arXiv subjects

G. Dubner

Publications and source records attributed to G. Dubner.

At least 19 recordsLinked to original sources

arXiv2017

Morphological properties of the Crab Nebula: a detailed multiwavelength study based on new VLA, HST, Chandra and XMM-Newton images

We present a detailed analysis of the morphological properties of the Crab Nebula across the electromagnetic spectrum based on new and previous high-quality data from radio to X-rays. In the radio range we obtained an image of the entire nebula at 3 GHz with subarcsecond angular resolution using the VLA (NRAO) and an image at 100 GHz of the central region using the ALMA array. Simultaneously with the VLA observations we performed HST WFPC3 near infrared and Chandra X-ray observations of the central region of the nebula. In addition we produced a new UV image of the Crab nebula at 291 nm by co-adding 75 individual exposures of the Optical-UV Monitor on board XMM-Newton. The high-angular resolution and high-dynamic range radio image at 3 GHz allowed us to improve the detection and characterization of peculiar morphological features including arches with foot brightening and intercrossed loop-like structures, likely originating in plasma confined to magnetic field lines. Based on the new radio image, we carried out a detailed multiwavelength correlation. In the central area, the comparison of the almost simultaneous images confirms that the wisps in the three spectral ranges do not generally coincide in location, the radio emission being the most discordant, which is suggestive of the existence of two different synchrotron components. The X-ray pulsar jet does not have a radio counterpart. Instead, another jet-like feature is seen in radio, though with different curvature and starting point.

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arXiv2016

Feature-tailored spectroscopic analysis of the SNR Puppis A in X-rays

We introduce a distinct method to perform spatially-resolved spectral analysis of astronomical sources with highly structured X-ray emission. The method measures the surface brightness of neighbouring pixels to adaptively size and shape each region, thus the spectra from the bright and faint filamentary structures evident in the broadband images can be extracted. As a test case, we present the spectral analysis of the complete X-ray emitting plasma in the supernova remnant Puppis A observed with XMM-Newton and Chandra. Given the angular size of Puppis A, many pointings with different observational configurations have to be combined, presenting a challenge to any method of spatially-resolved spectroscopy. From the fit of a plane-parallel shocked plasma model we find that temperature, absorption column, ionization time scale, emission measure and elemental abundances of O, Ne, Mg, Si, S and Fe, are smoothly distributed in the remnant. Some regions with overabundances of O-Ne-Mg, previously characterized as ejecta material, were automatically selected by our method, proving the excellent response of the technique. This method is an advantageous tool for the exploitation of archival X-ray data.

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arXiv2016

Interactions of the Infrared bubble N4 with the surroundings

The physical mechanisms that induce the transformation of a certain mass of gas in new stars are far from being well understood. Infrared bubbles associated with HII regions have been considered to be good samples of investigating triggered star formation. In this paper we report on the investigation of the dust properties of the infrared bubble N4 around the HII region G11.898+0.747, analyzing its interaction with its surroundings and star formation histories therein, with the aim of determining the possibility of star formation triggered by the expansion of the bubble. Using Herschel PACS and SPIRE images with a wide wavelength coverage, we reveal the dust properties over the entire bubble. Meanwhile, we are able to identify six dust clumps surrounding the bubble, with a mean size of 0.50 pc, temperature of about 22 K, mean column density of 1.7 $\times10^{22}$ cm$^{-2}$, mean volume density of about 4.4 $\times10^{4}$ cm$^{-3}$, and a mean mass of 320 $M_{\odot}$. In addition, from PAH emission seen at 8 $μ$m, free-free emission detected at 20 cm and a probability density function in special regions, we could identify clear signatures of the influence of the HII region on the surroundings. There are hints of star formation, though further investigation is required to demonstrate that N4 is the triggering source.

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arXiv2016

Deep morphological and spectral study of the SNR RCW 86 with Fermi-LAT

RCW 86 is a young supernova remnant (SNR) showing a shell-type structure at several wavelengths and is thought to be an efficient cosmic-ray (CR) accelerator. Earlier \textit{Fermi} Large Area Telescope results reported the detection of $γ$-ray emission coincident with the position of RCW 86 but its origin (leptonic or hadronic) remained unclear due to the poor statistics. Thanks to 6.5 years of data acquired by the \textit{Fermi}-LAT and the new event reconstruction Pass 8, we report the significant detection of spatially extended emission coming from RCW 86. The spectrum is described by a power-law function with a very hard photon index ($Γ= 1.42 \pm 0.1_{\rm stat} \pm 0.06_{\rm syst}$) in the 0.1--500 GeV range and an energy flux above 100 MeV of ($2.91$ $\pm$ $0.8_{\rm stat}$ $\pm$ $0.12_{\rm syst}$) $\times$ $10^{-11}$ erg cm$^{-2}$ s$^{-1}$. Gathering all the available multiwavelength (MWL) data, we perform a broadband modeling of the nonthermal emission of RCW 86 to constrain parameters of the nearby medium and bring new hints about the origin of the $γ$-ray emission. For the whole SNR, the modeling favors a leptonic scenario in the framework of a two-zone model with an average magnetic field of 10.2 $\pm$ 0.7 $μ$G and a limit on the maximum energy injected into protons of 2 $\times$ 10$^{49}$ erg for a density of 1 cm$^{-3}$. In addition, parameter values are derived for the North-East (NE) and South-West (SW) regions of RCW 86, providing the first indication of a higher magnetic field in the SW region.

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arXiv2015

The southern molecular environment of SNR G18.8+0.3

In a previous paper we have investigated the molecular environment towards the eastern border of the SNR G18.8+0.3. Continuing with the study of the surroundings of this SNR, in this work we focus on its southern border, which in the radio continuum emission shows a very peculiar morphology with a corrugated corner and a very flattened southern flank. We observed two regions towards the south of SNR G18.8+0.3 using the Atacama Submillimeter Telescope Experiment (ASTE) in the 12CO J=3-2. One of these regions was also surveyed in 13CO and C18O J=3-2. The angular and spectral resolution of these observations were 22", and 0.11 km/s. We compared the CO emission to 20 cm radio continuum maps obtain as part of the Multi-Array Galactic Plane Imaging Survey (MAGPIS) and 870 um dust emission extracted from the APEX Telescope Large Area Survey of the Galaxy. We discovered a molecular feature with a good morphological correspondence with the SNR's southernmost corner. In particular, there are indentations in the radio continuum map that are complemented by protrusions in the molecular CO image, strongly suggesting that the SNR shock is interacting with a molecular cloud. Towards this region we found that the 12CO peak is not correlated with the observed 13CO peaks, which are likely related to a nearby \hii~region. Regarding the most flattened border of SNR G18.8+0.3, where an interaction of the SNR with dense material was previously suggested, our 12CO J=3-2 map show no obvious indication that this is occurring.

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arXiv2015

e-MERLIN observations of the puzzling TeV source HESS J1943+213

HESS J1943+213 is a TeV source close to the Galactic plane proposed to be a BL Lac object. Our high resolution EVN observation failed to recover two thirds of the source flux density detected simultaneously by the WSRT. Our recent e-MERLIN observations in L and C bands show only a point source with flux density comparable to the EVN detection. Thus the structure responsible for the missing flux density has to be larger than 2". It may be related to the presumed extragalactic source (thus would have a kpc-scale size), or to the Galactic foreground material close to the line of sight to the source.

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arXiv2015

HII region G46.5-0.2: the interplay between ionizing radiation, molecular gas and star formation

HII regions are particularly interesting because they can generate dense layers of gas and dust, elongated columns or pillars of gas pointing towards the ionizing sources, and cometary globules of dense gas, where triggered star formation can occur. Understanding the interplay between the ionizing radiation and the dense surrounding gas is very important to explain the origin of these peculiar structures, and hence to characterize triggered star formation. G46.5-0.2 (G46), a poorly studied galactic HII region located at about 4 kpc, is an excellent target to perform this kind of studies. Using public molecular data extracted from the Galactic Ring Survey (13CO J=1-0) and from the James Clerk Maxwell Telescope data archive (12CO, 13CO, C18O J=3-2, HCO+ and HCN J=4-3), and infrared data from the GLIMPSE and MIPSGAL surveys, we perform a complete study of G46, its molecular environment and the young stellar objects placed around it. We found that G46, probably excited by an O7V star, is located close to the edge of the GRSMC G046.34-00.21 molecular cloud. It presents a horse-shoe morphology opening in direction of the cloud. We observed a filamentary structure in the molecular gas likely related to G46 and not considerable molecular emission towards its open border. We found that about 10' towards the southwest of G46 there are some pillar-like features, shining at 8 um and pointing towards the HII region open border. We propose that the pillar-like features were carved and sculpted by the ionizing flux from G46. We found several young stellar objects likely embedded in the molecular cloud grouped in two main concentrations: one, closer to the G46 open border consisting of Class II type sources, and other one mostly composed by Class I type YSOs located just ahead the pillars-like features, strongly suggesting an age gradient in the YSOs distribution.

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arXiv2014

Search for radio counterpart of the Fermi/VERITAS PWN candidate in the SNR CTA 1

We present new high angular resolution and high sensitivity radio observations toward the neutron star RX J0007.0+7303, carried out with the Karl G. Jansky Very Large Array at 1.5 GHz. This source powers a pulsar wind nebula (PWN) only detected in the X-ray and gamma-ray domains. The new high quality radio observations do not show any evidence of a source, either point-like or extended, that could be interpreted as the radio counterpart of the high energy PWN, down to a noise level of 15 mJy/beam.

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arXiv2014

Supernova Remnants Interacting with Molecular Clouds: X-ray and Gamma-ray Signatures

The giant molecular clouds (MCs) found in the Milky Way and similar galaxies play a crucial role in the evolution of these systems. The supernova explosions that mark the death of massive stars in these regions often lead to interactions between the supernova remnants (SNRs) and the clouds. These interactions have a profound effect on our understanding of SNRs. Shocks in SNRs should be capable of accelerating particles to cosmic ray (CR) energies with efficiencies high enough to power Galactic CRs. X-ray and gamma-ray studies have established the presence of relativistic electrons and protons is some SNRs and provided strong evidence for diffusive shock acceleration as the primary acceleration mechanism, including strongly amplified magnetic fields, temperature and ionization effects on the shock-heated plasmas, and modifications to the dynamical evolution of some systems. Because protons dominate the overall energetics of the CRs, it is crucial to understand this hadronic component even though electrons are much more efficient radiators and it can be difficult to identify the hadronic component. However, near MCs the densities are sufficiently high to allow the gamma-ray emission to be dominated by protons. Thus, these interaction sites provide some of our best opportunities to constrain the overall energetics of these particle accelerators. Here we summarize some key properties of interactions between SNRs and MCs, with an emphasis on recent X-ray and gamma-ray studies that are providing important constraints on our understanding of cosmic rays in our Galaxy.

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arXiv2014

The Supernova Remnant W44: confirmations and challenges for cosmic-ray acceleration

The middle-aged supernova remnant (SNR) W44 has recently attracted attention because of its relevance regarding the origin of Galactic cosmic-rays. The gamma-ray missions AGILE and Fermi have established, for the first time for a SNR, the spectral continuum below 200 MeV which can be attributed to neutral pion emission. Confirming the hadronic origin of the gamma-ray emission near 100 MeV is then of the greatest importance. Our paper is focused on a global re-assessment of all available data and models of particle acceleration in W44, with the goal of determining on a firm ground the hadronic and leptonic contributions to the overall spectrum. We also present new gamma-ray and CO NANTEN2 data on W44, and compare them with recently published AGILE and Fermi data. Our analysis strengthens previous studies and observations of the W44 complex environment and provides new information for a more detailed modeling. In particular, we determine that the average gas density of the regions emitting 100 MeV - 10 GeV gamma-rays is relatively high (n= 250 - 300 cm^-3). The hadronic interpretation of the gamma-ray spectrum of W44 is viable, and supported by strong evidence. It implies a relatively large value for the average magnetic field (B > 10^2 microG) in the SNR surroundings, sign of field amplification by shock-driven turbulence. Our new analysis establishes that the spectral index of the proton energy distribution function is p1 = 2.2 +/- 0.1 at low energies and p2 = 3.2 +/- 0.1 at high energies. We critically discuss hadronic versus leptonic-only models of emission taking into account simultaneously radio and gamma-ray data. We find that the leptonic models are disfavored by the combination of radio and gamma-ray data. Having determined the hadronic nature of the gamma-ray emission on firm ground, a number of theoretical challenges remains to be addressed.

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arXiv2014

Shock-cloud interaction and particle acceleration in SN 1006

The supernova remnant SN 1006 is a powerful source of high-energy particles and evolves in a relatively tenuous and uniform environment, though interacting with an atomic cloud in its northwestern limb. The X-ray image of SN 1006 reveals an indentation in the southwestern part of the shock front and the HI maps show an isolated cloud (southwestern cloud) having the same velocity as the northwestern cloud and whose morphology fits perfectly in the indentation. We performed spatially resolved spectral analysis of a set of small regions in the southwestern nonthermal limb and studied the deep X-ray spectra obtained within the XMM-Newton SN 1006 Large Program. We also analyzed archive HI data, obtained combining single dish and interferometric observations. We found that the best-fit value of the N_H derived from the X-ray spectra significantly increases in regions corresponding to the southwestern cloud, while the cutoff energy of the synchrotron emission decreases. The amount of the N_H variations corresponds perfectly with the HI column density of the southwestern cloud, as measured from the radio data. The decrease in the cutoff energy at the indentation clearly reveals that the back side of the cloud is actually interacting with the remnant. The southwestern limb therefore presents a unique combination of efficient particle acceleration and high ambient density, thus being the most promising region for gamma-ray hadronic emission in SN 1006. We estimate that such emission will be detectable with the Fermi telescope within a few years.

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arXiv2013

A hadronic scenario for HESS J1818-154

Aims: G15.4+0.1 is a faint supernova remnant (SNR) that has recently been associated with the gamma-ray source HESS J1818-154. We investigate a hadronic scenario for the production of the gamma-ray emission. Methods: Molecular 13CO (J=1-0) taken from the Galactic Ring Survey (GRS) and neutral hydrogen (HI) data from the Southern Galactic Plane Survey (SGPS) have been used in combination with new 1420 MHz radio continuum observations carried out with the Giant Metrewave Radio Telescope (GMRT). Results: From the new observations and analysis of archival data we provided for the first time a reliable estimate for the distance to the SNR G15.4+0.1 and discovered molecular clouds located at the same distance. On the basis of HI absorption features, we estimate the distance to G15.4+0.1 in 4.8+/-1.0 kpc. The 13CO observations clearly show a molecular cloud about 5 arcmin in size with two bright clumps, labeled A and B, clump A positionally associated with the location of HESS J1818-154 and clump B in coincidence with the brightest northern border of the radio SNR shell. The HI absorption and the 13CO emission study indicates a possible interaction between the molecular material and the remnant. We estimate the masses and densities of the molecular gas as (1.2+/-0.5)X10^3 M_sun and (1.5+/-0.4)X10^3 cm^-3 for clump A and (3.0+/-0.7)X10^3 M_sun and (1.1+/-0.3)X10^3 cm^-3 for clump B. Calculations show that the average density of the molecular clump A is sufficient to produce the detected gamma-ray flux, thus favoring a hadronic origin for the high-energy emission.

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arXiv2013

A bright-rimmed cloud sculpted by the H ii region Sh2-48

To characterize a bright-rimmed cloud embedded in the HII region Sh2-48 searching for evidence of triggered star formation. We carried out observations towards a region of 2'x2' centered at RA=18h 22m 11.39s, dec.=-14deg 35m 24.81s (J2000) using the Atacama Submillimeter Telescope Experiment (ASTE; Chile) in the 12CO J=3-2,13CO J=3-2, HCO+ J=4-3, and CS J=7-6 lines with an angular resolution of about 22". We also present radio continuum observations at 5 GHz carried out with the Jansky Very Large Array (JVLA; EEUU) interferometer with a synthetized beam of 7"x5". The analysis of our molecular observations reveals the presence of a relatively dense clump with n(H_2)~3x10^3 cm^-3, located in projection onto the interior of the HII region Sh2-48. The emission distribution of the four observed molecular transitions has, at VLSR~38 kms^-1, morphological anti-correlation with the bright-rimmed cloud as seen in the optical emission. From the new radio continuum observations we identify a thin layer of ionized gas located at the border of the clump which is facing to the ionizing star. The ionized gas has an electron density of about 73 cm^-3 which supports the hypothesis that the clump is being photoionized by the nearby star, BD-14 5014. From the evaluation of the pressure balance between the ionized and molecular gas, we conclude that the clump would be in a pre-pressure balance state with the shocks being driven into the surface layer. Two YSO candidates (class I), are placed slightly beyond the bright rim suggesting that their formation could have been triggered via the radiation-driven implosion process.

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arXiv2013

The most complete and detailed X-ray view of the SNR Puppis A

With the purpose of producing the first detailed full view of Puppis A in X-rays, we carried out new XMM-Newton observations covering the missing regions in the southern half of the supernova remnant (SNR) and combined them with existing XMM-Newton and Chandra data. The new images were produced in the 0.3-0.7, 0.7-1.0 and 1.0-8.0 energy bands. We investigated the SNR morphology in detail, carried out a multi-wavelength analysis and estimated the flux density and luminosity of the whole SNR. The complex structure observed across the remnant confirms that Puppis A evolves in an inhomogeneous, probably knotty interstellar medium. The southwestern corner includes filaments that perfectly correlate with radio features suggested to be associated with shock/cloud interaction. In the northern half of Puppis A the comparison with Spitzer infrared images shows an excellent correspondence between X-rays and 24 and 70 microns emission features, while to the south there are some matched and other unmatched features. X-ray flux densities of 12.6 X 10^-9, 6.2 X 10^-9, and 2.8 X 10^-9 erg cm^-2 s^-1 were derived for the 0.3-0.7, 0.7-1.0 and 1.0-8.0 keV bands, respectively. At the assumed distance of 2.2 kpc, the total X-ray luminosity between 0.3 and 8.0 keV is 1.2 X 10^37 erg s^-1. We also collected and updated the broad-band data of Puppis A between radio and GeV gamma-ray range, producing its spectral energy distribution. To provide constraints to the high-energy emission models, we re-analyzed radio data, estimating the energy content in accelerated particles to be Umin=4.8 X 10^49 erg and the magnetic field strength B=26 muG.

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arXiv2012

VLBI search for the radio counterpart of HESS J1943+213

HESS J1943+213, a TeV point source close to the Galactic plane recently discovered by the H.E.S.S. collaboration, was proposed to be an extreme BL Lacertae object, though a pulsar wind nebula (PWN) nature could not be completely discarded. To investigate its nature, we performed high-resolution radio observations with the European Very Long Baseline Interferometry Network (EVN) and reanalyzed archival continuum and H {\sc i} data. The EVN observations revealed a compact radio counterpart of the TeV source. The low brightness temperature and the resolved nature of the radio source are indications against the beamed BL Lacertae hypothesis. The radio/X-ray source appears immersed in a $\sim$ 1\arcmin elliptical feature suggesting a possible galactic origin (PWN nature) for the HESS source. We found that HESS\,J1943+213 is located in the interior of a $\sim1\degr$ diameter H {\sc i} feature, and explored the possibility of they being physically related.

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arXiv2012

The molecular clump towards the eastern border of SNR G18.8+0.3

The eastern border of the SNR G18.8+0.3, close to an HII regions complex, is a very interesting region to study the molecular gas that it is probably in contact with the SNR shock front. We observed the aforementioned region using the Atacama Submillimeter Telescope Experiment (ASTE) in the 12CO J=3-2, 13CO J=3-2, HCO+ J=4-3, and CS J=7-6 lines with an angular resolution of 22". To complement these observations, we analyzed IR, submillimeter and radio continuum archival data. In this work, we clearly show that the radio continuum "protrusion" that was early thought to belong to the SNR is an HII regions complex deeply embedded in a molecular clump. The new molecular observations reveal that this dense clump, belonging to an extended molecular cloud that surrounds the SNR southeast border, is not physically in contact with SNR G18.8+0.3, suggesting that the SNR shock front have not yet reached it or maybe they are located at different distances. We found some young stellar objects embedded in the molecular clump, suggesting that their formation should be approximately coeval with the SN explosion.

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arXiv2012

Outflowing activity in the UCHII region G045.47+0.05

Aims. This work aims at investigating the molecular gas in the surroundings of the ultra-compact HII region G045.47+0.05 looking for evidence of molecular outflows. Methods. We carried out observations towards a region of 2 arcmin x 2 arcmin centered at RA=19h 14m 25.6s, dec.= 11deg 09m 27.6s (J2000) using the Atacama Submillimeter Telescope Experiment (ASTE; Chile) in the 12CO J=3-2, 13CO J=3-2, HCO+ J=4-3 and CS J=7-6 lines with an angular resolution of 22 arcsec. We complement these observations with public infrared data. Results. We characterize the physical parameters of the molecular clump where G045.47+0.0 is embedded. The detection of the CS J=7-6 line emission in the region reveals that the ultra-compact HII region G045.47+0.0 has not completely disrupted the dense gas where it was born. The HCO+ abundance observed towards G045.47+0.0 suggests the presence of molecular outflow activity in the region. From the analysis of the 12CO J=3-2 transition we report the presence of bipolar molecular outflows with a total mass of about 300 solar masses . We derive a dynamical time (flow's age) of about 10^5 yr for the outflow gas, in agreement with the presence of an ultra-compact HII region. We identify the source 2MASS 19142564+1109283 as the massive protostar candidate to drive the molecular outflows. Based on the analysis of its spectral energy distribution we infer that it is an early B-type star of about 15 solar masses. The results of this work support the scenario where the formation of massive stars, at least up to early B-type stars, is similar to that of low mass stars.

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arXiv2011

Neutral pion emission from accelerated protons in the supernova remnant W44

We present the AGILE gamma-ray observations in the energy range 50 MeV - 10 GeV of the supernova remnant (SNR) W44, one of the most interesting systems for studying cosmic-ray production. W44 is an intermediate-age SNR (20, 000 years) and its ejecta expand in a dense medium as shown by a prominent radio shell, nearby molecular clouds, and bright [SII] emitting regions. We extend our gamma-ray analysis to energies substantially lower than previous measurements which could not conclusively establish the nature of the radiation. We find that gamma-ray emission matches remarkably well both the position and shape of the inner SNR shocked plasma. Furthermore, the gamma-ray spectrum shows a prominent peak near 1 GeV with a clear decrement at energies below a few hundreds of MeV as expected from neutral pion decay. Here we demonstrate that: (1) hadron-dominated models are consistent with all W44 multiwavelength constraints derived from radio, optical, X-ray, and gamma-ray observations; (2) ad hoc lepton-dominated models fail to explain simultaneously the well-constrained gamma-ray and radio spectra, and require a circumstellar density much larger than the value derived from observations; (3) the hadron energy spectrum is well described by a power-law (with index s = 3.0 \pm 0.1) and a low-energy cut-off at Ec = 6 \pm 1 GeV. Direct evidence for pion emission is then established in an SNR for the first time.

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