arXiv subjects

R. M. Wagner

Publications and source records attributed to R. M. Wagner.

At least 37 records · Page 2Linked to original sources

arXiv2014

Discovery and Observations of ASASSN-13db, an EX Lupi-Type Accretion Event on a Low-Mass T Tauri Star

We discuss ASASSN-13db, an EX Lupi-type ("EXor") accretion event on the young stellar object (YSO) SDSS J051011.01$-$032826.2 (hereafter SDSSJ0510) discovered by the All-Sky Automated Survey for SuperNovae (ASAS-SN). Using archival photometric data of SDSSJ0510 we construct a pre-outburst spectral energy distribution (SED) and find that it is consistent with a low-mass class II YSO near the Orion star forming region ($d \sim 420$ pc). We present follow-up photometric and spectroscopic observations of the source after the $ΔV \sim-$5.4 magnitude outburst that began in September 2013 and ended in early 2014. These data indicate an increase in temperature and luminosity consistent with an accretion rate of $\sim10^{-7}$ $\rm{M}_\odot$ yr$^{-1}$, three or more orders of magnitude greater than in quiescence. Spectroscopic observations show a forest of narrow emission lines dominated by neutral metallic lines from Fe I and some low-ionization lines. The properties of ASASSN-13db are similar to those of the EXor prototype EX Lupi during its strongest observed outburst in late 2008.

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arXiv2013

The 7.1 hour X-ray-UV-NIR period of the gamma-ray classical Nova Monocerotis 2012

Nova Mon 2012 is the third gamma-ray transient identified with a thermonuclear runaway on a white dwarf, that is, a nova event. Swift monitoring has revealed the distinct evolution of the harder and super-soft X-ray spectral components, while Swift-UV and V and I-band photometry show a gradual decline with subtle changes of slope. During the super-soft emission phase, a coherent 7.1 hr modulation was found in the soft X-ray, UV, optical and near-IR data, varying in phase across all wavebands. Assuming this period to be orbital, the system has a near-main sequence secondary, with little appreciable stellar wind. This distinguishes it from the first GeV nova, V407 Cyg, where the gamma-rays were proposed to form through shock-accelerated particles as the ejecta interacted with the red giant wind. We favor a model in which the gamma-rays arise from the shock of the ejecta with material close to the white dwarf in the orbital plane. This suggests that classical novae may commonly be GeV sources. We ascribe the orbital modulation to a raised section of an accretion disk passing through the line of sight, periodically blocking and reflecting much of the emission. The disk must, therefore, have reformed by day 150 after outburst.

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arXiv2011

Solid-phase C_60 in the peculiar binary XX Oph?

We present infrared spectra of the binary XX Oph obtained with the Infrared Spectrograph on the Spitzer Space Telescope. The data show some evidence for the presence of solid C_60 - the first detection of C_60 in the solid phase - together with the well-known ``Unidentified Infrared'' emission features. We suggest that, in the case of XX Oph, the C_60 is located close to the hot component, and that in general it is preferentially excited by stars having effective temperatures in the range 15,000-30,000K. C_60 may be common in circumstellar environments, but un-noticed in the absence of a suitable exciting source.

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arXiv2010

Multiple Major Outbursts from a Restless Luminous Blue Variable in NGC 3432

We present new photometric and spectroscopic observations of an unusual luminous blue variable (LBV) in NGC 3432, covering three major outbursts in October 2008, April 2009 and November 2009. Previously, this star experienced an outburst also in 2000 (known as SN 2000ch). During outbursts the star reached an absolute magnitude between -12.1 and -12.8. Its spectrum showed H, He I and Fe II lines with P-Cygni profiles during and soon after the eruptive phases, while only intermediate-width lines in pure emission (including He II 4686A were visible during quiescence. The fast-evolving light curve soon after the outbursts, the quasi-modulated light curve, the peak magnitude and the overall spectral properties are consistent with multiple episodes of variability of an extremely active LBV. However, the widths of the spectral lines indicate unusually high wind velocities (1500-2800 km/s), similar to those observed in Wolf-Rayet stars. Although modulated light curves are typical of LBVs during the S-Dor variability phase, the luminous maxima and the high frequency of outbursts are unexpected in S-Dor variables. Such extreme variability may be associated with repeated ejection episodes during a giant eruption of an LBV. Alternatively, it may be indicative of a high level of instability shortly preceding the core-collapse or due to interaction with a massive, binary companion. In this context, the variable in NGC 3432 shares some similarities with the famous stellar system HD 5980 in the Small Magellanic Cloud, which includes an erupting LBV and an early Wolf-Rayet star.

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arXiv2010

The peculiar dust shell of Nova DZ Cru (2003)

We present Spitzer Space Telescope observations of the "peculiar variable" DZ Cru, identified by Rushton et al. (2008, MNRAS, 386, 289) as a classical nova. A dust shell, on which are superimposed a number of features, is prominent in the 5-35micron range some 4 years after eruption. We suggest that the dust in DZ Cru is primarily hydrogenated amorphous carbon in which aliphatic bands currently predominate, and which may either become predominantly aromatic as the dust is photo-processed by ultraviolet radiation from the stellar remnant, or more likely completely destroyed.

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arXiv2009

Pinpointing the TeV gamma-ray emission region in M87 using TeV and 43 GHz radio monitoring

The TeV radio galaxy M87 is the first radio galaxy detected in the TeV regime. The structure of its jet, which is not pointing towards our line of sight, is spatially resolved in X-ray (by Chandra), optical and radio observations. In 2008, the three main Atmospheric Cherenkov Telescope observatories VERITAS, MAGIC and H.E.S.S. coordinated their observations in a joint campaign from January to May with a total observation time of approx. 120 hours. In February, strong and rapid day-scale TeV flares were detected. VLBA monitoring observations during the same period showed that the 43 GHz radio flux density of the unresolved core began to rise at the time of the TeV flares and eventually reached levels above any previously seen with VLBI. New jet components appeared during the flare. The localization accuracy of the TeV instruments of many arcseconds, even for strong sources, is inadequate to constrain the origin of the emission in the inner jets of AGNs. For M87, with a 6 billion solar mass black hole and a distance of 16.7 Mpc, the VLBA resolution instead corresponds to 30 by 60 Schwarzschild radii. This is starting to resolve the jet collimation region. The temporal coincidence of the TeV and radio flares indicates that they are related and provides the first direct evidence that the TeV radiation from this source is produced within a few tens of RS of the radio core, thought to be coincident to within the VLBA resolution with the black hole.

astro-ph.HE↗

arXiv2009

The CTA Observatory

In recent years, ground-based very-high-energy (VHE; E>100 GeV) gamma-ray astronomy has experienced a major breakthrough with the impressive astrophysical results obtained mainly by the current generation experiments like H.E.S.S., MAGIC, MILAGRO and VERITAS. The ground-based Imaging Air Cherenkov Technique for detecting VHE gamma-rays has matured, and a fast assembly of inexpensive and robust telescopes is possible. The goal for the next generation of instruments is to increase their sensitivity by a factor >10 compared to current facilities, to extend the accessible gamma-ray energies from a few tens of GeV to a hundred TeV, and to improve on other parameters like the energy and angular resolution (improve the point-spread function by a factor 4-5 w.r.t. current instruments). The Cherenkov Telescope Array (CTA) project is an initiative to build the next generation ground-based gamma-ray instrument, will serve as an observatory to a wide astrophysics community. I discuss the key physics goals and resulting design considerations for CTA, the envisaged technical solutions chosen, and the organizational and operational requirements for operating such a large-scale facility as well as the specific needs of VHE gamma-ray astronomy.

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arXiv2009

A first joint M87 campaign in 2008 from radio to TeV gamma-rays

M87, the central galaxy of the Virgo cluster, is the first radio galaxy detected in the TeV regime. The structure of its jet, which is not pointing toward the line of sight, is spatially resolved in X-ray (by Chandra), in optical and in radio observations. Time correlation between the TeV flux and emission at other wavelengths provides a unique opportunity to localize the very high energy gamma-ray emission process occurring in AGN. For 10 years, M87 has been monitored in the TeV band by atmospheric Cherenkov telescopes. In 2008, the three main atmospheric Cherenkov telescope observatories (H.E.S.S., MAGIC and VERITAS) coordinated their observations in a joint campaign from January to May with a total observation time of approx. 120 hours. The campaign largely overlapped with an intensive VLBA project monitoring the core of M87 at 43 GHz every 5 days. In February, high TeV activities with rapid flares have been detected. Contemporaneously, M87 was observed with high spatial resolution instruments in X-rays (Chandra). We discuss the results of the joint observation campaign in 2008.

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arXiv2009

Monitoring of bright blazars with MAGIC telescope

Blazars, a class of Active Galactic Nuclei (AGN) characterized by a close orientation of their relativistic outflows (jets) towards the line of sight, are a well established extragalactic TeV $γ$-ray emitters. Since 2006, three nearby and TeV bright blazars, Markarian (Mrk) 421, Mrk 501 and 1ES 1959+650, are regularly observed by the MAGIC telescope with single exposures of 30 to 60 minutes. The sensitivity of MAGIC allows to establish a flux level of 30% of the Crab flux for each such observation. In a case of Mrk 421 strong flux variability in different time scales and a high correlation between X-ray/TeV emissions have been observed. In addition, preliminary results on measured light curves from Mrk 501 and 1ES1959+650 in 2007/8 are shown.

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arXiv2009

MWL observations of VHE blazars in 2006

In 2006 the MAGIC telescope observed the well known very high energy (VHE, > 80 GeV) blazars Mrk 421 and Mrk 501 in the course of multi-wavelength campaigns, comprising measurements in the optical, X-ray and VHE regime. MAGIC performed additional snapshot observations on Mrk 421 around the MWL campaigns and detected the source each night with high significance, establishing once more flux variability on nightly scales for this object. For certain nights, the integral flux exceeded the one of Crab significantly, whereas the truly simultaneous observations have been conducted in a rather low flux state. The MAGIC observations contemporaneous to XMM-Newton revealed clear intra-night variability. No significant correlation between the spectral index and the flux could be found for the nine days of observations. The VHE observations of Mrk 501 have been conducted during one of the lowest flux states ever measured by MAGIC for this object. The VHE and optical light curves do not show significant variability, whereas the flux in X-rays increased by about 50 %. In this contribution, the results of the MAGIC observations will be presented in detail.

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arXiv2009

The strong flaring activity of M87 in early 2008 as observed by the MAGIC telescope

M87 is the first known radio galaxy to emit very high energy (VHE) gamma-rays. During a monitoring program of M87, a rapid flare in VHE gamma-rays was detected by the MAGIC telescope in early 2008. The flux was found to be variable on a timescale as short as 1 day, reaching 15% of the Crab Nebula flux above 350 GeV. In contrast, the flux at lower energies (150 GeV to 350 GeV) is compatible with being constant. We present light curves and energy spectra, and argue that the observed day-scale flux variability favours the M87 core as source of the gamma-ray emission rather than the bright know HST-1 in the jet of M87.

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arXiv2009

The June 2008 flare of Markarian 421 from optical to TeV energies

We present optical to very-high energy (VHE) gamma-ray observations of Mrk 421 between 2008 May 24 and June 23. A high-energy (HE) gamma-ray signal was detected by AGILE-GRID during June 9-15, brighter than the average flux observed by EGRET in Mrk 421 by a factor of approx. 1.5. In 20-60 keV X-rays, a large-amplitude 5-day flare (June 9-15) was resolved with a maximum flux of approx. 55 mCrab. SuperAGILE, RXTE/ASM and Swift/BAT data show a clearly correlated flaring structure between soft and hard X-rays, with a high flux/amplitude variability in hard X-rays. Hints of the same flaring behavior is also detected in the simultaneously recorded GASP-WEBT optical data. A target of opportunity observation by Swift near the flare maximum on June 12-13 revealed the highest 2-10 keV flux ever observed (>100 mCrab) and a peak synchrotron energy of approx. 3 keV, a large shift from typical values of 0.5-1 keV. Observations at VHE (E>200 GeV) gamma-rays during June 6-8 show the source flux peaking in a bright state, well correlated with the simultaneous peak in the X-rays. The gamma-ray flare can be interpreted within the framework of the Synchrotron Self Compton model in terms of a rapid acceleration of leptons in the jet.

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arXiv2009

Wide and deep near-UV (360nm) galaxy counts and the extragalactic background light with the Large Binocular Camera

Deep multicolour surveys are the main tool to explore the formation and evolution of the faint galaxies which are beyond the spectroscopic limit with the present technology. The photometric properties of these faint galaxies are usually compared with current renditions of semianalytical models to provide constraints on the fundamental physical processes involved in galaxy formation and evolution, namely the mass assembly and the star formation. Galaxy counts over large sky areas in the near-UV band are important because they are difficult to obtain given the low efficiency of near-UV instrumentation, even at 8m class telescopes. A large instrumental field of view helps in minimizing the biases due to the cosmic variance. We have obtained deep images in the 360nm U band provided by the blue channel of the Large Binocular Camera at the prime focus of the Large Binocular Telescope. We have derived over an area of ~0.4 sq. deg. the galaxy number counts down to U=27 in the Vega system (corresponding to U=27.86 in the AB system) at a completeness level of 30% reaching the faintest current limit for this wavelength and sky area. The shape of the galaxy counts in the U band can be described by a double power-law, the bright side being consistent with the shape of shallower surveys of comparable or greater areas. The slope bends over significantly at U>23.5 ensuring the convergence of the contribution by star forming galaxies to the EBL in the near-UV band to a value which is more than 70% of the most recent upper limits derived for this band. We have jointly compared our near-UV and K band counts collected from the literature with few selected hierarchical CDM models emphasizing critical issues in the physical description of the galaxy formation and evolution.

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arXiv2009

Go Long, Go Deep: Finding Optical Jet Breaks for Swift-Era GRBs with the LBT

Using the 8.4m Large Binocular Telescope, we observed six GRB afterglows from 2.8 hours to 30.8 days after the burst triggers to systematically probe the late time behaviors of afterglows including jet breaks, flares, and supernova bumps. We detected five afterglows with Sloan r' magnitudes ranging from 23.0-26.3 mag. The depth of our observations allows us to extend the temporal baseline for measuring jet breaks by another decade in time scale. We detected two jet breaks and a third candidate, all of which are not detectable without deep, late time optical observations. In the other three cases, we do not detect the jet breaks either because of contamination from the host galaxy light, the presence of a supernova bump, or the intrinsic faintness of the optical afterglow. This suggests that the basic picture that GRBs are collimated is still valid and that the apparent lack of Swift jet breaks is due to poorly sampled afterglow light curves, particularly at late times.

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arXiv2008

The drive system of the Major Atmospheric Gamma-ray Imaging Cherenkov Telescope

The MAGIC telescope is an imaging atmospheric Cherenkov telescope, designed to observe very high energy gamma-rays while achieving a low energy threshold. One of the key science goals is fast follow-up of the enigmatic and short lived gamma-ray bursts. The drive system for the telescope has to meet two basic demands: (1) During normal observations, the 72-ton telescope has to be positioned accurately, and has to track a given sky position with high precision at a typical rotational speed in the order of one revolution per day. (2) For successfully observing GRB prompt emission and afterglows, it has to be powerful enough to position to an arbitrary point on the sky within a few ten seconds and commence normal tracking immediately thereafter. To meet these requirements, the implementation and realization of the drive system relies strongly on standard industry components to ensure robustness and reliability. In this paper, we describe the mechanical setup, the drive control and the calibration of the pointing, as well as present measurements of the accuracy of the system. We show that the drive system is mechanically able to operate the motors with an accuracy even better than the feedback values from the axes. In the context of future projects, envisaging telescope arrays comprising about 100 individual instruments, the robustness and scalability of the concept is emphasized.

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arXiv2008

A model for delayed emission in a very-high energy gamma-ray flare in Markarian 501

Recently, the MAGIC collaboration reported evidence for a delay in the arrival times of photons of different energies during a gamma-ray flare from the blazar Markarian 501 on 2005 July 9. We apply a homogeneous synchrotron self-Compton (SSC) model under the assumption that the blob containing relativistic electrons was observed in its acceleration phase. This modified SSC model predicts the appearance of a gamma-ray flare first at lower energies and subsequently at higher energies. Based on the reported time delay of approx. 240 s between the flare observed at 190 GeV and 2.7 TeV, we predict a delay on the order of 1 h if observed between 10 GeV and 100 GeV. Such delay timescales can be tested in the future by simultaneous flare observations with the Gamma Ray Large Area Space Telescope (GLAST) and Cherenkov telescopes.

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arXiv2008

The performance of the blue prime focus Large Binocular Camera at the Large Binocular Telescope

We present the characteristics and some early scientific results of the first instrument at the Large Binocular Telescope (LBT), the Large Binocular Camera (LBC). Each LBT telescope unit will be equipped with similar prime focus cameras. The blue channel is optimized for imaging in the UV-B bands and the red channel for imaging in the VRIz bands. The corrected field-of-view of each camera is approximately 30 arcminutes in diameter, and the chip area is equivalent to a 23x23 arcmin2 field. In this paper we also present the commissioning results of the blue channel. The scientific and technical performance of the blue channel was assessed by measurement of the astrometric distortion, flat fielding, ghosts, and photometric calibrations. These measurements were then used as input to a data reduction pipeline applied to science commissioning data. The measurements completed during commissioning show that the technical performance of the blue channel is in agreement with original expectations. Since the red camera is very similar to the blue one we expect similar performance from the commissioning that will be performed in the following months in binocular configuration. Using deep UV image, acquired during the commissioning of the blue camera, we derived faint UV galaxy-counts in a ~500 sq. arcmin. sky area to U(Vega)=26.5. These galaxy counts imply that the blue camera is the most powerful UV imager presently available and in the near future in terms of depth and extent of the field-of-view. We emphasize the potential of the blue camera to increase the robustness of the UGR multicolour selection of Lyman break galaxies at redshift z~3.

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arXiv2008

V723 Cassiopeia still on in X-rays: A bright Super Soft Source 12 years after outburst

We find that the classical nova V723 Cas (1995) is still an active X-ray source more than 12 years after outburst and analyze seven X-ray observations carried out with Swift between 2006 January 31 and 2007 December 3. The average count rate is 0.022+/-0.01 cts s^-1 but the source is variable within a factor of two of the mean and does not show any signs of turning off. We present supporting optical observations which show that between 2001 and 2006 an underlying hot source was present with steadily increasing temperature. In order to confirm that the X-ray emission is from V723 Cas, we extract a ROSAT observation taken in 1990 and find that there was no X-ray source at the position of the nova. The Swift XRT spectra resemble those of the Super Soft X-ray binary Sources (SSS) which is confirmed by RXTE survey data which show no X-ray emission above 2 keV between 1996 and 2007. Using blackbody fits we constrain the effective temperature to between T_eff=(2.8-3.8)x10^5 K and a bolometric luminosity >5x10^36 erg s^-1 and caution that luminosities from blackbodies are generally overestimated and temperatures underestimated. We discuss a number of possible explanations for the continuing X-ray activity, including the intriguing possibility of steady hydrogen burning due to renewed accretion.

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