173 research outputs found
The Distribution of H2O Maser Emission in the Nucleus of NGC 4945
We present the first interferometer map of the water maser emission in the
active nucleus of NGC 4945. Although the declination of the galaxy is about -49
degrees, we were able to make the observations with the southernmost antennas
of the Very Long Baseline Array. Strong maser emission is present in three
velocity ranges, one near the systemic velocity and two shifted roughly
symmetrically by +/-(100-150) km/s. This is the first detection of highly
blue-shifted water emission in NGC 4945. We determined the position of the
maser to be RA(B1950)= 13 02 32.28 +/- 0.02 ; Dec(B1950)= -49 12 01.9 +/- 0.1.
The uncertainties in earlier estimates are at least several arcseconds. The
maser lies within 2'' (36 pc at a distance of 3.7 Mpc) of the peaks in 1.4 GHz
continuum and 1.6 micron emission from the nucleus. The mappable maser emission
is distributed roughly linearly over about 40 milliarcseconds (0.7 pc) at a
position angle of about 45 degrees, which is close to the 43 +/- 2 degree
position angle of the galactic disk. The red and blue-shifted emission
symmetrically stradle the systemic emission on the sky, which suggests material
in edge-on circular motion around a central object. The position-velocity
structure indicates a binding mass of about one million Suns, within a volume
of radius about 0.3 pc. This implies that the central engine radiates on the
order of 10% of its Eddington luminosity.Comment: 18 pages, including 5 Postscript figures. Accepted for publication in
ApJ Letter
LOTIS Upper Limits and the Prompt OT from GRB 990123
GRB 990123 established the existence of prompt optical emission from
gamma-ray bursts (GRBs). The Livermore Optical Transient Imaging System (LOTIS)
has been conducting a fully automated search for this kind of simultaneous low
energy emission from GRBs since October 1996. Although LOTIS has obtained
simultaneous, or near simultaneous, coverage of the error boxes obtained with
BATSE, IPN, XTE, and BeppoSAX for several GRBs, image analysis resulted in only
upper limits. The unique gamma-ray properties of GRB 990123, such as very large
fluence (top 0.4%) and hard spectrum, complicate comparisons with more typical
bursts. We scale and compare gamma-ray properties, and in some cases afterglow
properties, from the best LOTIS events to those of GRB 990123 in an attempt to
determine whether the prompt optical emission of this event is representative
of all GRBs. Furthermore, using LOTIS upper limits in conjunction with the
relativistic blast wave model, we weakly constrain the GRB and afterglow
parameters such as density of the circumburster medium and bulk Lorentz factor
of the ejecta.Comment: 5 pages, 2 figures, To appear in Proceedings of the 5th Huntsville
Gamma-Ray Burst Symposiu
LOTIS Search for Early Time Optical Afterglows: GRB 971227
We report on the very early time search for an optical afterglow from GRB
971227 with the Livermore Optical Transient Imaging System (LOTIS). LOTIS began
imaging the `Original' BATSE error box of GRB 971227 approximately 14 s after
the onset of gamma-ray emission. Continuous monitoring of the position
throughout the evening yielded a total of 499 images (10 s integration).
Analysis of these images revealed no steady optical afterglow brighter than
R=12.3 +- 0.2 in any single image. Coaddition of different combinations of the
LOTIS images also failed to uncover transient optical emission. In particular,
assuming a constant early time flux, no optical afterglow brighter than R=14.2
+- 0.2 was present within the first 1200 s and no optical afterglow brighter
than R=15.0 +- 0.2 was present in the first 6.0 h.
Follow up observations by other groups revealed a likely X-ray afterglow and
a possible optical afterglow. Although subsequent deeper observations could not
confirm a fading source, we show that these transients are not inconsistent
with our present knowledge of the characteristics of GRB afterglows. We also
demonstrate that with the upgraded thermoelectrically cooled CCDs, LOTIS is
capable of either detecting very early time optical afterglow or placing
stringent constraints on the relationship between the gamma-ray emission and
the longer wavelength afterglow in relativistic blast wave models.Comment: 17 pages, 3 eps figures, revisions based on reviewers comment
Sco X-1: The Evolution and Nature of the Twin Compact Radio Lobes
The radio components associated with the LMXB Sco X-1 have been monitored
with extensive VLBI imaging at 1.7 and 5.0 GHz over four years, including a
56-hour continuous VLBI observation in 1999 June. We often detected one strong
and one weak compact radio component, moving in opposite directions from the
radio core. We suggest that the moving components are radio lobes generated by
the disruption of energy flow in a twin-beam from the binary system. The
average lifetime of a lobe-pair, the space motion of the lobes and the measured
energy flow in the beam are discussed in arXiv:astro-ph/0104325. The lobe has a
flux density that is variable over a time-scale of one hour, a measured minimum
size of 1 mas (2.8 au), and is extended perpendicular to its motion. This short
electron radiative lifetime may be caused by synchrotron losses if the lobe
magnetic field is 300 G, or by adiabatic expansion of the electrons as soon as
they are produced at the working surface. The lobes also show periods of slow
expansion and a steepening radio spectrum, perhaps related to the
characteristics of the beam energy flow. The radio morphology for Sco X-1 is
more simple than for most other Galactic jet sources. The lobes of Sco X-1 are
similar to hot-spots found in many extragalactic double sources. Scaling the
phenomena observed in Sco X-1 to extragalactic sources implies hot-spot
variability time-scales of 10^4 yr and hot-spot lifetimes of 10^5 yr. The
recurrent formation of lobes in Sco X-1 probably does not occur for
extragalactic radio sources.Comment: 22 pages of text + 16 figures. ApJ, in pres
CO-bandhead spectroscopy of IC 342: mass and age of the nuclear star cluster
We have used the NASA Infra-Red Telescope Facility (IRTF) to observe the
nuclear stellar cluster in the nearby, face-on, giant Scd spiral IC 342. From
high resolution (R = 21500) spectra at the 12CO (2-0) bandhead at 2.3 micron we
derive a line-of-sight stellar velocity dispersion sigma = (33 +- 3) km/s.
To interpret this observation we construct dynamical models based on the
Jeans equation for a spherical system. The light distribution of the cluster is
modeled using an isophotal analysis of an HST V-band image from the HST Data
Archive, combined with new ground-based K-band imaging. Under the assumption of
an isotropic velocity distribution, the observed kinematics imply a K-band
mass-to-light ratio M/L_K = 0.05, and a cluster mass M ~ 6 times 10^6 Msun. We
model the mass-to-light ratio with the `starburst99' stellar population
synthesis models of Leitherer and collaborators, and infer a best-fitting
cluster age in the range 63-630 Myears. Although this result depends somewhat
on a number of uncertainties in the modeling (e.g., the assumed extinction
along the line-of-sight towards the nucleus, the IMF of the stellar population
model, and the velocity dispersion anisotropy of the cluster), none of these
can be plausibly modified to yield a significantly larger age. We discuss the
implications of this result on possible scenarios for the frequency of nuclear
starbursts and their impact on secular evolution of spiral galaxy nuclei.
As a byproduct of our analysis, we infer that IC 342 cannot have any central
black hole more massive than 0.5 million solar masses. This is ~ 6 times less
massive than the black hole inferred to exist in our Galaxy, consistent with
the accumulating evidence that galaxies with less massive bulges harbor less
massive black holes.Comment: 27 pages, incl. 9 figures, submitted to The Astronomical Journa
Instruments of RT-2 Experiment onboard CORONAS-PHOTON and their test and evaluation III: Coded Aperture Mask and Fresnel Zone Plates in RT-2/CZT Payload
Imaging in hard X-rays of any astrophysical source with high angular
resolution is a challenging job. Shadow-casting technique is one of the most
viable options for imaging in hard X-rays. We have used two different types of
shadow-casters, namely, Coded Aperture Mask (CAM) and Fresnel Zone Plate (FZP)
pair and two types of pixellated solid-state detectors, namely, CZT and CMOS in
RT-2/CZT payload, the hard X-ray imaging instrument onboard the CORONAS-PHOTON
satellite. In this paper, we present the results of simulations with different
combinations of coders (CAM & FZP) and detectors that are employed in the
RT-2/CZT payload. We discuss the possibility of detecting transient Solar
flares with good angular resolution for various combinations. Simulated results
are compared with laboratory experiments to verify the consistency of the
designed configuration.Comment: 27 pages, 16 figures, Accepted for publication in Experimental
Astronomy (in press
New Constraints on Simultaneous Optical Emission from Gamma-Ray Bursts Measured by the Livermore Optical Transient Imaging System Experiment
LOTIS is a gamma-ray burst optical counterpart search experiment located near Lawrence Livermore National Laboratory in California. Since operations began in 1996 October, LOTIS has responded to five triggers as of 1997 July 30, which occurred during good weather conditions. GRB 970223 (BATSE trigger 6100) was an exceptionally strong burst, lasting »30 s with a peak at »8 s. LOTIS began imaging the error box »11 s after the burst began and achieved simultaneous optical coverage of 100% of the region enclosed by the BATSE 3 j error circle and the interplanetary network annulus. No optical transients were observed brighter than the m » 11 completeness limit of the resulting images, providing a new V upper limit on the ratio of simultaneous optical to gamma-ray fluence of R \u3c 1024 and on the ratio of L simultaneous optical (at 700 nm) to gamma-ray (at 100 keV) flux density of R ! 305 for a B-type spectrum and R \u3c 475 for an M-type spectrum
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Instrumentation of LOTIS: Livermore Optical Transient Imaging System; a fully automated wide field of view telescope system searching for simultaneous optical counterparts of gamma ray bursts
LOTIS is a rapidly slewing wide-field-of-view telescope which was designed and constructed to search for simultaneous gamma-ray burst (GRB) optical counterparts. This experiment requires a rapidly slewing ({lt} 10 sec), wide-field-of-view ({gt} 15{degrees}), automatic and dedicated telescope. LOTIS utilizes commercial tele-photo lenses and custom 2048 x 2048 CCD cameras to view a 17.6 x 17.6{degrees} field of view. It can point to any part of the sky within 5 sec and is fully automated. It is connected via Internet socket to the GRB coordinate distribution network which analyzes telemetry from the satellite and delivers GRB coordinate information in real-time. LOTIS started routine operation in Oct. 1996. In the idle time between GRB triggers, LOTIS systematically surveys the entire available sky every night for new optical transients. This paper will describe the system design and performance
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