supernova radiation in the shock wave.Comment: To be published in Astronomy Letter
The asymmetric radio remnant of SN 1987A
We present seven years of radio observations of SN 1987A made with the
Australia Telescope Compact Array. At 1.4, 2.4, 4.8 and 8.6 GHz, the flux
density of the radio remnant has increased monotonically since emission was
redetected 1200 days after the explosion. On day 3200, the remnant was
expanding at 2800 +/- 400 km/s, which we interpret as indicating significant
deceleration of the fastest moving ejecta. Since day 1787 the spectral index
has remained constant at alpha = -0.95 +/- 0.04. These observations are all
consistent with the shock having encountered a denser, shocked, component of
the progenitor's stellar wind. At the current rate of expansion, the shock is
expected to encounter the inner optical ring in the year 2006 +/- 3.
Using super-resolution, we have also obtained 9 GHz images of the remnant
(resolution approx 0".5) at four epochs. The emission is distributed around the
rim of a near-circular shell, but has become increasingly asymmetric with time.
There are two "hotspots" to the east and west, aligned along the major axis of
the optical ring. This morphology is most likely indicative of an axisymmetric
circumstellar medium into which the shock is expanding, consistent with present
understanding of the progenitor star and its environment.Comment: 45 pages, LaTeX, including 15 PostScript figures. To appear in "The
Astrophysical Journal", volume 479 (20 Apr 1997
Monitoring the Evolution of the X-ray Remnant of SN 1987A
We report on the results of our monitoring program of the remnant of SN 1987A
with the Advanced CCD Imaging Spectrometer (ACIS) on board the {\it Chandra
X-ray Observatory}. Two new observations have been performed in AO2, bringing
the total to four monitoring observations over the past two years. Over this
time period, new techniques for correction of ``Charge Transfer Inefficiency
(CTI)'' and for use of charge spreading to provide angular resolution somewhat
better than the pixel size of the CCD detector have become available at Penn
State. We have processed all four observations using sub-pixel resolution to
obtain the highest possible angular resolution, and using our CTI correction
software to provide more reliable spectral analysis and flux estimations.
The high angular resolution images indicate that the X-ray bright knots are
convincingly correlated with the optical spots, primarily at \la1 keV, while
higher energy photons are very well correlated with radio images. Our data also
provide marginal evidence for radial expansion of the X-ray remnant at a rate
of 5200 2100 km s. The X-ray flux appears to linearly increase by
60% over the 18 month period of these observations. The spectrum is
dominated by broad complexes of atomic emission lines and can be fit with a
simple model of a plane-parallel shock with electron temperatures of
2 4 keV and a postshock electron density of 210
420 cm. The implied 0.5 10 keV band luminosity in 2001 April is
1.3 10 ergs s; as of that date, we still observe
no direct evidence for the central point source, with an upper limit on the
{\it observed} luminosity of 5.5 10 ergs
s in the 2 10 keV band.Comment: 29 pages, 5 figures (2 color images), Accepted for Ap
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