238 research outputs found
Echoes of multiple outbursts of Sagittarius A* revealed by Chandra
The relatively rapid spatial and temporal variability of the X-ray radiation
from some molecular clouds near the Galactic center shows that this emission
component is due to the reflection of X-rays generated by a source that was
luminous in the past, most likely the central supermassive black hole,
Sagittarius A*. Studying the evolution of the molecular cloud reflection
features is therefore a key element to reconstruct Sgr A*'s past activity. The
aim of the present work is to study this emission on small angular scales in
order to characterize the source outburst on short time scales. We use Chandra
high-resolution data collected from 1999 to 2011 to study the most rapid
variations detected so far, those of clouds between 5' and 20' from Sgr A*
towards positive longitudes. Our systematic spectral-imaging analysis of the
reflection emission, notably of the Fe Kalpha line at 6.4 keV and its
associated 4-8 keV continuum, allows us to characterize the variations down to
15" angular scale and 1-year time scale. We reveal for the first time abrupt
variations of few years only and in particular a short peaked emission, with a
factor of 10 increase followed by a comparable decrease, that propagates along
the dense filaments of the Bridge cloud. This 2-year peaked feature contrasts
with the slower 10-year linear variations we reveal in all the other molecular
structures of the region. Based on column density constraints, we argue that
these two different behaviors are unlikely to be due to the same illuminating
event. The variations are likely due to a highly variable active phase of Sgr
A* sometime within the past few hundred years, characterized by at least two
luminous outbursts of a few-year time scale and during which the Sgr A*
luminosity went up to at least 10^39 erg/s.Comment: 17 pages, 16 figures, Accepted for publication in Astronomy &
Astrophysic
From soil to plant, the journey of P through trophic relationships and ectomycorrhizal association
Phosphorus (P) is essential for plant growth and productivity. It is one of the most limiting macronutrients in soil because it is mainly present as unavailable, bound P whereas plants can only use unbound, inorganic phosphate (Pi), which is found in very low concentrations in soil solution. Some ectomycorrhizal fungi are able to release organic compounds (organic anions or phosphatases) to mobilize unavailable P. Recent studies suggest that bacteria play a major role in the mineralization of nutrients such as P through trophic relationships as they can produce specific phosphatases such as phytases to degrade phytate, the main form of soil organic P. Bacteria are also more effective than other microorganisms or plants at immobilizing free Pi. Therefore, bacterial grazing by grazers, such as nematodes, could release Pi locked in bacterial biomass. Free Pi may be taken up by ectomycorrhizal fungus by specific phosphate transporters and transferred to the plant by mechanisms that have not yet been identified. This mini-review aims to follow the phosphate pathway to understand the ecological and molecular mechanisms responsible for transfer of phosphate from the soil to the plant, to improve plant P nutrition
Multi-wavelength observations of 1RXH J173523.7-354013: revealing an unusual bursting neutron star
On 2008 May 14, the Burst Alert Telescope aboard the Swift mission triggered
on a type-I X-ray burst from the previously unclassified ROSAT object 1RXH
J173523.7-354013, establishing the source as a neutron star X-ray binary. We
report on X-ray, optical and near-infrared observations of this system. The
X-ray burst had a duration of ~2 h and belongs to the class of rare,
intermediately long type-I X-ray bursts. From the bolometric peak flux of
~3.5E-8 erg/cm^2/s, we infer a source distance of D<9.5 kpc. Photometry of the
field reveals an optical counterpart that declined from R=15.9 during the X-ray
burst to R=18.9 thereafter. Analysis of post-burst Swift/XRT observations, as
well as archival XMM-Newton and ROSAT data suggests that the system is
persistent at a 0.5-10 keV luminosity of ~2E35 (D/9.5 kpc)^2 erg/s. Optical and
infrared photometry together with the detection of a narrow Halpha emission
line (FWHM=292+/-9 km/s, EW=-9.0+/-0.4 Angstrom) in the optical spectrum
confirms that 1RXH J173523.7-354013 is a neutron star low-mass X-ray binary.
The Halpha emission demonstrates that the donor star is hydrogen-rich, which
effectively rules out that this system is an ultra-compact X-ray binary.Comment: Accepted for publication in MNRAS, 13 pages, 6 figures, 5 table
Simultaneous Multi-Wavelength Observations of Sgr A* during 2007 April 1-11
We report the detection of variable emission from Sgr A* in almost all
wavelength bands (i.e. centimeter, millimeter, submillimeter, near-IR and
X-rays) during a multi-wavelength observing campaign. Three new moderate flares
are detected simultaneously in both near-IR and X-ray bands. The ratio of X-ray
to near-IR flux in the flares is consistent with inverse Compton scattering of
near-IR photons by submillimeter emitting relativistic particles which follow
scaling relations obtained from size measurements of Sgr A*. We also find that
the flare statistics in near-IR wavelengths is consistent with the probability
of flare emission being inversely proportional to the flux. At millimeter
wavelengths, the presence of flare emission at 43 GHz (7mm) using VLBA with
milli-arcsecond spatial resolution indicates the first direct evidence that
hourly time scale flares are localized within the inner 3070
Schwarzschild radii of Sgr A*. We also show several cross correlation plots
between near-IR, millimeter and submillimeter light curves that collectively
demonstrate the presence of time delays between the peaks of emission up to
three hours. The evidence for time delays at millimeter and submillimeter
wavelengths are consistent with the source of emission being optically thick
initially followed by a transition to an optically thin regime. In particular,
there is an intriguing correlation between the optically thin near-IR and X-ray
flare and optically thick radio flare at 43 GHz that occurred on 2007 April 4.
This would be the first evidence of a radio flare emission at 43 GHz delayed
with respect to the near-IR and X-ray flare emission.Comment: replaced with revised version 57 pages, 28 figures, ApJ (in press
Model Systems to Study the Mechanism of Vascular Aging
Cardiovascular diseases are the leading cause of death globally. Within cardiovascular aging, arterial aging holds significant importance, as it involves structural and functional alterations in arteries that contribute substantially to the overall decline in cardiovascular health during the aging process. As arteries age, their ability to respond to stress and injury diminishes, while their luminal diameter increases. Moreover, they experience intimal and medial thickening, endothelial dysfunction, loss of vascular smooth muscle cells, cellular senescence, extracellular matrix remodeling, and deposition of collagen and calcium. This aging process also leads to overall arterial stiffening and cellular remodeling. The process of genomic instability plays a vital role in accelerating vascular aging. Progeria syndromes, rare genetic disorders causing premature aging, exemplify the impact of genomic instability. Throughout life, our DNA faces constant challenges from environmental radiation, chemicals, and endogenous metabolic products, leading to DNA damage and genome instability as we age. The accumulation of unrepaired damages over time manifests as an aging phenotype. To study vascular aging, various models are available, ranging from in vivo mouse studies to cell culture options, and there are also microfluidic in vitro model systems known as vessels-on-a-chip. Together, these models offer valuable insights into the aging process of blood vessels.</p
Fading hard X-ray emission from the Galactic Centre molecular cloud Sgr B2
The centre of our Galaxy harbours a 4 million solar mass black hole that is
unusually quiet: its present X-ray luminosity is more than 10 orders of
magnitude less than its Eddington luminosity. The observation of iron
fluorescence and hard X-ray emission from some of the massive molecular clouds
surrounding the Galactic Centre has been interpreted as an echo of a past
flare. Alternatively, low-energy cosmic rays propagating inside the clouds
might account for the observed emission, through inverse bremsstrahlung of low
energy ions or bremsstrahlung emission of low energy electrons. Here we report
the observation of a clear decay of the hard X-ray emission from the molecular
cloud Sgr B2 during the past 7 years thanks to more than 20 Ms of INTEGRAL
exposure. The measured decay time is compatible with the light crossing time of
the molecular cloud core . Such a short timescale rules out inverse
bremsstrahlung by cosmic-ray ions as the origin of the X ray emission. We also
obtained 2-100 keV broadband X-ray spectra by combining INTEGRAL and XMM-Newton
data and compared them with detailed models of X-ray emission due to
irradiation of molecular gas by (i) low-energy cosmic-ray electrons and (ii)
hard X-rays. Both models can reproduce the data equally well, but the time
variability constraints and the huge cosmic ray electron luminosity required to
explain the observed hard X-ray emission strongly favor the scenario in which
the diffuse emission of Sgr B2 is scattered and reprocessed radiation emitted
in the past by Sgr A*. Using recent parallax measurements that place Sgr B2 in
front of Sgr A*, we find that the period of intense activity of Sgr A* ended
between 75 and 155 years ago.Comment: Accepted for publication in ApJ. 10 pages, 5 figure
Discovery of a superluminal Fe K echo at the Galactic Center: The glorious past of Sgr A* preserved by molecular clouds
We present the result of a study of the X-ray emission from the Galactic
Centre (GC) Molecular Clouds (MC) within 15 arcmin from Sgr A*. We use
XMM-Newton data (about 1.2 Ms of observation time) spanning about 8 years. The
MC spectra show all the features characteristic of reflection: i) intense Fe
Kalpha, with EW of about 0.7-1 keV, and the associated Kbeta line; ii) flat
power law continuum and iii) a significant Fe K edge (tau~0.1-0.3). The diffuse
low ionisation Fe K emission follows the MC distribution, nevertheless not all
MC are Fe K emitters. The long baseline monitoring allows the characterisation
of the temporal evolution of the MC emission. A complex pattern of variations
is shown by the different MC, with some having constant Fe K emission, some
increasing and some decreasing. In particular, we observe an apparent
super-luminal motion of a light front illuminating a Molecular nebula. This
might be due to a source outside the MC (such as Sgr A* or a bright and long
outburst of a X-ray binary), while it cannot be due to low energy cosmic rays
or a source located inside the cloud. We also observe a decrease of the X-ray
emission from G0.11-0.11, behaviour similar to the one of Sgr B2. The line
intensities, clouds dimensions, columns densities and positions with respect to
Sgr A*, are consistent with being produced by the same Sgr A* flare. The
required high luminosity (about 1.5~10^39 erg/s) can hardly be produced by a
binary system, while it is in agreement with a flare of Sgr A* fading about 100
years ago. The low intensity of the Fe K emission coming from the 50 and the 20
km/s MC places an upper limit of 10^36 erg/s to the mean luminosity of Sgr A*
in the last 60-90 years. The Fe K emission and variations from these MC might
have been produced by a single flare of Sgr A*.Comment: ApJ in press 17 pages, 14 Figures, 3 table
The two states of Sgr A* in the near-infrared: bright episodic flares on top of low-level continuous variability
In this paper we examine properties of the variable source Sgr A* in the
near-infrared (NIR) using a very extensive Ks-band data set from NACO/VLT
observations taken 2004 to 2009. We investigate the variability of Sgr A* with
two different photometric methods and analyze its flux distribution. We find
Sgr A* is continuously emitting and continuously variable in the near-infrared,
with some variability occurring on timescales as long as weeks. The flux
distribution can be described by a lognormal distribution at low intrinsic
fluxes (<~5 mJy, dereddened with A_{Ks}=2.5). The lognormal distribution has a
median flux of approximately 1.1 mJy, but above 5 mJy the flux distribution is
significantly flatter (high flux events are more common) than expected for the
extrapolation of the lognormal distribution to high fluxes. We make a general
identification of the low level emission above 5 mJy as flaring emission and of
the low level emission as the quiescent state. We also report here the
brightest Ks-band flare ever observed (from August 5th, 2008) which reached an
intrinsic Ks-band flux of 27.5 mJy (m_{Ks}=13.5). This flare was a factor 27
increase over the median flux of Sgr A*, close to double the brightness of the
star S2, and 40% brighter than the next brightest flare ever observed from
Sgr~A*.Comment: 14 pages, 6 figures, accepted for publication in Ap
Bursting behavior of the Galactic Center faint X-ray transient GRS 1741.9-2853
The neutron star low-mass X-ray binary GRS 1741.9-2853 is a known type-I
burster of the Galactic Center. It is transient, faint, and located in a very
crowded region, only 10 arcmin from the supermassive black hole Sgr A*.
Therefore, its bursting behavior has been poorly studied so far. In particular,
its persistent emission has rarely been detected between consecutive bursts,
due to lack of sensitivity or confusion. This is what made GRS 1741.9-2853 one
of the nine "burst-only sources" identified by BeppoSAX a few years ago. The
physical properties of GRS 1741.9-2853 bursts are yet of great interest since
we know very little about the nuclear regimes at stake in low accretion rate
bursters. We examine here for the first time several bursts in relation with
the persistent emission of the source, using INTEGRAL, XMM-Newton, and Swift
observations. We investigate the source flux variability and bursting behavior
during its 2005 and 2007 long outbursts. The persistent luminosity of GRS
1741.9-2853 varied between ~1.7 and 10.5 10^36 erg s^-1, i.e. 0.9-5.3% of the
Eddington luminosity. The shape of the spectrum as described by an absorbed
power-law remained with a photon index Gamma ~ 2 and a column density $N_{\rm
H} ~ 12 10^22 cm^-2 throughout the outbursts. We discovered 11 type-I bursts
with INTEGRAL, and inspected 4 additional bursts: 2 recorded by XMM-Newton and
2 by Swift. From the brigthest burst, we derive an upper limit on the source
distance of ~7 kpc. The observed bursts characteristics and source accretion
rate suggest pure helium explosions igniting at column depths y_{ign} ~ 0.8-4.8
10^8 g cm^-1, for typical energy releases of ~1.2-7.4 10^39 erg.Comment: 11 pages, 7 figures, accepted for publication in A&
INTEGRAL, Swift, and RXTE observations of the 518 Hz accreting transient pulsar Swift J1749.4-2807
The burst-only Swift J1749.4-2807 source was discovered in a high
X-ray-active state, while during an {INTEGRAL observations of the Galactic
bulge on 2010 April 10. Pulsations at 518 Hz were discovered in the RXTE data,
confirming previous suggestions of possible associations between burst-only
sources and accreting millisecond X-ray pulsars. The subsequent discovery of
X-ray eclipses made Swift J1749.42807 the first eclipsing accreting
millisecond X-ray pulsar. We obtain additional information on Swift
J1749.4-2807 and other burst-only sources. We report on the results of a
monitoring campaign on the source, carried out for about two weeks with the
Swift, INTEGRAL, and RXTE satellites.
The observations showed that the X-ray spectrum (energy range 0.5-40 keV) of
Swift J1749.4-2807 during the entire event was accurately modeled by an
absorbed power-law model (N_H~3e2 cm^-2, Gamma~1.7). X-ray eclipses were also
detected in the Swift data and provides a clear evidence of a dust-scattering
halo located along the line of sight to the source. Only one type-I X-ray burst
was observed throughout the two-weeks long monitoring. The X-ray flux of Swift
J1749.4-2807 decayed below the detection threshold of Swift/XRT about 11 days
after the discovery, in a exponential fashion (e-folding time of tau=12^+7_-3
days). We compare the properties of the outburst observed from Swift
J1749.4-2807 with those of the previously known millisecond X-ray pulsars and
other transient low mass X-ray binaries.Comment: Accepted for publication on A&
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