1,819 research outputs found
Gamma-ray emission from globular clusters
Over the last few years, the data obtained using the Large Area Telescope
(LAT) aboard the Fermi Gamma-ray Space Telescope has provided new insights on
high-energy processes in globular clusters, particularly those involving
compact objects such as Millisecond Pulsars (MSPs). Gamma-ray emission in the
100 MeV to 10 GeV range has been detected from more than a dozen globular
clusters in our galaxy, including 47 Tucanae and Terzan 5. Based on a sample of
known gamma-ray globular clusters, the empirical relations between gamma-ray
luminosity and properties of globular clusters such as their stellar encounter
rate, metallicity, and possible optical and infrared photon energy densities,
have been derived. The measured gamma-ray spectra are generally described by a
power law with a cut-off at a few gigaelectronvolts. Together with the
detection of pulsed gamma-rays from two MSPs in two different globular
clusters, such spectral signature lends support to the hypothesis that
gamma-rays from globular clusters represent collective curvature emission from
magnetospheres of MSPs in the clusters. Alternative models, involving
Inverse-Compton (IC) emission of relativistic electrons that are accelerated
close to MSPs or pulsar wind nebula shocks, have also been suggested.
Observations at >100 GeV by using Fermi/LAT and atmospheric Cherenkov
telescopes such as H.E.S.S.-II, MAGIC-II, VERITAS, and CTA will help to settle
some questions unanswered by current data.Comment: 11 pages, 7 figures, 2 tables, J. Astron. Space Sci., in pres
X-ray Localization of the Globular Cluster G1 with XMM-Newton
We present an accurate X-ray position of the massive globular cluster G1 by
using XMM-Newton and the Hubble Space Telescope (HST). The X-ray emission of G1
has been detected recently with XMM-Newton. There are two possibilities for the
origin of the X-ray emission. It can be either due to accretion of the central
intermediate-mass black hole, or by ordinary low-mass X-ray binaries. The
precise location of the X-ray emission might distinguish between these two
scenarios. By refining the astrometry of the XMM-Newton and HST data, we
reduced the XMM-Newton error circle to 1.5". Despite the smaller error circle,
the precision is not sufficient to distinguish an intermediate-mass black hole
and luminous low-mass X-ray binaries. This result, however, suggests that
future Chandra observations may reveal the origin of the X-ray emission.Comment: 4 pages, 2 figures; accepted for publication in Ap
Swift Observations of the Be/X-ray Transient System 1A 1118-615
We report results of Swift observations for the high mass Be/X-ray binary
system 1A 1118-615, during an outburst stage in January, 2009 and at a flaring
stage in March, 2009. Using the epoch-folding method, we successfully detected
a pulsed period of 407.69(2) sec in the outburst of January and of 407.26(1)
sec after the flare detection in March. We find that the spectral detection for
the source during outburst can be described by a blackbody model with a high
temperature (kT ~ 1-3 keV) and a small radius (R ~ 1 km), indicating that the
emission results from the polar cap of the neutron star. On the other hand, the
spectra obtained after the outburst can further be described by adding an
additional component with a lower temperature (kT ~ 0.1-0.2 keV) and a larger
emission radius (R ~ 10-500 km), which indicates the emission from around the
inner region of an accretion disk. We find that the thermal emission from the
hot spot of the accreting neutron star dominates the radiation in outburst; the
existence of both this X-ray contribution and the additional soft component
suggest that the polar cap and the accretion disk emission might co-exist after
the outburst. Because the two-blackbody signature at the flaring stage is a
unique feature of 1A 1118-615, our spectral results may provide a new insight
to interpret the X-ray emission for the accreting neutron star. The time
separation between the three main outbursts of this system is ~17 years and it
might be related to the orbital period. We derive and discuss the associated
physical properties by assuming the elongated orbit for this specific Be/X-ray
transient.Comment: 9 pages, 3 figures and 4 tables; accepted by MNRA
First Detection of A Sub-kpc Scale Molecular Outflow in the Starburst Galaxy NGC 3628
We successfully detected a molecular outflow with a scale of 370-450 pc in
the central region of the starburst galaxy NGC 3628 through deep CO(1-0)
observations by using the Nobeyama Millimeter Array (NMA). The mass of the
outflowing molecular gas is ~2.8x10^7 M_sun, and the outflow velocity is
~90(+/-10) km s^{-1}. The expansion timescale of the outflow is 3.3-6.8 Myr,
and the molecular gas mass flow rate is 4.1-8.5 M_sun yr^{-1}. It requires
mechanical energy of (1.8-2.8)x10^{54} erg to create this sub-kpc scale
molecular outflow. In order to understand the evolution of the molecular
outflow, we compare the physical properties between the molecular outflow
observed from our NMA CO(1-0) data and the plasma gas from the soft X-ray
emission of the Chandra X-ray Observatory (CXO) archival data. We found that
the distribution between the molecular outflow and the strong plasma outflow
seems to be in a similar region. In this region, the ram pressure and the
thermal pressure of the plasma outflow are 10^{-(8-10)} dyne cm^{-2}, and the
thermal pressure of molecular outflow is 10^{-(11-13)} dyne cm^{-2}. This
implies the molecular outflow is still expanding outward. The molecular gas
consumption timescale is estimated as 17-27 Myr, and the total starburst
timescale is 20-34 Myr. The evolutionary parameter is 0.11-0.25, suggesting
that the starburst activity in NGC 3628 is still in a young stage.Comment: 15 pages, 14 figures, accepted by Ap
XMM-Newton observations of the spiral galaxy M74 (NGC 628)
The face-on spiral galaxy M74 (NGC 628) was observed by XMM on 2002 February
2. In total, 21 sources are found in the inner 5' from the nucleus (after
rejection of a few sources associated to foreground stars). Hardness ratios
suggest that about half of them belong to the galaxy. The higher-luminosity end
of the luminosity function is fitted by a power-law of slope -0.8. This can be
interpreted as evidence of ongoing star formation, in analogy with the
distributions found in disks of other late-type galaxies. A comparison with
previous Chandra observations reveals a new ultraluminous X-ray transient (L_x
\~ 1.5 x 10^39 erg/s in the 0.3--8 keV band) about 4' North of the nucleus. We
find another transient black-hole candidate (L_x ~ 5 x 10^38 erg/s) about 5'
North-West of the nucleus. The UV and X-ray counterparts of SN 2002ap are also
found in this XMM observation.Comment: submitted to ApJL. Based on publicly available data, see
http://xmm.vilspa.esa.es/external/xmm_news/items/sn_2002_ap/index.shtm
Quiescent X-Ray/Optical Counterparts of the Black Hole Transient H 1705-250
We report the result of a new Chandra observation of the black hole X-ray
transient H 1705-250 in quiescence. H 1705-250 was barely detected in the new
50 ks Chandra observation. With 5 detected counts, we estimate the source
quiescent luminosity to be Lx~9.1e30 erg/s in the 0.5-10 keV band (adopting a
distance of 8.6 kpc). This value is in line with the quiescent luminosities
found among other black hole X-ray binaries with similar orbital periods. By
using images taken with the Faulkes Telescope North, we derive a refined
position of H 1705-250. We also present the long-term lightcurve of the optical
counterpart from 2006 to 2012, and show evidence for variability in quiescence.Comment: 5 pages, 2 figures; Accepted for publication in MNRA
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