High Energy Astrophysical Phenomena (astro-ph.HE): Recent submissions
Now showing items 21-40 of 98
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Exceptional outburst of the blazar CTA 102 in 2012: the GASP-WEBT campaign and its extensionAfter several years of quiescence, the blazar CTA 102 underwent an exceptional outburst in 2012 September-October. The flare was tracked from γ-ray to near-infrared (NIR) frequencies, including Fermi and Swift data as well as photometric and polarimetric data from several observatories. An intensive Glast-Agile support programme of the Whole Earth Blazar Telescope (GASP-WEBT) collaboration campaign in optical and NIR bands, with an addition of previously unpublished archival data and extension through fall 2015, allows comparison of this outburst with the previous activity period of this blazar in 2004-2005. We find remarkable similarity between the optical and γ-ray behaviour of CTA 102 during the outburst, with a time lag between the two light curves of ≈1 h, indicative of cospatiality of the optical and γ-ray emission regions. The relation between the γ-ray and optical fluxes is consistent with the synchrotron self-Compton (SSC) mechanism, with a quadratic dependence of the SSC γ-ray flux on the synchrotron optical flux evident in the post-outburst stage. However, the γ-ray/optical relationship is linear during the outburst; we attribute this to changes in the Doppler factor. A strong harder-when-brighter spectral dependence is seen both the in γ-ray and optical non-thermal emission. This hardening can be explained by convexity of the UV-NIR spectrum that moves to higher frequencies owing to an increased Doppler shift as the viewing angle decreases during the outburst stage. The overall pattern of Stokes parameter variations agrees with a model of a radiating blob or shock wave that moves along a helical path down the jet.
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Constraining the progenitor evolution of GW 150914One of the largest surprises from the LIGO results regarding the first gravitational wave detection (GW 150914) was the fact the black holes (BHs) were heavy, of order 30 - 40 M<SUB>⊙</SUB>. The most promising explanation for this obesity is that the BH-BH merger occurred at low metallicity (Z): when the iron (Fe) contents is lower this is expected to result in weaker mass loss during the Wolf-Rayet (WR) phase. We therefore critically evaluate the claims for the reasons of heavy BHs as a function of Z in the literature. Furthermore, weaker stellar winds might lead to more rapid stellar rotation, allowing WR and BH progenitor evolution in a chemically homogeneous manner. However, there is as yet no empirical evidence for more rapid rotation amongst WR stars in the low Z environment of the Magellanic Clouds. Due to the intrinsic challenge of determining WR rotation rates from emission lines, the most promising avenue to constrain rotation-rate distributions amongst various WR subgroups is through the utilisation of their emission lines in polarised light. We thus provide an overview of linear spectro-polarimetry observations of both single and binary WRs in the Galaxy, as well as the Large and Small Magellanic Clouds, at 50% and 20% of solar Z, respectively. Initial results suggest that the route of chemically homogeneous evolution (CHE) through stellar rotation is challenging, whilst the alternative of a post-LBV or common envelope evolution is more likely.
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Natal molecular cloud of SNR Kes 41. Complete characterisationUsing high-resolution data of the <SUP>12</SUP>CO and <SUP>13</SUP>CO (J = 1-0) line emission from the Mopra Southern Galactic Plane CO Survey in conjunction with neutral hydrogen observations from the Southern Galactic Plane Survey (SGPS) and mid-infrared Spitzer data, we have explored the large-scale environment of the supernova remnant Kes 41. On the basis of these data, we identified for the first time the parent cloud of Kes 41 in its whole extension and surveyed the HII regions, masers, and the population of massive young stellar objects in the cloud. The whole unveiled giant cloud, located at the kinematic distance of 12.0 ± 3.6 kpc, whose average total mass and size are 10-30 × 10<SUP>5</SUP> M<SUB>⊙</SUB> and 26', also shines in γ-rays, as revealed by the Large Area Telescope on board the Fermi satellite. We determined a high average proton density 500-1000 cm<SUP>-3</SUP> in the large molecular complex, of which protons from the neutral atomic and ionised gases comprise only 15%.
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Molecular shocks and the gamma-ray clouds of the W28 supernova remnantInterstellar medium clouds in the W28 region are emitting gamma-rays and it is likely that the W28 supernova remnant is responsible, making W28 a prime candidate for the study of cosmic-ray acceleration and diffusion. Understanding the influence of both supernova remnant shocks and cosmic rays on local molecular clouds can help to identify multi-wavelength signatures of probable cosmic-ray sources. To this goal, transitions of OH, SiO, NH<SUB>3</SUB>, HCO<SUP>+</SUP> and CS have complemented CO in allowing a characterisation of the chemically rich environment surrounding W28. This remnant has been an ideal test-bed for techniques that will complement arcminute-scale studies of cosmic-ray source candidates with future GeV-PeV gamma-ray observations.
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Unidentified γ-ray emission towards the SNR Kes 41 revisitedKes 41 is one of the Galactic supernova remnants (SNRs) that are proposed to be physically linked to γ-ray emission at GeV energies. The nature of the γ-ray photons has been explained, but inconclusively, as hadronic collisions of particles accelerated at the SNR blast wave with target protons in an adjacent molecular clump. We performed an analysis of Fermi-Large Area Telescope (LAT) data of about nine years to assess the origin of the γ-ray emission. To investigate this matter, we also used spectral modelling constraints from the physical properties of the interstellar medium towards the γ-ray emitting region along with a revised radio continuum spectrum of Kes 41 (α = -0.54 ± 0.10, S ∝ ν<SUP>α</SUP>). We demonstrate that the γ-ray fluxes in the GeV range can be explained through bremsstrahlung emission from electrons interacting with the surrounding medium. We also considered a model in which the emission is produced by pion decay after hadronic collisions, and confirm that this mechanism cannot be excluded.
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Massive star winds and HMXB donorsUnderstanding the complex behavior of High Mass X-ray binaries (HMXBs) is not possible without detailed information about their donor stars. While crucial, this turns out to be a challenge on multiple fronts. First, multi-wavelength spectroscopy is vital. As such systems can be highly absorbed, this is often already hard to accomplish. Secondly, even if the spectroscopic data is available, the determination of reliable stellar parameters requires sophisticated model atmospheres that accurately describe the outermost layers and the wind of the donor star.
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A Study of the Interstellar Medium Towards the Unidentified Dark TeV γ-Ray Sources HESS J1614-518 and HESS J1616-508HESS J1614-518 and HESS J1616-508 are two tera-electron volt γ-ray sources that are not firmly associated with any known counterparts at other wavelengths. We investigate the distribution of interstellar medium towards the tera-electron volt γ-ray sources using results from a 7-mm-wavelength Mopra study, the Mopra Southern Galactic Plane CO Survey, the Millimetre Astronomer's Legacy Team-45 GHz survey and [C i] data from the HEAT telescope. Data in the CO(1-0) transition lines reveal diffuse gas overlapping the two tera-electron volt sources at several velocities along the line of sight, while observations in the CS(1-0) transition line reveal several interesting dense gas features. To account for the diffuse atomic gas, archival H i data was taken from the Southern Galactic Plane Survey. The observations reveal gas components with masses 10<SUP>3</SUP> to 10<SUP>5</SUP> M<SUB>⊙</SUB> and with densities 10<SUP>2</SUP> to 10<SUP>3</SUP> cm<SUP>-3</SUP> overlapping the two tera-electron volt sources. Several origin scenarios potentially associated with the tera-electron volt γ-ray sources are discussed in light of the distribution of the local interstellar medium. We find no strong convincing evidence linking any counterpart with HESS J1614-518 or HESS J1616-508.
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Ammonia excitation imaging of shocked gas towards the W28 gamma-ray source HESS J1801-233We present 12 mm Mopra observations of the dense (>10<SUP>3</SUP> cm<SUP>-3</SUP>) molecular gas towards the north-east of the W28 supernova remnant (SNR). This cloud is spatially well matched to the TeV gamma-ray source HESS J1801-233 and is known to be an SNR-molecular cloud interaction region. Shock-disruption is evident from broad NH<SUB>3</SUB> (1,1) spectral linewidths in regions towards the W28 SNR, while strong detections of spatially extended NH<SUB>3</SUB> (3,3), NH<SUB>3</SUB>(4,4) and NH<SUB>3</SUB>(6,6) inversion emission towards the cloud strengthen the case for the existence of high temperatures within the cloud. Velocity dispersion measurements and NH<SUB>3</SUB>(n,n)/(1,1) ratio maps, where n = 2, 3, 4 and 6, indicate that the source of disruption is from the side of the cloud nearest to the W28 SNR, suggesting that it is the source of cloud-disruption. Towards part of the cloud, the ratio of ortho to para-NH<SUB>3</SUB> is observed to exceed 2, suggesting gas-phase NH<SUB>3</SUB> enrichment due to NH<SUB>3</SUB> liberation from dust-grain mantles. The measured NH<SUB>3</SUB> abundance with respect to H<SUB>2</SUB> is ∼(1.2 ± 0.5) × 10<SUP>-9</SUP>, which is not high, as might be expected for a hot, dense molecular cloud enriched by sublimated grain-surface molecules. The results are suggestive of NH<SUB>3</SUB> sublimation and destruction in this molecular cloud, which is likely to be interacting with the W28 SNR shock.
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Detection of a 23.6 min periodic modulation in the optical counterpart of 3XMMJ051034.6-670353We present high speed optical photometric observations made using the NTT and ULTRACAM of the optical counterpart of 3XMMJ051034.6-670353, which was recently identified as an X-ray source showing a modulation on a period of 23.6 min. Although the optical counterpart is faint (g = 21.4), we find that the u'g'r' light curves show a periodic modulation on a period which is consistent with the X-ray period. We also obtained three low resolution spectra of 3XMMJ051034.6-670353 using the Gemini South Telescope and GMOS. There is no evidence for strong emission lines in the optical spectrum of 3XMMJ051034.6-670353. We compare and contrast the optical and X-ray observations of 3XMMJ051034.6-670353 with the ultra compact binaries HM Cnc and V407 Vul. We find we can identify a distribution of binary masses in which stable direct impact accretion can occur.
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Swift observations of the 2015 outburst of AG Peg - from slow nova to classical symbiotic outburstSymbiotic stars often contain white dwarfs with quasi-steady shell burning on their surfaces. However, in most symbiotics, the origin of this burning is unclear. In symbiotic slow novae, however, it is linked to a past thermonuclear runaway. In 2015 June, the symbiotic slow nova AG Peg was seen in only its second optical outburst since 1850. This recent outburst was of much shorter duration and lower amplitude than the earlier eruption, and it contained multiple peaks - like outbursts in classical symbiotic stars such as Z And. We report Swift X-ray and UV observations of AG Peg made between 2015 June and 2016 January. The X-ray flux was markedly variable on a time-scale of days, particularly during four days near optical maximum, when the X-rays became bright and soft. This strong X-ray variability continued for another month, after which the X-rays hardened as the optical flux declined. The UV flux was high throughout the outburst, consistent with quasi-steady shell burning on the white dwarf. Given that accretion discs around white dwarfs with shell burning do not generally produce detectable X-rays (due to Compton-cooling of the boundary layer), the X-rays probably originated via shocks in the ejecta. As the X-ray photoelectric absorption did not vary significantly, the X-ray variability may directly link to the properties of the shocked material. AG Peg's transition from a slow symbiotic nova (which drove the 1850 outburst) to a classical symbiotic star suggests that shell burning in at least some symbiotic stars is residual burning from prior novae.
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V729 Sgr: a long period dwarf nova showing negative superhumps during quiescenceWe report K2 observations of the eclipsing cataclysmic variable V729 Sgr which covered nearly 80 d in duration. We find five short outbursts and two long outbursts, one of which shows a clear plateau phase in the rise to maximum brightness. The mean time between successive short outbursts is ∼10 d while the time between the two long outbursts is ∼38 d. The frequency of these outbursts is unprecedented for a cataclysmic variable (CV) above the orbital period gap. We find evidence that the mid-point of the eclipse occurs systematically earlier in outburst than in quiescence. During five of the six quiescent epochs we find evidence for a second photometric period which is roughly 5 per cent shorter than the 4.16 h orbital period which we attribute to negative superhumps. V729 Sgr is therefore one of the longest period CVs to show negative superhumps during quiescence.
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Interstellar gas towards the TeV γ-ray sources HESS J1640-465 and HESS J1641-463We present a detailed analysis of the interstellar medium towards the tera electron volt (TeV) γ-ray sources HESS J1640-465 and HESS J1641-463 using results from the Mopra Southern Galactic Plane CO Survey and from a Mopra 7 mm-wavelength study. The γ-ray sources are positionally coincident with two supernova remnants (SNRs) G338.3-0.0 and G338.5+0.1, respectively. A bright complex of H II regions connect the two SNRs and TeV objects. Observations in the CO(1-0) transition lines reveal substantial amounts of diffuse gas positionally coincident with the γ-ray sources at multiple velocities along the line of sight, while 7 mm observations in CS, SiO, HC<SUB>3</SUB>N and CH<SUB>3</SUB>OH transition lines reveal regions of dense, shocked gas. Archival H I data from the Southern Galactic Plane Survey was used to account for the diffuse atomic gas. Physical parameters of the gas towards the TeV sources were calculated from the data. We find that for a hadronic origin for the γ-ray emission, the cosmic ray enhancement rates are ∼10<SUP>3</SUP> and 10<SUP>2</SUP> times the local solar value for HESS J1640-465 and HESS J1641-463, respectively.
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The Superoutburst Duration versus Orbital Period Relation for AM CVn StarsWe examine the relationship between superoutburst duration t <SUB>dur</SUB> and orbital period P <SUB>orb</SUB> in AM CVn ultra-compact binary systems. We show that the previously determined steep relation derived by Levitan et al. was strongly influenced by the inclusion of upper limits for systems with a relatively long orbital period in their fit. Excluding the upper limit values and including t <SUB>dur</SUB> values for three systems at long P <SUB>orb</SUB> that were not considered previously, then d{log}({t}<SUB>dur</SUB>})/d{log}({P}<SUB>orb</SUB>}) is flat as predicted by Cannizzo & Nelemans.
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Vela X-1 as a laboratory for accretion in high-mass X-ray binariesVela X-1 is an eclipsing high mass X-ray binary (HMXB) consisting of a 283s accreting X-ray pulsar in a close orbit of 8.964 days around the B0.5Ib supergiant HD77581 at a distance of just 2.4 kpc. The system is considered a prototype of wind-accreting HMXB and it has been used as a baseline in different theoretical or modelling studies. <P />We discuss the observational properties of the system and the use of the observational data as laboratory to test recent developments in modelling the accretion process in High-Mass X-ray Binaries \citep[e.g.,][]{Sander:2018,El-Mellah:2018}, which range from detailed descriptions of the wind acceleration to modelling of the structure of the flow of matter close to the neutron star and its variations.
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Mass loss and stellar superwindsMass loss bridges the gap between massive stars and supernovae (SNe) in two major ways: (i) theoretically, it is the amount of mass lost that determines the mass of the star prior to explosion and (ii) observations of the circumstellar material around SNe may teach us the type of progenitor that made the SN. Here, I present the latest models and observations of mass loss from massive stars, both for canonical massive O stars, as well as very massive stars that show Wolf-Rayet type features. <P />This article is part of the themed issue 'Bridging the gap: from massive stars to supernovae'.
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Searching for an interstellar medium association for HESS J1534 - 571The Galactic supernova remnant HESS J1534-571 (also known as G323.7 - 1.0) has a shell-like morphology in TeV gamma-ray emission and is a key object in the study of cosmic ray origin. Little is known about its distance and local environment. We examine Mopra <SUP>12</SUP>CO/<SUP>13</SUP>CO(1-0) data, Australian Telescope Compact Array H I, and Parkes H I data towards HESS J1534-571. We trace molecular clouds in at least five velocity ranges, including clumpy interstellar medium structures near a dip in H I emission at a kinematic velocity consistent with the Scutum-Crux arm at ∼3.5 kpc. This feature may be a cavity blown-out by the progenitor star, a scenario that suggests HESS J1534-571 resulted from a core-collapse event. By employing parametrizations fitted to a sample of supernova remnants of known distance, we find that the radio continuum brightness of HESS J1534-571 is consistent with the ∼3.5 kpc kinematic distance of the Scutum-Crux arm H I dip. Modelling of the supernova evolution suggests a ∼8-24 kyr age for HESS J1534-571 at this distance.
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A Supernova Remnant Counterpart for HESS J1832-085We examine the new Galactic supernova remnant (SNR) candidate, G23.11+0.18, as seen by the Murchison Widefield Array radio telescope. We describe the morphology of the candidate and find a spectral index of -0.63 ± 0.05 in the 70-170 MHz domain. Coincident TeV gamma-ray detection in High Energy Stereoscopic System (HESS) data supports the SNR nature of G23.11+0.18 and suggests that G23.11+0.18 is accelerating particles beyond TeV energies, thus making this object a promising new cosmic-ray hadron source candidate. The remnant cannot be seen in current optical, infrared and X-ray data sets. We do find, however, a dip in CO-traced molecular gas at a line-of-sight velocity of ∼85 km s<SUP>-1</SUP>, suggesting the existence of a G23.11+0.18 progenitor wind-blown bubble. Furthermore, the discovery of molecular gas clumps at a neighboring velocity toward HESS J1832-085 adheres to the notion that a hadronic gamma-ray production mechanism is plausible toward the north of the remnant. Based on these morphological arguments, we propose an interstellar medium association for G23.11+0.18 at a kinematic distance of 4.6 ± 0.8 kpc.
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Wolf-Rayet spin at low metallicity and its implication for black hole formation channelsContext. The spin of Wolf-Rayet (WR) stars at low metallicity (Z) is most relevant for our understanding of gravitational wave sources, such as GW 150914, and of the incidence of long-duration gamma-ray bursts (GRBs). Two scenarios have been suggested for both phenomena: one of them involves rapid rotation and quasi-chemical homogeneous evolution (CHE) and the other invokes classical evolution through mass loss in single and binary systems. <BR /> Aims: The stellar spin of WR stars might enable us to test these two scenarios. In order to obtain empirical constraints on black hole progenitor spin we infer wind asymmetries in all 12 known WR stars in the Small Magellanic Cloud (SMC) at Z = 1 / 5 Z<SUB>⊙</SUB> and within a significantly enlarged sample of single and binary WR stars in the Large Magellanic Cloud (LMC at Z = 1 / 2 Z<SUB>⊙</SUB>), thereby tripling the sample of Vink from 2007. This brings the total LMC sample to 39, making it appropriate for comparison to the Galactic sample. <BR /> Methods: We measured WR wind asymmetries with VLT-FORS linear spectropolarimetry, a tool that is uniquely poised to perform such tasks in extragalactic environments. <BR /> Results: We report the detection of new line effects in the LMC WN star BAT99-43 and the WC star BAT99-70, along with the well-known WR LBV HD 5980 in the SMC, which might be undergoing a chemically homogeneous evolution. With the previous reported line effects in the late-type WNL (Ofpe/WN9) objects BAT99-22 and BAT99-33, this brings the total LMC WR sample to four, I.e. a frequency of 10%. Perhaps surprisingly, the incidence of line effects amongst low Z WR stars is not found to be any higher than amongst the Galactic WR sample, challenging the rotationally induced CHE model. <BR /> Conclusions: As WR mass loss is likely Z-dependent, our Magellanic Cloud line-effect WR stars may maintain their surface rotation and fulfill the basic conditions for producing long GRBs, both via the classical post-red supergiant or luminous blue variable channel, or resulting from CHE due to physics specific to very massive stars.
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Arcminute-scale studies of the interstellar gas towards HESS J1804-216: Still an unidentified TeV γ-ray sourceThe Galactic TeV ${γ}$ -ray source ${HESS J1804{-}216}$ is currently an unidentified source. In an attempt to unveil its origin, we present here the most detailed study of interstellar gas using data from the Mopra Southern Galactic Plane CO Survey, 7- and 12-mm wavelength Mopra surveys and Southern Galactic Plane Survey of HI. Several components of atomic and molecular gas are found to overlap ${HESS J1804{-}216}$ at various velocities along the line of sight. The CS(1-0) emission clumps confirm the presence of dense gas. Both correlation and anti-correlation between the gas and TeV ${γ}$ -ray emission have been identified in various gas tracers, enabling several origin scenarios for the TeV ${γ}$ -ray emission from ${HESS J1804{-}216}$ . For a hadronic scenario, ${SNR G8.7{-}0.1}$ and the progenitor supernova remnant (SNR) of ${PSR J1803{-}2137}$ require cosmic ray (CR) enhancement factors of ${\mathord{∼} 50}$ times the solar neighbour CR flux value to produce the TeV ${γ}$ -ray emission. Assuming an isotropic diffusion model, CRs from both these SNRs require a slow diffusion coefficient, as found for other TeV SNRs associated with adjacent ISM gas. The morphology of gas located at 3.8 kpc (the dispersion measure distance to ${PSR J1803{-}2137}$ ) tends to anti-correlate with features of the TeV emission from ${HESS J1804{-}216}$ , making the leptonic scenario possible. Both pure hadronic and pure leptonic scenarios thus remain plausible.
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TESS observations of the asynchronous polar CD Ind: mapping the changing accretion geometryWe present the results of near continuous TESS optical observations of the asynchronous polar CD Ind (RX J2115-5840). The 27.9 d long light curve, with 2 min resolution, reveals remarkable changes in the magnetic accretion geometry of the system over the 7.3 d beat period. We have modelled the changes in the optical spin period pulse shape using a cyclotron emission mapping technique. The resulting cyclotron emission maps of the magnetic white dwarf reveal how the accretion geometry changes from single- to two-pole accretion and back over the beat cycle. Finally, we present the results from particle-based numerical magnetic accretion simulations, which agree with our interpretation of the changing accretion scenario.
