Detection Of Hard X-ray Emission Research Articles

The transient event in NGC 1566 from 2017 to 2019

Context.NGC 1566 is a local face-on Seyfert galaxy and is known for exhibiting recurrent outbursts that are accompanied by changes in spectral type. The most recent transient event occurred from 2017 to 2019 and was reported to be accompanied by a change in Seyfert classification from Seyfert 1.8 to Seyfert 1.2.Aims.We aim to study the transient event in detail by analyzing the variations in the optical broad-line profiles. In particular, we intend to determine the structure and kinematics of the broad-line region.Methods.We analyzed data from an optical spectroscopic variability campaign of NGC 1566 taken with the 9.2 m Southern African Large Telescope (SALT) between July 2018 and October 2019 triggered by the detection of hard X-ray emission in June 2018. We supplemented this data set with optical to near-infrared (NIR) spectroscopic archival data taken by VLT/MUSE in September 2015 and October 2017, and investigated the emission from different line species during the event.Results.NGC 1566 exhibits pronounced spectral changes during the transient event. We observe the emergence and fading of a strong power-law-like blue continuum as well as strong variations in the Balmer, He I, and He IIlines and the coronal lines [Fe VII], [Fe X], and [Fe XI]. Moreover, we detect broad double-peaked emission line profiles of O Iλ8446 and the Ca IIλλ8498, 8542, 8662 triplet. This is the first time that genuine double-peaked O Iλ8446 and Ca IIλλ8498, 8542, 8662 emission in AGN is reported in the literature. All broad lines show a clear redward asymmetry with respect to their central wavelength and we find indications for a significant blueward drift of the total line profiles during the transient event. The profiles and the FWHM of the Balmer lines remain largely constant during all observations. We show that the double-peaked emission line profiles are well approximated by emission from a low-inclination, relativistic eccentric accretion disk, and that single-peaked profiles can be obtained by broadening due to scale-height-dependent turbulence. Small-scale features in the O Iand Ca IIlines suggest the presence of inhomogeneities in the broad-line region.Conclusions.We conclude that the broad-line region in NGC 1566 is dominated by the kinematics of a relativistic eccentric accretion disk. The broad-line region can be modeled to be vertically stratified with respect to scale-height turbulence with O Iand Ca IIbeing emitted close to the disk in a region with high (column) density, while the Balmer and helium lines are emitted at greater scale height above the disk. The observed blueward drift might be attributed to a low-optical-depth wind launched during the transient event. Except for this wind, the observed kinematics of the broad-line region remain largely unchanged during the transient event.

Hot X-ray onsets of solar flares

ABSTRACT The study of the localized plasma conditions before the impulsive phase of a solar flare can help us understand the physical processes that occur leading up to the main flare energy release. Here, we present evidence of a hot X-ray ‘onset’ interval of enhanced isothermal plasma temperatures in the range of 10–15 MK over a period of time prior to the flare’s impulsive phase. This ‘hot onset’ interval occurs during the initial soft X-ray increase and definitely before any detectable hard X-ray emission. The isothermal temperatures, estimated by the Geostationary Operational Environmental Satellite X-ray sensor, and confirmed with data from the Reuven Ramaty High Energy Solar Spectroscopic Imager, show no signs of gradual increase, and the ‘hot onset’ phenomenon occurs regardless of flare classification or configuration. In a small sample of four representative flare events, we tentatively identify this early hot onset soft X-ray emission to occur within footpoint and low-lying loop regions, rather than in coronal structures, based on images from the Atmospheric Imaging Assembly. We confirm this via limb occultation of a flaring region. These hot X-ray onsets appear before there is evidence of collisional heating by non-thermal electrons, and hence challenge the standard modelling techniques.

WHERE IS THE CHROMOSPHERIC RESPONSE TO CONDUCTIVE ENERGY INPUT FROM A HOT PRE-FLARE CORONAL LOOP?

Before the onset of a flare is observed in hard X-rays there is often a prolonged pre-flare or pre-heating phase with no detectable hard X-ray emission but pronounced soft X-ray emission suggesting that energy is being released and deposited into the corona and chromosphere already at this stage. This work analyses the temporal evolution of coronal source heating and the chromospheric response during this pre-heating phase to investigate the origin and nature of early energy release and transport during a solar flare. Simultaneous X-ray, EUV, and microwave observations of a well observed flare with a prolonged pre-heating phase are analysed to study the time evolution of the thermal emission and to determine the onset of particle acceleration. During the 20 minutes duration of the pre-heating phase we find no hint of accelerated electrons, neither in hard X-rays nor in microwave emission. However, the total energy budget during the pre-heating phase suggests that energy must be supplied to the flaring loop to sustain the observed temperature and emission measure. Under the assumption of this energy being transported toward the chromosphere via thermal conduction, significant energy deposition at the chromosphere is expected. However, no detectable increase of the emission in the AIA wavelength channels sensitive to chromospheric temperatures is observed. The observations suggest energy release and deposition in the flaring loop before the onset of particle acceleration, yet a model in which energy is conducted to the chromosphere and subsequent heating of the chromosphere is not supported by the observations.

A Hot Microflare Observed with RHESSI and Hinode

RHESSI and Hinode observations of a GOES B-class flare are combined to investigate the origin of 15 MK plasma. The absence of any detectable hard X-ray emission coupled with weak blueshifted emission lines (indicating upward velocities averaging only 14 km/s) suggests that this was a result of direct heating in the corona, as opposed to nonthermal electron precipitation causing chromospheric evaporation. These findings are in agreement with a recent hydrodynamical simulation of microflare plasmas which found that higher temperatures can be attained when less energy is used to accelerate electrons out of the thermal distribution. In addition, unusual redshifts in the 2 MK Fe XV line (indicating downward velocities of 14 km/s) were observed cospatial with one of the flare ribbons during the event. Downflows of such high temperature plasma are not predicted by any common flare model.

BeppoSAX observation of 4U 1705-44: detection of hard X-ray emission in the soft state

Context. 4U 1705-44 is one of the best-studied type I X-ray burster and atoll sources. Aims. Since it covers a wide range in luminosity (from a few to 50 × 10 36 erg s −1 ) and shows clear spectral state transitions, it represents a good laboratory for testing the accretion models proposed for atoll sources. Methods. We analyzed the energy spectrum accumulated with BeppoSAX observations (43.5 ks) in August 2000 when the source was in a soft spectral state. Results. The continuum of the wide-band energy spectrum is well-described by the sum of a blackbody (kTbb ∼ 0.56 keV) and a Comptonized component (seed-photon temperature kTW ∼ 1 keV, electron temperature kTe ∼ 2.7 keV, and optical depth τ ∼ 11). A hard tail was detected at energies above ∼25 keV. The latter can be modeled by a power law having a photon index ∼2.9, which contributes ∼11% of the total flux in the range 0.1–200 keV. A broad emission line, possibly from a relativistic accretion disk, models the feature in the Fe K line region of the spectrum. Conclusions. This is the first time that a high-energy tail has been observed during a soft state of the source.

X-Ray Spectrum of `the Cap' of M82

The extended X-ray emission from ‘the Cap’ region located 11.6 kpc above the disk of the starburst galaxy M82 has been observed with Suzaku and XMM-Newton. We have clearly detected K-shell emission lines from O VII, O VIII, Ne X, Mg XI, Mg XII and the Fe-L complex. Two optically-thin thermal plasma components are required to fit the observed Xray spectra. We have determined the metal abundances of O, Ne, Mg, Si and Fe in this region for the first time. Their metal abundance ratios agree well with those of metal-poor stars and the model prediction of metals synthesized by type-II supernovae, but are inconsistent with those of type-Ia supernovae. This result is support for the idea that the origin of the metals in the Cap is type-II supernovae explosions occurring in the starburst regions in the M82 galaxy. We discuss the contribution from sputtered dust grains to the metals in the Cap and suggestion of the charge-exchange processes. We also report the detection of hard X-ray emission above 2 keV extending from M82 along the north-east and north-west directions.

Hard X‐Rays from Ultracompact HiiRegions in W49A

We report the Chandra detection of hard X-ray emission from the Welch ring in W49A, an organized structure of ultracompact (UC) H II regions containing a dozen nascent early-type stars. Two UC H II regions are associated with hard X-ray emission in a deep Advanced CCD Imaging Spectrometer (ACIS) image exposed for ~96.7 ks. One of the two X-ray sources has no near-infrared counterpart and is extended by ~5'', or ~0.3 pc, at a distance of ~11.4 kpc, which is spatially aligned with the cometary radio continuum emission associated with the UC H II region. The X-ray spectrum of the emission, when fit with a thermal model, indicates a heavily absorbed plasma with extinction of ~5 × 1023 cm-2, temperature ~7 keV, and X-ray luminosity in the 3.0-8.0 keV band of ~3×1033 ergs s-1. Both the luminosity and the size of the emission resemble the extended hard emission found in UC H II regions in Sagittarius B2, yet they are smaller by an order of magnitude than the emission found in massive star clusters such as NGC 3603. Three possibilities are discussed for the cause of the hard extended emission in the Welch ring: an ensemble of unresolved point sources, shocked interacting winds of the young O stars, and a wind-blown bubble interacting with ambient cold matter.

INTEGRAL detection of hard X-rays from NGC 6334: nonthermal emission from colliding winds or an AGN?

We report the detection of hard X-ray emission from the field of the star-forming region NGC 6334 with the the International Gamma-Ray Astrophysics Laboratory INTEGRAL. The JEM-X monitor and ISGRI imager aboard INTEGRAL and Chandra ACIS imager were used to construct 3-80 keV images and spectra of NGC 6334. The 3-10 keV and 10-35 keV images made with JEM-X show a complex structure of extended emission from NGC 6334. The ISGRI source detected in the energy ranges 20-40 keV and 40-80 keV coincides with the NGC 6334 ridge. The 20-60 keV flux from the source is (1.8+-0.37)*10(-11) erg cm(-2) s(-1). Spectral analysis of the source revealed a hard power-law component with a photon index about 1. The observed X-ray fluxes are in agreement with extrapolations of X-ray imaging observations of NGC 6334 by Chandra ACIS and ASCA GIS. The X-ray data are consistent with two very different physical models. A probable scenario is emission from a heavily absorbed, compact and hard Chandra source that is associated with the AGN candidate radio source NGC 6334B. Another possible model is the extended Chandra source of nonthermal emission from NGC 6334 that can also account for the hard X-ray emission observed by INTEGRAL. The origin of the emission in this scenario is due to electron acceleration in energetic outflows from massive early type stars. The possibility of emission from a young supernova remnant, as suggested by earlier infrared observations of NGC 6334, is constrained by the non-detection of 44Ti lines.

A study of the expanding envelope of Nova V1974 Cyg 1992 based on IUE high resolution spectroscopy

We have carried out a detailed analysis of the IUE archival high resolution spectra of the classical nova V1974 Cyg 1992. The main UV resonance lines show P Cygni profiles in the first days, which change into symmetric pure emission lines, and then slowly become fainter and narrower. Lines of higher ionization species reach their peak luminosity later than those of low ionization. This can be explained by a fast wind which is optically thick in the early days, when the pseudo- photosphere is located inside the wind. As the mass loss decreases, the radius of the pseudo-photosphere schrinks. This has three effects that explain the observed changes: (1) the deeper accelerating layers of the wind become visible where the emission lines are formed by collisional excitation and/or recombination; (2) as the mass loss rate decreases the emission comes from deeper regions of the wind where the velocities are smaller; (3) the effective temperature and the degree of ionization increase. In addition to the P Cygni and emission lines, we could identify two shortward shifted absorption systems which originate in two separate expanding shells, outside the wind layers where the emission lines are formed. The velocity of both shells increase with time. The outer main shell, containing most of the matter ejected at the outburst, produces the so-called absorption line and the inner faster moving second shell produces the so-called absorption line system. The acceleration of the two shells is the result of increasing line-radiation pressure due to the UV-brightening of the star as the effective radius decreases. Around day 60 the second shell has overtaken the slower moving principal system shell, and merged with it. This explains: the sudden disappearance of the diffuse line system near that date, the upward jump of ∆v = 240 km s −1 in velocity of the principal system and the first detection of hard X-ray emission on day 63. This velocity jump indicates that the main shell is ≈4 times more massive than the second shell. The deceleration suffered by the diffuse-enhanced system after the shock provides a shock temperature Tshock ≈ 1.6 keV, in fairly good agreement with the temperature of the observed hard X-ray emission. The UV observations are interpreted through an empirical model in which the pre-nova slow wind phase is followed by the ejection of two shells, where the principal and the diffuse-enhanced absorption systems are formed, and by a phase of fast continuous lower density wind. Our empirical expansion velocity law for the principal system, together with Hα interferometric observations of the angular radius on day 10 are used to determine the distance to the nova, which is found to be 2.9 ± 0.2 kpc, in agreement with HST imaging and with the absolute magnitude versus rate of decline relationship.

BeppoSAX observations of CF Tucanae and TY Pyxidis

We present the results of BeppoSAX observations of the RS CVn binary systems CF Tuc and TY Pyx. A long- durationflare was detected on CF Tucjust at the beginning of the observation; a second smaller flareoccurred about half a period later. The light curve of TY Pyx shows a significant orbital modulation of the quiescent emission with possible evidence for eclipses, and strong flares. We find that the coronae of the two stars are very similar: both systems have quiescent temperatures of ∼10 and 20-25 MK, with only a factor of two difference in the emission measures. During the flares, temperatures of ∼30- 40 MK are reached; these temperatures are not high enough to give detectable hard X-ray emission in the PDS. Both stars have subsolar coronal metallicities: we confirm the very low coronal abundance of CF Tuc (Z ∼ 0.1−0.2Z� ), while for TY Pyx we find Z ∼ 0.5−0.6Z� . For both stars the coronal metallicities are in good agreement with their measured photospheric abundances.

Hard X-ray emission from a young massive star-forming cluster

We report the detection of hard X-ray emission (>2 keV) from a number of point sources associated with the very young massive star-forming region IRAS 19410+2336. The X-ray emission is detected from several sources located around the central and most deeply embedded mm continuum source, which remains undetected in the X-ray regime. All X-ray sources have K-band counterparts, and those likely belonging to the evolving massive cluster show near-infrared colors in the 2MASS data indicative of pre-main-sequence stages. The X-ray luminosities around 10^{31} erg/s are at the upper end of luminosities known for low-mass pre-main-sequence sources, and mass estimates based on the infrared data indicate that at least some of the X-ray detected sources are intermediate-mass objects. Therefore, we conclude that the X-ray emission is due to intermediate-mass pre-main-sequence Herbig Ae/Be stars or their precursors. The emission process is possibly due to magnetic star-disk interaction as proposed for their low-mass counterparts.

BeppoSAX detection and follow-up of GRB 980425

We present BeppoSAX GRBM and WFC light curves of GRB 980425 and NFI follow-up data taken in 1998 April, May, and November. The first NFI observation has detected within the radius error box of the GRB an X-ray source positionally consistent with the supernova SN 1998bw, exploded within a day of GRB 980425, and a fainter X-ray source, not consistent with the position of the supernova. The former source is detected in the following NFI pointings and exhibits a decline of a factor of two in six months. If it is associated with SN 1998bw, this is the first detection of hard X-ray emission from a Type I supernova. The latter source exhibits only marginally significant variability. Based on these data, it is not possible to select either source as a firm candidate for the GRB counterpart.

On Search and Detection of Hard X-Ray Emission from Orion-Like Complexes Produced by a Flux of Subrelativistic Nuclei

We analyze the origin of the Orion γ-rays discovered by the COMPTEL telescope and possibilities to search Orion-like sources with X-ray telescopes. We have shown that if these γ-rays are produced by a flux of carbon and oxygen subrelativistic nuclei (as it is usually assumed) then we expect a flux of continuous and line X-ray emission. We showed that in addition to the bremsstrahlung and inverse bremsstrahlung radiation a significant part of the continuous X-ray flux is generated by radiative capture of background electrons. The calculations showed also that the 6.4 keV iron line is the most prominent in the Orion X-ray spectrum, if the nuclei fill the whole volume of the Orion complex. The line flux is estimated by Lx ∼ 4 · 1040 ph s−1. Since the region of the emission is expected to be extended (the total area is about 100 square degrees) the intensity of the line near Earth is 4.6 · 10−3 ph cm−2 s−1 sr−1. This value is below the 3σ level of the ASCA telescope. Therefore, a search of distant (point-like) Orion-like candidates with X-ray telescopes seems to be more perspective. As an example, we analyze the flux of the 6.4 keV iron line observed with ASCA in the direction of the galactic center. If this line is emitted by an Orion-like object in the center of the Galaxy then we expect that there is a powerful source of subrelativistic cosmic rays with energies ≤ 25 MeV n−1 as well as γ-rays in the range 3 to 7 MeV. Their luminosities are Ln ∼ 4 · 1043 n s−1 and Lγ ∼ 1041 ph s−1, respectively.

Sigma detection of hard X-ray emission from the soft transient type I X-ray burster KS 1731 - 260

view Abstract Citations (57) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Sigma Detection of Hard X-Ray Emission from the Soft Transient Type I X-Ray Burster KS 1731-260 Barret, D. ; Bouchet, L. ; Mandrou, P. ; Roques, J. P. ; Cordier, B. ; Laurent, Ph. ; Lebrun, F. ; Paul, J. ; Sunyaev, R. ; Churazov, E. ; Gilfanov, M. ; Diachkov, A. ; Khavenson, N. ; Novikov, B. ; Chulkov, I. ; Kuznetsov, A. Abstract Observations of KS 1731 - 260 by the French hard X-ray/soft gamma-ray telescope Sigma reveal the existence of a hard state for which the spectrum extends to 150 keV, with a power-law shape characterized by a photon spectral index of about 2.9 (+/-0.8). Consequently, KS 1731 - 260 must be added to the list of the hard X-ray sources located within the 100 sq deg around the Galactic center. The result also supports the idea that power-law tails are not an exclusive property of the ultrasoft transients, which are black hole candidates, but may also be a temporary characteristic of weak magnetized neutron stars. Publication: The Astrophysical Journal Pub Date: August 1992 DOI: 10.1086/171615 Bibcode: 1992ApJ...394..615B Keywords: Emission Spectra; Neutron Stars; X Ray Stars; Astronomical Spectroscopy; Compton Effect; X Ray Telescopes; Astrophysics; X-RAYS: BURSTS; STARS: INDIVIDUAL ALPHANUMERIC: KS 1731-260; X-RAYS: STARS full text sources ADS | data products SIMBAD (9)

A new hard X-ray source 15 arcmin away from 3C 273?

The detection of hard X-ray emission (40-80 keV) from a location 15' away from 3C 273, at a 5.5 σ confidence level during a 16 hr observation of a 5 × 5 deg 2 sky region containing the QSO 3C 273 and at a less significance level (∼ 3 σ) in two other pointings, is reported. The analysis done on the SIGMA images is briefly described in order to assess the reality of this detection. The source is certainly variable in the SIGMA operational band by a factor of 2. The search for likely counterparts at other wavelengths has indicated interesting objects, especially in the X-ray band

Active galactic nuclei with ultrasoft X-ray emission

The discovery of a class of AGN with clearly distinguishable soft X-ray components below 0.5 keV is reported. These AGN were detected through equivalent hydrogen column densities that are less than a few times 10 exp 20/sq cm. At least three kinds of AGN comprise this sample: low-redshift quasars and Seyfert galaxies that have only ultrasoft emission (these objects lack detectable hard X-ray emission); AGN that have both ultrasoft as well as hard X-ray components; and high-redshift quasars whose soft emission may originate, as proposed for the lensed quasar 0957 + 561, in an extended source or, alternately, in a source that is luminosity-enhanced due to relativistic beaming.

OSO-7 observations of solar x-rays in the energy range 10?100 keV

The solar X-ray experiment on the satellite OSO-7 has provided extensive observations of hard and soft X-ray bursts. We give a general description of the hard X-ray data here, in parallel with the description of the soft X-ray data already published (Datlowe et al., 1974). The data for this study consist of 123 hard X-ray bursts which occurred between 10 October 1971 and 6 June 1972. We examine the behavior of a typical event in terms of its spectral and flux variations. For the whole data sample, we find that 2/3 of the soft X-ray bursts have detectable hard X-ray emission. We present the distributions of frequency of occurrence of peak flux, spectral index, collisional energy loss, burst duration and the duration at half maximum of the flux profile. No correlation was found between the flux and the spectral slope of an individual data sample, nor was there a correlation between the peak flux and the full width at half maximum of a burst. We have utilized Hα flare identifications to study longitude variations. No statistically significant variation was found in the relative frequency of hard or soft X-ray bursts with longitude. However, the spectral slope does exhibit a center-to-limb variation, with limb spectra tending to be softer. We have compared the growth of energy in the hot flare plasma (soft X-ray source) with the collisional energy deposition (hard X-ray source) for the entire sample. This analysis shows that collision loss within the hot plasma is not the principal source of its heating.