共查询到20条相似文献,搜索用时 60 毫秒
1.
Rajmal Jain Arun Kumar Awasthi Arvind Singh Rajpurohit Markus J. Aschwanden 《Solar physics》2011,270(1):137-149
We report solar flare plasma to be multi-thermal in nature based on the theoretical model and study of the energy-dependent
timing of thermal emission in ten M-class flares. We employ high-resolution X-ray spectra observed by the Si detector of the
“Solar X-ray Spectrometer” (SOXS). The SOXS onboard the Indian GSAT-2 spacecraft was launched by the GSLV-D2 rocket on 8 May
2003. Firstly we model the spectral evolution of the X-ray line and continuum emission flux F(ε) from the flare by integrating a series of isothermal plasma flux. We find that the multi-temperature integrated flux F(ε) is a power-law function of ε with a spectral index (γ)≈−4.65. Next, based on spectral-temporal evolution of the flares we find that the emission in the energy range E=4 – 15 keV is dominated by temperatures of T=12 – 50 MK, while the multi-thermal power-law DEM index (δ) varies in the range of −4.4 and −5.7. The temporal evolution of the X-ray flux F(ε,t) assuming a multi-temperature plasma governed by thermal conduction cooling reveals that the temperature-dependent cooling
time varies between 296 and 4640 s and the electron density (n
e) varies in the range of n
e=(1.77 – 29.3)×1010 cm−3. Employing temporal evolution technique in the current study as an alternative method for separating thermal from nonthermal
components in the energy spectra, we measure the break-energy point, ranging between 14 and 21±1.0 keV. 相似文献
2.
If massive sterile neutrinos exist, their decays into photons and/or electron-positron pairs may give rise to observable consequences.
We consider the possibility that MeV sterile neutrino decays lead to the diffuse positron annihilation line in the Milky Way
center, and we thus obtain bounds on the sterile neutrino decay rate Γ
e
≥10−28 s−1 from relevant astrophysical/cosmological data. Also, we expect a soft gamma flux of 1.2×10−4–9.7×10−4 ph cm−2 s−1 from the Milky Way center which shows up as a small MeV bump in the background photon spectrum. Furthermore, we estimate
the flux of active neutrinos produced by sterile neutrino decays to be 0.02–0.1 cm−2 s−1 passing through the earth. 相似文献
3.
P. Goldoni M. Ribó T. Di Salvo J. M. Paredes V. Bosch-Ramon M. Rupen 《Astrophysics and Space Science》2007,309(1-4):293-297
LS 5039 is the only X-ray binary persistently detected at TeV energies by the Cherenkov HESS telescope. It is moreover a γ-ray emitter in the GeV and possibly MeV energy ranges. To understand important aspects of jet physics, like the magnetic
field content or particle acceleration, and emission processes, such as synchrotron and inverse Compton (IC), a complete modeling
of the multiwavelength data is necessary. LS 5039 has been detected along almost all the electromagnetic spectrum thanks to
several radio, infrared, optical and soft X-ray detections. However, hard X-ray detections above 20 keV have been so far elusive
and/or doubtful, partly due to source confusion for the poor spatial resolution of hard X-ray instruments. We report here
on deep (∼300 ks) serendipitous INTEGRAL hard X-ray observations of LS 5039, coupled with simultaneous VLA radio observations. We obtain a 20–40 keV flux of 1.1±0.3 mCrab
(5.9 (±1.6) ×10−12 erg cm−2 s−1), a 40–100 keV upper limit of 1.5 mCrab (9.5×10−12 erg cm−2 s−1), and typical radio flux densities of ∼25 mJy at 5 GHz. These hard X-ray fluxes are significantly lower than previous estimates
obtained with BATSE in the same energy range but, in the lower interval, agree with extrapolation of previous RXTE measurements. The INTEGRAL observations also hint to a break in the spectral behavior at hard X-rays. A more sensitive characterization of the hard
X-ray spectrum of LS 5039 from 20 to 100 keV could therefore constrain key aspects of the jet physics, like the relativistic
particle spectrum and the magnetic field strength. Future multiwavelength observations would allow to establish whether such
hard X-ray synchrotron emission is produced by the same population of relativistic electrons as those presumably producing
TeV emission through IC. 相似文献
4.
Markus Ackermann 《Astrophysics and Space Science》2007,309(1-4):421-427
The AMANDA-II telescope, operated by the IceCube collaboration, is currently the world’s most sensitive telescope to fluxes
of neutrinos from individual sources. A data sample of 4282 neutrino induced events collected in 1001 days of detector livetime
during the years 2000–2004 have now been analyzed looking for a neutrino signal from point-like sources. A sensitivity to
fluxes of
of d
Φ/dE=1.0×10−10(E/TeV)−2 TeV−1 cm−2s−1 was reached in the energy range between 1.7 TeV and 2.4 PeV. So far no statistically significant localized excess of events
over the background of atmospheric neutrinos has been found, which would be ascribed to a neutrino source. However, the flux
upper limits derived from the non-observation of a signal are comparable to observed fluxes of high energy gamma rays from
blazars and within the range of current models for neutrino emission from selected sources. Possible constraints on these
models are discussed.
相似文献
5.
A. D. Kaminker D. G. Yakovlev A. Y. Potekhin N. Shibazaki P. S. Shternin O. Y. Gnedin 《Astrophysics and Space Science》2007,308(1-4):423-430
We model thermal evolution of magnetars with a phenomenological heat source in a spherical internal layer and compare the
results with observations of persistent thermal radiation from magnetars. We show that the heat source should be located in
the outer magnetar’s crust, at densities ρ≲5×1011 g cm−3, and the heating rate should be ∼1020 erg cm−3 s−1. Heating deeper layers is extremely inefficient because the thermal energy is mainly radiated away by neutrinos and does
not warm up the surface to the magnetar’s level. This deep heating requires too much energy; it is inconsistent with the energy
budget of neutron stars.
相似文献
6.
We present an analysis of hard X-ray imaging observations from one of the first solar flares observed with the Reuven Ramaty
High-Energy Solar Spectroscopic Imager (RHESSI) spacecraft, launched on 5 February 2002. The data were obtained from the 22
February 2002, 11:06 UT flare, which occurred close to the northwest limb. Thanks to the high energy resolution of the germanium-cooled
hard X-ray detectors on RHESSI we can measure the flare source positions with a high accuracy as a function of energy. Using
a forward-fitting algorithm for image reconstruction, we find a systematic decrease in the altitudes of the source centroids
z(ε) as a function of increasing hard X-ray energy ε, as expected in the thick-target bremsstrahlung model of Brown. The altitude
of hard X-ray emission as a function of photon energy ε can be characterized by a power-law function in the ε=15–50 keV energy
range, viz., z(ε)≈2.3(ε/20 keV)−1.3 Mm. Based on a purely collisional 1-D thick-target model, this height dependence can be inverted into a chromospheric density
model n(z), as derived in Paper I, which follows the power-law function n
e(z)=1.25×1013(z/1 Mm)−2.5 cm−3. This density is comparable with models based on optical/UV spectrometry in the chromospheric height range of h≲1000 km, suggesting that the collisional thick-target model is a reasonable first approximation to hard X-ray footpoint sources.
At h≈1000–2500 km, the hard X-ray based density model, however, is more consistent with the `spicular extended-chromosphere model' inferred from radio sub-mm observations, than with standard models based on hydrostatic equilibrium. At coronal heights,
h≈2.5–12.4 Mm, the average flare loop density inferred from RHESSI is comparable with values from hydrodynamic simulations
of flare chromospheric evaporation, soft X-ray, and radio-based measurements, but below the upper limits set by filling-factor
insensitive iron line pairs. 相似文献
7.
We investigate the properties of acoustic events (AEs), defined as spatially concentrated and short duration energy flux,
in the quiet Sun, using observations of a 2D field of view (FOV) with high spatial and temporal resolution provided by the
Solar Optical Telescope (SOT) onboard Hinode. Line profiles of Fe i 557.6 nm were recorded by the Narrow-band Filter Imager (NFI) on a 82″×82″ FOV during 75 min with a time step of 28.75 s
and 0.08″ pixel size. Vertical velocities were computed at three atmospheric levels (80, 130, and 180 km) using the bisector
technique, allowing the determination of energy flux to be made in the range 3 – 10 mHz using two complementary methods (Hilbert
transform and Fourier power spectrum). Horizontal velocities were computed using local correlation tracking (LCT) of continuum
intensities providing divergences. We found that the net energy flux is upward. In the range 3 – 10 mHz, a full FOV space
and time averaged flux of 2700 W m−2 (lower layer 80 – 130 km) and 2000 W m−2 (upper layer 130 – 180 km) is concentrated in less than 1 % of the solar surface in the form of narrow (0.3″) AE. Their total
duration (including rise and decay) is of the order of 103 s. Inside each AE, the mean flux is 1.6×105 W m−2 (lower layer) and 1.2×105 W m−2 (upper). Each event carries an average energy (flux integrated over space and time) of 2.5×1019 J (lower layer) to 1.9×1019 J (upper). More than 106 events could exist permanently on the Sun, with a birth and decay rate of 3500 s−1. Most events occur in intergranular lanes, downward velocity regions, and areas of converging motions. 相似文献
8.
We impose the requirement that the spatial distribution of pulsars deduced from their dispersion measures using a model of
the galactic electron density (n
e
) should be consistent with cylindrical symmetry around the galactic centre (assumed to be 10 kpc from the Sun). Using a carefully
selected subsample of the pulsars detected by the II Molonglo Survey (II MS), we test a number of simple models and conclude
that (i) the effective mean 〈ne〉) for the whole galaxy is 0.037-0.012
+0.020 cm-3, (ii) the scale height of electrons is greater than 300 pc and probably about 1 kpc or more, and (iii) there is little evidence
for variation of ne with galactic radius RGC for RGc ≳ 5 kpc. Further, we make a detailed analysis of the contribution to ne from H II regions. Combining the results of a number of relatively independent calculations, we propose a model for the galactic
electron density of the formn
e
(z) = 0.030 + 0.020 exp (- |z|/70) cm-3 where z(pc) is the height above the galactic plane and the second term describes the contribution from H II regions. We believe
the statistical uncertainties in the parameters of this model are quite small. 相似文献
9.
Optical CCD imaging with Hα and [SII] filters and spectroscopic observations of the galactic supernova remnant G85.9-0.6 have been performed for the
first time. The CCD image data are taken with the 1.5 m Russian-Turkish Telescope (RTT150) at TüBİTAK National Observatory
(TUG) and spectral data are taken with the Bok 2.3 m telescope on Kitt Peak, AZ.
The images are taken with narrow-band interference filters Hα, [SII] and their continuum. [SII]/Hα ratio image is performed. The ratio obtained from [SII]/Hα is found to be ∼0.42, indicating that the remnant interacts with HII regions. G85.9-0.6 shows diffuse-shell morphology. [SII]λ
λ6716/6731 average flux ratio is calculated from the spectra, and the electron density N
e
is obtained to be 395 cm−3. From [OIII]/Hβ ratio, shock velocity has been estimated, pre-shock density of n
c
=14 cm−3, explosion energy of E=9.2×1050 ergs, interstellar extinction of E(B−V)=0.28, and neutral hydrogen column density of N(HI)=1.53×1021 cm−2 are reported. 相似文献
10.
We consider sterile neutrinos as a component of dark matter in the Milky Way and clusters, and compare their rest mass, decay
rate and the mixing angle. A radiative decaying rate of order Γ∼10−19 s−1 for sterile neutrino rest mass m
s
=18–19 keV can satisfactorily account for the cooling flow problem and heating source in Milky Way center simultaneously.
Also, these ranges of decay rate and rest mass match the prediction of the mixing angle sin 22θ∼10−3 with a low reheating temperature in the inflation model, which enables the sterile-active neutrino oscillation to be visible
in future experiments. However, decaying sterile neutrinos have to be ruled out as a major component of dark matter because
of the high decay rate. 相似文献
11.
Elena DzifčÁKOVÁ 《Solar physics》2006,234(2):243-256
Changes in the excitation equilibrium of Feviii – Fe xv in the solar corona due to the electron non-thermal κ-distribution
are studied. The shape of the distribution affects the electron excitation rate and thus the relative intensities of the spectral
lines. Since the shape of the electron distribution function influences also the ionization equilibrium of Fe, both effects
change the final intensities of the lines. Possibilities for diagnostics of the shape of the electron distribution in the
solar corona are discussed. Synthetic spectrum of Fe for T = 1.58× 106 K and ne = 108 cm−3 is shown together with the synthetic spectra computed with DEM for the quiet Sun. 相似文献
12.
N. Barrière P. von Ballmoos H. Halloin N. Abrosimov J. M. Alvarez K. Andersen P. Bastie S. Boggs P. Courtois T. Courvoisier M. Harris M. Hernanz J. Isern P. Jean J. Knödlseder G. Skinner B. Smither P. Ubertini G. Vedrenne G. Weidenspointner C. Wunderer 《Experimental Astronomy》2005,20(1-3):269-278
The next generation of instrumentation for nuclear astrophysics will have to achieve a factor of 10–100 improvement in sensitivity over present technologies. With the focusing gamma-ray telescope MAX we take up this challenge: combining unprecedented sensitivity with high spectral and angular resolution, and the capability of measuring the polarization of the incident photons. The feasibility of such a crystal diffraction gamma-ray lens has recently been demonstrated with the prototype lens CLAIRE. MAX is a proposed mission which will make use of satellite formation flight to achieve 86 m focal length, with the Laue lens being carried by one satellite and the detector by the other. In the current design, the Laue diffraction lens of MAX will consist of 13740 copper and germanium (Ge1−x
Si
x
, x ∼ 0.02) crystal tiles arranged on 36 concentric rings. It simultaneously focuses in two energy bands, each centred on one of the main scientific objectives of the mission: the 800–900 keV band is dedicated to the study of nuclear gamma-ray lines from type Ia supernovae (e.g. 56 Co decay line at 847 keV) while the 450–530 keV band focuses on electron-positron annihilation (511 keV emission) from the Galactic centre region with the aim of resolving potential point sources. MAX promises a breakthrough in the study of point sources at gamma-ray energies by combining high narrow-line sensitivity (better than 10−6 cm−2 s−1) and high energy resolution (E/dE ∼ 500). The mission has successfully undergone a pre-phase A study with the French Space Agency CNES, and continues to evolve: new diffracting materials such as bent or composite crystals seem very promising.
PACS: 95.55.Ka, 29.30.Kv, 61.10.-i 相似文献
13.
J. Tueller H. A. Krimm T. Okajima S. D. Barthelmy S. M. Owens P. J. Serlemitsos Y. Soong K.-W. Chan Y. Ogasaka R. Shibata K. Tamura A. Furuzawa Y. Tawara H. Kunieda K. Yamashita 《Experimental Astronomy》2005,20(1-3):121-129
InFOCμS is a new generation balloon-borne hard X-ray telescope with focusing optics and spectroscopy. We had a successful 22.5-hour flight from Fort Sumner, NM on September 16,17, 2004. In this paper, we present the performance of the hard X-ray telescope, which consists of a depth-graded platinum/carbon multilayer mirror and a CdZnTe detector. The telescope has an effective area of 49 cm2 at 30 keV, an angular resolution of 2.4 arcmin (HPD), and a field of view of 11 arcmin (FWHM) depending on energies. The CdZnTe detector is configured with a 12 × 12 segmented array of detector pixels. The pixels are 2 mm square, and are placed on 2.1 mm centers. An averaged energy resolution is 4.4 keV at 60 keV and its standard deviation is 0.36 keV over 128 pixels. The detector is surrounded by a 3-cm thick CsI anti coincidence shield to reduce background from particles and photons not incident along the mirror focal direction. The inflight background is 2.9 × 10−4 cts cm−2 sec−1 keV−1 in the 20–50 keV band. 相似文献
14.
The fluxes and spectra of galactic and extragalactic neutrinos at energy 1011–1019 eV are calculated. In particular, the neutrino flux from the normal galaxies is calculated taking into account the spectral index distribution. The only assumption that seriously affects the calculated neutrino flux atE
v
1017 eV is the power-like generation spectrum of protons in the entire considered energy region.The normal galaxies with the accepted parameters generate the metagalactic equivalent electron component (electrons+their radiation) with energy density
e8.5×10–7 eV cm–3, while the density of the observed diffuse X-ray radiation alone is 100 times higher. This requires the existence of other neutrino sources and we found the minimized neutrino flux under two limitations: (1) the power-law generation spectrum of protons and (2) production of the observed energy density of the diffuse X-an -radiation. These requirements are met in the evolutionary model of origin of the metagalactic cosmic rays with modern energy density
M83.6×10–7 eV cm–3.The possibility of experiments with cosmic neutrinos of energyE
v
3×1017 eV is discussed. The upper bound on neutrino-nucleon cross-section <2.2×10–29 cm2 is obtained in evolutionary model from the observed zenith angular distribution of extensive air showers.In Appendix 2 the diffuse X-and -ray flux arising together with neutrino flux is calculated. It agrees with observed flux in the entire energy range from 1 keV up to 100 MeV. 相似文献
15.
We present the results of a search for carbon recombination lines in the Galaxy at 34.5 MHz (C575α) made using the dipole
array at Gauribidanur near Bangalore. Observations made towards 32 directions resulted in detections of lines, in absorption
at nine positions. Followup observations at 328 MHz (C272α) using the Ooty Radio Telescope detected these lines in emission.
A VLA D-array observation of one of the positions at 330 MHz yielded no detection implying a lower limit of 10′ for the angular
size of the line forming region.
The longitude-velocity distribution of the observed carbon lines indicate that the line forming regions are located mainly
between 4 kpc and 7 kpc from the Galactic centre. Combining our results with published carbon recombination line data near
76 MHz (Erickson, McConnell & Anantharamaiah 1995), we obtain constraintson the physical parameters of the line forming regions.
We find thatif the angular size of the line forming regions is ≥ 4°, then the range of parameters that fit the data are:T
e
=20–40 K,n
e
∼ 0.1–0.3 cm−3 and pathlengths ∼ 0.07–0.9 pc which may correspond to thin photodissociated regions around molecular clouds. On the other
hand, if the line forming regions are ∼ 2° in extent, then warmer gas (T
e
∼ 60–300 K) with lower electron densities (n
e
∼ 0.03–0.05 cm−3) extending over several tens of parsecs along the line of sight and possibly associated with atomic HI gas can fit the data.
Based on the range of derived parameters, we suggest that the carbon line regions are most likely associated with photo-dissociation
regions. 相似文献
16.
We studied the evolution of a small eruptive flare (GOES class C1) from its onset phase using multi-wavelength observations
that sample the flare atmosphere from the chromosphere to the corona. The main instruments involved were the Coronal Diagnostic
Spectrometer (CDS) aboard SOHO and facilities at the Dunn Solar Tower of the National Solar Observatory/Sacramento Peak. Transition
Region and Coronal Explorer (TRACE) together with Ramaty High-Energy Spectroscopic Imager (RHESSI) also provided images and
spectra for this flare. Hα and TRACE images display two loop systems that outline the pre-reconnection and post-reconnection magnetic field lines and
their topological changes revealing that we are dealing with an eruptive confined flare. RHESSI data do not record any detectable
emission at energies ≥25 keV, and the observed count spectrum can be well fitted with a thermal plus a non-thermal model of
the photon spectrum. A non-thermal electron flux F ≈ 5 × 1010 erg cm−2 s−1 is determined. The reconstructed images show a very compact source whose peak emission moves along the photospheric magnetic
inversion line during the flare. This is probably related to the motion of the reconnection site, hinting at an arcade of
small loops that brightens successively. The analysis of the chromospheric spectra (Ca II K, He I D3 and Hγ, acquired with a four-second temporal cadence) shows the presence of a downward velocity (between 10 and 20 km s−1) in a small region intersected by the spectrograph slit. The region is included in an area that, at the time of the maximum
X-ray emission, shows upward motions at transition region (TR) and coronal levels. For the He I 58.4 and O v 62.97 lines, we determine a velocity of ≈−40 km s−1 while for the Fe XIX 59.22 line a velocity of ≈−80 km s−1 is determined with a two-component fitting. The observations are discussed in the framework of available hydrodynamic simulations
and they are consistent with the scenario outlined by Fisher (1989). No explosive evaporation is expected for a non-thermal
electron beam of the observed characteristics, and no gentle evaporation is allowed without upward chromospheric motion. It
is suggested that the energy of non-thermal electrons can be dissipated to heat the high-density plasma, where possibly the
reconnection occurs. The consequent conductive flux drives the evaporation process in a regime that we can call sub-explosive. 相似文献
17.
E. Sánchez-Ayaso J. A. Combi J. F. Albacete Colombo J. López-Santiago J. Martí A. J. Mu?oz-Arjonilla 《Astrophysics and Space Science》2012,337(2):573-579
We report XMM-Newton observations of the Galactic supernova remnant G296.8-0.3, together with complementary radio and infrared data. The spatial
and spectral properties of the X-ray emission, detected towards G296.8-0.3, was investigated in order to explore the possible
evolutionary scenarios and the physical connexion with its unusual morphology detected at radio frequencies. G296.8-0.3 displays
diffuse X-ray emission correlated with the peculiar radio morphology detected in the interior of the remnant and with the
shell-like radio structure observed to the northwest side of the object. The X-ray emission peaks in the soft/medium energy
range (0.5–3.0 keV). The X-ray spectral analysis confirms that the column density is high (N
H∼0.64×1022 cm−2) which supports a distant location (d>9 kpc) for the SNR. Its X-ray spectrum can be well represented by a thermal (PSHOCK) model, with kT∼0.86 keV, an ionization timescale of 6.1×1010 cm−3 s, and low abundance (∼0.12 Z
⊙). The 24 μm observations show shell-like emission correlated with part of the northwest and southeast boundaries of the SNR.
In addition a point-like X-ray source is also detected close to the geometrical center of the radio SNR. The object presents
some characteristics of the so-called compact central objects (CCO). Its X-ray spectrum is consistent with those found at
other CCOs and the value of N
H is consistent with that of G296.8-0.3, which suggests a physical connexion with the SNR. 相似文献
18.
E × B-drifting jets have been generally ignored for the past 25 years even though they may well describe all the astrophysical
jet sources, both on galactic and stellar scales. Here we present closed-form solutions for their joint field-and-particle
distribution, argue that the observed jets are near equipartition, with extremely relativistic, monoenergetic e±-pairs of bulk Lorentz factor γ ≲ 104, and are first-order stable. We describe plausible mechanisms for the jets’ (i) formation, (ii) propagation, and (iii) termination. Wherever a beam meets with resistance, its frozen-in Poynting flux transforms the delta-shaped energy distribution
of the pairs into an almost white power law,E
2
N
E ∼E
−∫ with ∫ ≳ 0, via single-step falls through the huge convected potential. 相似文献
19.
Based on currently available observations of 28 maser sources in 25 star-forming regions with measured trigonometric parallaxes,
proper motions, and radial velocities, we have constructed the rotation curve of the Galaxy. Taking different distances to
the Galactic center R
0, we have estimated the peculiar velocity of the Sun, the angular velocity of Galactic rotation, and its three derivatives.
For R
0 = 8 kpc, we have found the circular velocity of the Sun to be V
0 = 243 ± 16 km s−1, which corresponds to a revolution period of 202 ± 10 Myr. We have obtained the Oort constants A = 16.9 ± 1.2 km s−1 kpc−1 and B = −13.5 ± 1.4 km s−1 kpc−1. Our simulation of the influence of a spiral density wave has shown that the peculiar velocity of the Sun with respect to
the local standard of rest and the component (V
⊙)LSR depend significantly on the Sun’s phase in the spiral wave. 相似文献
20.
A compact structure of a low-mass Type I presupernovae is assumed to be an essential feature of the hydrodynamical problem
dealing with the supernova Type I (SNI) envelope outbursts. This structure is characterized by a degenerate carbon-oxygen
core, which suffers a thermonuclear explosion of carbon fuel (M
0≃1.40M
⊙), and by a compact lowmass envelope (M
e
≲0.1M
⊙) with external radiusR
e≃109 cm. The parameters, of this hydrostatic envelope are specified and then, for a relatively small explosion energy, ofW
0≃(2–10)×1049 erg, hydrodynamic problem of the envelope ejection is solved numerically. This energy comes from neutrino-induced detonative
carbon burning. The resulting structure of the SNI atmosphere expanding with the velocity gradient can be employed for an
interpretation of the observed SNI spectra. In accordance with our previous papers, the SNI light curves are considered to
occur due to an additional slow (with time-scale 106–107 s) release of the bulk of the SNI energy,W≃1051, erg. The slow energy release does not, however, affect the structure of the outermost expanding layers of the envelope which
are responsible for the SNI spectra.
A short (Δt≃10−2 s) burst of soft (2–10 keV) X-rays with total radiated energy of about 1040 erg is found to appear 10–20 days before the SNI optical maximum. 相似文献