117 research outputs found
The Orbital Period of the Ultraluminous X-ray Source in M82
The ultraluminous x-ray source (ULX) in the galaxy M82 has been identified as
a possible intermediate-mass black hole. We have found that the x-ray flux from
M82 is modulated with a peak-to-peak amplitude corresponding to an isotropic
luminosity of 2.4x10^40 erg/s in M82 and a period of 62.0 +/- 2.5 days, which
we interpret as the orbital period of the ULX binary. This orbital period
implies that the mass-donor star must be a giant or supergiant. Large
mass-transfer rates, sufficient to fuel the ULX, are expected for a giant-phase
mass donor in an x-ray binary. The giant phase has a short lifetime, indicating
that we see the ULX in M82 in a brief and unusual period of its evolution.Comment: 3 pages, appeared in Scienc
VLA Detection of the Ionized Stellar Winds Arising from Massive Stars in the Galactic Center Arches Cluster
The Galactic center Arches stellar cluster, detected and studied until now
only in the near-infrared, is comprised of at least one hundred massive (M>20
Msun) stars. Here we report the detection at centimeter wavelengths of radio
continuum emission from eight radio sources associated with the cluster. Seven
of these radio sources have rising spectral indices between 4.9 and 8.5 GHz and
coincide spatially with the brightest stars in the cluster, as determine from
JHK photometry and Brackett alpha and Brackett Gamma spectroscopy. Our results
confirm the presence of powerful ionized winds in these stars. The eighth radio
source has a nonthermal spectrum and its nature is yet unclear, but it could be
associated with a lower mass young star in the cluster.Comment: 6 pages, 2 embedded figures, accepted to ApJLetter
A VLA Study of Newly-Discovered Southern Latitude Non-Thermal Filaments in the Galactic Center: Radio Continuum Total-intensity and Spectral Index Properties
The non-thermal filament (NTF) radio structures clustered within a few
hundred parsecs of the Galactic Center (GC) are apparently unique to this
region of the Galaxy. Recent radio images of the GC using MeerKAT at 1 GHz have
revealed a multitude of faint, previously unknown NTF bundles (NTFBs), some of
which are comprised of as many as 10 or more individual filaments. In this work
we present Very Large Array (VLA) observations at C- and X-bands (4 - 12 GHz)
at arcsecond-scale resolutions of three of these newly-discovered NTFBs, all
located at southern Galactic latitudes. These observations allow us to compare
their total-intensity properties with those of the larger NTF population. We
find that these targets generally possess properties similar to what is
observed in the larger NTF population. However, the larger NTF population
generally has steeper spectral index values than what we observe for our chosen
targets. The results presented here based on the total-intensity properties of
these structures indicate that the NTFs are likely all formed from Cosmic Rays
(CRs). These CRs are either generated by a nearby compact source and then
diffuse along the NTF lengths or are generated by extended, magnetized
structures whose magnetic field undergoes reconnection with the NTF magnetic
field.Comment: 18 pages, 13 figures, 2 tables. Submitted to ApJ for peer-revie
Multiwavelength Observations of Massive Stellar Cluster Candidates in the Galaxy
The Galaxy appears to be richer in young, massive stellar clusters than
previously known, due to advances in infrared surveys which have uncovered
deeply embedded regions of star formation. Young, massive clusters can
significantly impact the surrounding interstellar medium (ISM) and hence radio
observations can also be an important tracer of their activity. Several hundred
cluster candidates are now known by examining survey data. Here we report on
multiwavelength observations of six of these candidates in the Galaxy. We
carried out 4.9 and 8.5 GHz VLA observations of the radio emission associated
with these clusters to obtain the physical characteristics of the surrounding
gas, including the Lyman continuum photon flux and ionized gas mass. Spitzer
Infrared Array Camera observations were also made of these regions, and provide
details on the stellar population as well as the dust continuum and polycyclic
aromatic hydrocarbon emission. When compared to the known young, massive
clusters in the Galaxy, the six cluster candidates have less powerful Lyman
ionizing fluxes and ionize less of the H II mass in the surrounding ISM.
Therefore, these cluster candidates appear to be more consistent with
intermediate-mass clusters (10^3-10^4 Msun).Comment: 39 pages, 20 figures. Accepted in the Astronomical Journal; to be
published Fall 201
A Radio Transient 0.1 pc from Sagittarius A*
We report the discovery of a transient radio source 2.7 arcsec (0.1 pc
projected distance) South of the Galactic Center massive black hole,
Sagittarius A*. The source flared with a peak of at least 80 mJy in March 2004.
The source was resolved by the Very Large Array into two components with a
separation of ~0.7 arcsec and characteristic sizes of ~0.2 arcsec. The two
components of the source faded with a power-law index of 1.1 +/- 0.1. We detect
an upper limit to the proper motion of the Eastern component of ~3 x 10^3 km
s^-1 relative to Sgr A*. We detect a proper motion of ~10^4 km s^-1 for the
Western component relative to Sgr A*. The transient was also detected at X-ray
wavelengths with the Chandra X-ray Observatory and the XMM-Newton telescope and
given the designation CXOGC J174540.0-290031. The X-ray source falls in between
the two radio components. The maximum luminosity of the X-ray source is ~10^36
erg s^-1, significantly sub-Eddington. The radio jet flux density predicted by
the X-ray/radio correlation for X-ray binaries is orders of magnitude less than
the measured flux density. We conclude that the radio transient is the result
of a bipolar jet originating in a single impulsive event from the X-ray source
and interacting with the dense interstellar medium of the Galactic Center.Comment: accepted in ApJ; 24 pages; 8 figure
A High-Resolution Survey of HI Absorption toward the Central 200 pc of the Galactic Center
We present an HI absorption survey of the central 250 pc of the Galaxy. Very
Large Array (VLA) observations were made at 21 cm in the DnC and CnB
configurations and have a resolution of ~15"(0.6 pc at the Galactic Center (GC)
distance) and a velocity resolution of ~2.5 km/s. This study provides HI data
with high spatial resolution, comparable with the many high resolution
observations which have been made of GC sources over the past ten years. Here
we present an overview of the HI absorption toward ~40 well-known continuum
sources and a detailed comparison of the ionized, atomic and molecular
components of the interstellar medium for the Sgr B, Radio Arc and Sgr C
regions. In these well-known regions, the atomic gas appears to be closely
correlated in both velocity and distribution to the ionized and molecular gas,
indicating that it resides in photo-dissociation regions related to the HII
regions in the GC. Toward the majority of the radio continuum sources, HI
absorption by the 3-kpc arm is detected, constraining these sources to lie
beyond a 5 kpc distance in the Galaxy.Comment: 59 pages, including 41 figures; accepted for publication in
Astrophysical Journal Supplement Series in December 201
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