arXiv subjects

A. Rau

Publications and source records attributed to A. Rau.

At least 109 records · Page 6Linked to original sources

arXiv2014

Discovery of SXP265, a Be/X-ray binary pulsar in the Wing of the Small Magellanic Cloud

We identify a new candidate for a Be/X-ray binary in the XMM-Newton slew survey and archival Swift observations that is located in the transition region of the Wing of the Small Magellanic Cloud and the Magellanic Bridge. We investigated and classified this source with follow-up XMM-Newton and optical observations. We model the X-ray spectra and search for periodicities and variability in the X-ray observations and the OGLE I-band light curve. The optical counterpart has been classified spectroscopically, with data obtained at the SAAO 1.9 m telescope, and photometrically, with data obtained using GROND at the MPG 2.2 m telescope. The X-ray spectrum is typical of a high-mass X-ray binary with an accreting neutron star. We detect X-ray pulsations, which reveal a neutron-star spin period of P = (264.516+-0.014) s. The source likely shows a persistent X-ray luminosity of a few 10^35 erg/s and in addition type-I outbursts that indicate an orbital period of ~146 d. A periodicity of 0.867 d, found in the optical light curve, can be explained by non-radial pulsations of the Be star. We identify the optical counterpart and classify it as a B1-2II-IVe star. This confirms SXP 265 as a new Be/X-ray binary pulsar originating in the tidal structure between the Magellanic Clouds.

astro-ph.HE↗

arXiv2014

GROND coverage of the main peak of Gamma-Ray Burst 130925A

Prompt or early optical emission in gamma-ray bursts is notoriously difficult to measure, and observations of the dozen cases show a large variety of properties. Yet, such early emission promises to help us achieve a better understanding of the GRB emission process(es). We performed dedicated observations of the ultra-long duration (T90 about 7000 s) GRB 130925A in the optical/near-infrared with the 7-channel "Gamma-Ray Burst Optical and Near-infrared Detector" (GROND) at the 2.2m MPG/ESO telescope. We detect an optical/NIR flare with an amplitude of nearly 2 mag which is delayed with respect to the keV--MeV prompt emission by about 300--400 s. The decay time of this flare is shorter than the duration of the flare (500 s) or its delay. While we cannot offer a straightforward explanation, we discuss the implications of the flare properties and suggest ways toward understanding it.

astro-ph.HE↗

arXiv2013

Simultaneous optical/gamma-ray observations of GRB 121217's prompt emission

Since the advent of the Swift satellite it has been possible to obtain precise localisations of GRB positions of sub-arcsec accuracy within seconds, facilitating ground-based robotic telescopes to automatically slew to the target within seconds. This has yielded a plethora of observational data for the afterglow phase of the GRB, but the quantity of data (<2 keV) covering the initial prompt emission still remains small. Only in a handful of cases has it been possible obtain simultaneous coverage of the prompt emission in a multi-wavelength regime (gamma-ray to optical), as a result of: observing the field by chance prior to the GRB (e.g. 080319B/naked-eye burst), long-prompt emission (e.g., 080928, 110205A) or triggered on a pre-cursor (e.g., 041219A, 050820A, 061121). This small selection of bursts have shown both correlated and uncorrelated gamma-ray and optical light curve behaviour, and the multi-wavelength emission mechanism remains far from resolved (i.e. single population synchrotron self-Component, electron distributions, additional neutron components or residual collisions). Such multi-wavelength observations during the GRB prompt phase are pivotal in providing further insight on the poorly understood prompt emission mechanism. We add to this small sample the Swift burst 121217A that had two distinct periods of prompt emission separated by ~700 s, observed by Swift/BAT, Swift/XRT and Fermi/GBM. As a result of the time delay of the second emission, it enabled optical imaging (from 3 to 7 bands) to be taken with the GROND instrument to a resolution as fine as 10s. This multi-wavelength data will hopefully allow us to shed more light on the current picture of prompt emission physics.

astro-ph.HE↗

arXiv2013

Prompt emission of GRB 121217A from gamma-rays to the NIR

The mechanism that causes the prompt-emission episode of gamma-ray bursts (GRBs) is still widely debated despite there being thousands of prompt detections. The favoured internal shock model relates this emission to synchrotron radiation. However, it does not always explain the spectral indices of the shape of the spectrum, often fit with empirical functions. Multi-wavelength observations are therefore required to help investigate the possible underlying mechanisms that causes the prompt emission. We present GRB 121217A, for which we were able to observe its near-infrared (NIR) emission during a secondary prompt-emission episode with the Gamma-Ray Burst Optical Near-infrared Detector (GROND) in combination with the Swift and Fermi satellites, covering an energy range of 0.001 keV to 100 keV. We determine a photometric redshift of z=3.1+/-0.1 with a line-of-sight extinction of A_V~0 mag, utilising the optical/NIR SED. From the afterglow, we determine a bulk Lorentz factor of Gamma~250 and an emission radius of R<10^18 cm. The prompt-emission broadband spectral energy distribution is well fit with a broken power law with b1=-0.3+/-0.1, b2=0.6+/-0.1 that has a break at E=6.6+/-0.9 keV, which can be interpreted as the maximum injection frequency. Self-absorption by the electron population below energies of E_a<6 keV suggest a magnetic field strength of B~10^5 G. However, all the best fit models underpredict the flux observed in the NIR wavelengths, which also only rebrightens by a factor of ~2 during the second prompt emission episode, in stark contrast to the X-ray emission, which rebrightens by a factor of ~100, suggesting an afterglow component is dominating the emission. We present GRB 121217A one of the few GRBs for which there are multi-wavelength observations of the prompt-emission period and show that it can be understood with a synchrotron radiation model.

astro-ph.HE↗

arXiv2013

Time-resolved Analysis of Fermi GRBs with Fast and Slow-Cooled Synchrotron Photon Models

Time-resolved spectroscopy is performed on eight bright, long gamma-ray bursts (GRBs) dominated by single emission pulses that were observed with the {\it Fermi Gamma-ray Space Telescope}. Fitting the prompt radiation of GRBs by empirical spectral forms such as the Band function leads to ambiguous conclusions about the physical model for the prompt radiation. Moreover, the Band function is often inadequate to fit the data. The GRB spectrum is therefore modeled with two emission components consisting of optically thin nonthermal synchrotron radiation from relativistic electrons and, when significant, thermal emission from a jet photosphere, which is represented by a blackbody spectrum. To produce an acceptable fit, the addition of a blackbody component is required in 5 out of the 8 cases. We also find that the low-energy spectral index αis consistent with a synchrotron component with α= -0.81\pm 0.1. This value lies between the limiting values of α= -2/3 and α= -3/2 for electrons in the slow and fast-cooling regimes, respectively, suggesting ongoing acceleration at the emission site. The blackbody component can be more significant when using a physical synchrotron model instead of the Band function, illustrating that the Band function does not serve as a good proxy for a nonthermal synchrotron emission component. The temperature and characteristic emission-region size of the blackbody component are found to, respectively, decrease and increase as power laws with time during the prompt phase. In addition, we find that the blackbody and nonthermal components have separate temporal behaviors.

astro-ph.HE↗

arXiv2013

Afterglow rebrightenings as a signature of a long-lasting central engine activity? The emblematic case of GRB 100814A

In the past few years the number of well-sampled optical to NIR light curves of long Gamma-Ray Bursts (GRBs) has greatly increased particularly due to simultaneous multi-band imagers such as GROND. Combining these densely sampled ground-based data sets with the Swift UVOT and XRT space observations unveils a much more complex afterglow evolution than what was predicted by the most commonly invoked theoretical models. GRB 100814A represents a remarkable example of these interesting well-sampled events, showing a prominent late-time rebrightening in the optical to NIR bands and a complex spectral evolution. This represents a unique laboratory to test the different afterglow emission models. Here we study the nature of the complex afterglow emission of GRB 100814A in the framework of different theoretical models. Moreover, we compare the late-time chromatic rebrightening with those observed in other well-sampled long GRBs. We analysed the optical and NIR observations obtained with the seven-channel Gamma-Ray burst Optical and Near-infrared Detector at the 2.2 m MPG/ESO telescope together with the X-ray and UV data detected by the instruments onboard the Swift observatory. The broad-band afterglow evolution, achieved by constructing multi-instrument light curves and spectral energy distributions, will be discussed in the framework of different theoretical models. We find that the standard models that describe the broad-band afterglow emission within the external shock scenario fail to describe the complex evolution of GRB 100814A, and therefore more complex scenarios must be invoked. [abridged]

astro-ph.HE↗

arXiv2013

The ultraluminous GRB 110918A

GRB 110918A is the brightest long GRB detected by Konus-WIND during its 19 years of continuous observations and the most luminous GRB ever observed since the beginning of the cosmological era in 1997. We report on the final IPN localization of this event and its detailed multiwavelength study with a number of space-based instruments. The prompt emission is characterized by a typical duration, a moderare $E_{peak}$ of the time-integrated spectrum, and strong hard-to-soft evolution. The high observed energy fluence yields, at z=0.984, a huge isotropic-equivalent energy release $E_{iso}=(2.1\pm0.1)\times10^{54}$ erg. The record-breaking energy flux observed at the peak of the short, bright, hard initial pulse results in an unprecedented isotropic-equivalent luminosity $L_{iso}=(4.7\pm0.2)\times10^{54}$erg s$^{-1}$. A tail of the soft gamma-ray emission was detected with temporal and spectral behavior typical of that predicted by the synchrotron forward-shock model. Swift/XRT and Swift/UVOT observed the bright afterglow from 1.2 to 48 days after the burst and revealed no evidence of a jet break. The post-break scenario for the afterglow is preferred from our analysis, with a hard underlying electron spectrum and ISM-like circumburst environment implied. We conclude that, among multiple reasons investigated, the tight collimation of the jet must have been a key ingredient to produce this unusually bright burst. The inferred jet opening angle of 1.7-3.4 deg results in reasonable values of the collimation-corrected radiated energy and the peak luminosity, which, however, are still at the top of their distributions for such tightly collimated events. We estimate a detection horizon for a similar ultraluminous GRB of $z\sim7.5$ for Konus-WIND, and $z\sim12$ for Swift/BAT, which stresses the importance of GRBs as probes of the early Universe.

astro-ph.HE↗

arXiv2013

New constraints on GRB jet geometry and relativistic shock physics

We use high--quality, multi-band observations of Swift GRB120404A, from gamma-ray to radio frequencies, together with the new hydrodynamics code of van Eerten et al. (2012) to test the standard synchrotron shock model. The evolution of the radio and optical afterglow, with its prominent optical rebrightening at t_rest 260-2600 s, is remarkably well modelled by a decelerating jet viewed close to the jet edge, combined with some early re-energization of the shock. We thus constrain the geometry of the jet with half-opening and viewing angles of 23 and 21 deg respectively and suggest that wide jets viewed off-axis are more common in GRBs than previously thought. We also derive the fireball microphysics parameters epsilon_B=2.4e-4 and epsilon_e=9.3e-2 and a circumburst density of n=240 cm^-3. The ability to self-consistently model the microphysics parameters and jet geometry in this way offers an alternative to trying to identify elusive canonical jet breaks at late times. The mismatch between the observed and model-predicted X-ray fluxes is explained by the local rather than the global cooling approximation in the synchrotron radiation model, constraining the microphysics of particle acceleration taking place in a relativistic shock and, in turn, emphasising the need for a more realistic treatment of cooling in future developments of theoretical models. Finally, our interpretation of the optical peak as due to the passage of the forward shock synchrotron frequency highlights the importance of high quality multi-band data to prevent some optical peaks from being erroneously attributed to the onset of fireball deceleration.

astro-ph.HE↗

arXiv2013

The Cosmic Evolution of Fermi BL Lacertae Objects

Fermi has provided the largest sample of gamma-ray selected blazars to date. In this work we use a uniformly selected set of 211 BL Lacertae (BL Lac) objects detected by it Fermi during its first year of operation. We have obtained redshift constraints for 206 out of the 211 BL Lacs in our sample making it the largest and most complete sample of BL Lacs available in the literature. We use this sample to determine the luminosity function of BL Lacs and its evolution with cosmic time. We find that for most BL Lac classes, the evolution is positive with a space density peaking at modest redshift (z~1.2). The low-luminosity, high-synchrotron peaked (HSP) BL Lacs are an exception, showing strong negative evolution, with number density increasing for z$\lesssim$0.5. Since this rise corresponds to a drop-off in the density of flat-spectrum radio quasars (FSRQs), a possible interpretation is that these HSPs represent an accretion-starved end-state of an earlier merger-driven gas-rich phase. We additionally find that the known BL Lac correlation between luminosity and photon spectral index persists after correction for the substantial observational selection effects with implications for the so called `blazar sequence'. Finally, estimating the beaming corrections to the luminosity function, we find that BL Lacs have an average Lorentz factor of $γ=6.1^{+1.1}_{-0.8}$, and that most are seen within 10$^{\circ}$ of the jet axis.

astro-ph.CO↗

arXiv2013

NuSTAR detection of the blazar B2 1023+25 at redshift 5.3

B2 1023+25 is an extremely radio-loud quasar at z=5.3 which was first identified as a likely high-redshift blazar candidate in the SDSS+FIRST quasar catalog. Here we use the Nuclear Spectroscopic Telescope Array (NuSTAR) to investigate its non-thermal jet emission, whose high-energy component we detected in the hard X-ray energy band. The X-ray flux is ~5.5x10^(-14) erg cm^(-2)s^(-1) (5-10keV) and the photon spectral index is Gamma_X=1.3-1.6. Modeling the full spectral energy distribution, we find that the jet is oriented close to the line of sight, with a viewing angle of ~3deg, and has significant Doppler boosting, with a large bulk Lorentz factor ~13, which confirms the identification of B2 1023+25 as a blazar. B2 1023+25 is the first object at redshift larger than 5 detected by NuSTAR, demonstrating the ability of NuSTAR to investigate the early X-ray Universe and to study extremely active supermassive black holes located at very high redshift.

astro-ph.CO↗

arXiv2013

The Hot and Energetic Universe: The Wide Field Imager (WFI) for Athena+

The Wide Field Imager (WFI) is one of the two scientific instruments proposed for the Athena+ X-ray observatory. It will provide imaging in the 0.1-15 keV band over a wide field, simultaneously with spectrally and time-resolved photon counting. The instrument is designed to make optimal use of the grasp (collecting area times solid angle product) provided by the optical design of the Athena+ mirror system (Willingale et al. 2013), by combining a sensitive approx. 40' diameter field of view (baseline; 50' goal) DEPFET detector with a pixel size properly sampling the angular resolution of 5 arc sec on-axis (half energy width).This synthesis makes the WFI a very powerful survey instrument, significantly surpassing currently existing capabilities (Nandra et al. 2013; Aird et al. 2013). In addition, the WFI will provide unprecedented simultaneous high-time resolution and high count rate capabilities for the observation of bright sources with low pile-up and high efficiency. In this paper, we summarize the instrument design, the status of the technology development, and the baseline performance.

astro-ph.IM↗

arXiv2013

The low-extinction afterglow in the solar-metallicity host galaxy of GRB 110918A

Galaxies selected through long gamma-ray bursts (GRBs) could be of fundamental importance when mapping the star formation history out to the highest redshifts. Before using them as efficient tools in the early Universe, however, the environmental factors that govern the formation of GRBs need to be understood. Metallicity is theoretically thought to be a fundamental driver in GRB explosions and energetics, but is still, even after more than a decade of extensive studies, not fully understood. This is largely related to two phenomena: a dust-extinction bias, that prevented high-mass and thus likely high-metallicity GRB hosts to be detected in the first place, and a lack of efficient instrumentation, that limited spectroscopic studies including metallicity measurements to the low-redshift end of the GRB host population. The subject of this work is the very energetic GRB 110918A, for which we measure one of the largest host-integrated metallicities, ever, and the highest stellar mass for z<1.9. This presents one of the very few robust metallicity measurements of GRB hosts at z~1, and establishes that GRB hosts at z~1 can also be very metal rich. It conclusively rules out a metallicity cut-off in GRB host galaxies and argues against an anti-correlation between metallicity and energy release in GRBs.

astro-ph.HE↗

arXiv2013

IPN localizations of Konus short gamma-ray bursts

Between the launch of the \textit{GGS Wind} spacecraft in 1994 November and the end of 2010, the Konus-\textit{Wind} experiment detected 296 short-duration gamma-ray bursts (including 23 bursts which can be classified as short bursts with extended emission). During this period, the IPN consisted of up to eleven spacecraft, and using triangulation, the localizations of 271 bursts were obtained. We present the most comprehensive IPN localization data on these events. The short burst detection rate, $\sim$18 per year, exceeds that of many individual experiments.

astro-ph.HE↗

arXiv2013

The Interplanetary Network Supplement to the Fermi GBM Catalog of Cosmic Gamma-Ray Bursts

We present Interplanetary Network (IPN) data for the gamma-ray bursts in the first Fermi Gamma-Ray Burst Monitor (GBM) catalog. Of the 491 bursts in that catalog, covering 2008 July 12 to 2010 July 11, 427 were observed by at least one other instrument in the 9-spacecraft IPN. Of the 427, the localizations of 149 could be improved by arrival time analysis (or triangulation). For any given burst observed by the GBM and one other distant spacecraft, triangulation gives an annulus of possible arrival directions whose half-width varies between about 0.4' and 32 degrees, depending on the intensity, time history, and arrival direction of the burst, as well as the distance between the spacecraft. We find that the IPN localizations intersect the 1 sigma GBM error circles in only 52% of the cases, if no systematic uncertainty is assumed for the latter. If a 6 degree systematic uncertainty is assumed and added in quadrature, the two localization samples agree about 87% of the time, as would be expected. If we then multiply the resulting error radii by a factor of 3, the two samples agree in slightly over 98% of the cases, providing a good estimate of the GBM 3 sigma error radius. The IPN 3 sigma error boxes have areas between about 1 square arcminute and 110 square degrees, and are, on the average, a factor of 180 smaller than the corresponding GBM localizations. We identify two bursts in the IPN/GBM sample that did not appear in the GBM catalog. In one case, the GBM triggered on a terrestrial gamma flash, and in the other, its origin was given as uncertain. We also discuss the sensitivity and calibration of the IPN.

astro-ph.HE↗

arXiv2013

The low-extinction afterglow in the solar-metallicity host galaxy of gamma-ray burst 110918A

Metallicity is theoretically thought to be a fundamental driver in gamma-ray burst (GRB) explosions and energetics, but is still, even after more than a decade of extensive studies, not fully understood. This is largely related to two phenomena: a dust-extinction bias, that prevented high-mass and thus likely high-metallicity GRB hosts to be detected in the first place, and a lack of efficient instrumentation, that limited spectroscopic studies including metallicity measurements to the low-redshift end of the GRB host population. The subject of this work is the very energetic GRB 110918A, for which we measure a redshift of z=0.984. GRB 110918A gave rise to a luminous afterglow with an intrinsic spectral slope of b=0.70, which probed a sight-line with little extinction (A_V=0.16 mag) typical of the established distributions of afterglow properties. Photometric and spectroscopic follow-up observations of the galaxy hosting GRB 110918A, including optical/NIR photometry with GROND and spectroscopy with VLT/X-shooter, however, reveal an all but average GRB host in comparison to the z~1 galaxies selected through similar afterglows to date. It has a large spatial extent with a half-light radius of ~10 kpc, the highest stellar mass for z<1.9 (log(M_*/M_sol) = 10.68+-0.16), and an Halpha-based star formation rate of 41 M_sol/yr. We measure a gas-phase extinction of ~1.8 mag through the Balmer decrement and one of the largest host-integrated metallicities ever of around solar (12 + log(O/H) = 8.93+/-0.13). This presents one of the very few robust metallicity measurements of GRB hosts at z~1, and establishes that GRB hosts at z~1 can also be very metal rich. It conclusively rules out a metallicity cut-off in GRB host galaxies and argues against an anti-correlation between metallicity and energy release in GRBs.

astro-ph.HE↗

arXiv2013

Blazar candidates beyond redshift 4 observed with GROND

The search for extremely massive high redshift blazars is essential to put strong constraints on the supermassive black holes formation theories. Up to now, the few blazars known to have a redshift larger than 4 have been discovered serendipitously. We try a more systematic approach. Assuming radio-loudness as a proxy for the jet orientation, we select a sample of extremely radio-loud quasars. We measure their black hole masses with a method based on fitting the thermal emission from the accretion disc. We achieve a precision of a factor of two for our measures, thanks to the observations performed with the Gamma-Ray Burst Optical Near-Infrared Detector (GROND). The infrared to optical GROND data allow us to observe directly the peak of the disc emission, thus constraining the overall disc luminosity. We obtain a small range of masses, that peaks at 10^{9.3}Msun. If some of our candidates will be confirmed as blazars, these results would introduce interesting constraints on the mass function of extremely massive black holes at very high redshift. Moreover, all our blazar candidates have high accretion rates. This result, along with the high masses, opens an interesting view on the need of a fast growth of the heaviest black holes at very high redshift.

astro-ph.CO↗

arXiv2013

The unusual afterglow of the Gamma-Ray Burst 100621A

In order to constrain the broad-band spectral energy distribution of the afterglow of GRB 100621A, dedicated observations were performed in the optical/near-infrared with the 7-channel "Gamma-Ray Burst Optical and Near-infrared Detector" (GROND) at the 2.2m MPG/ESO telescope, in the sub-millimeter band with the large bolometer array LABOCA at APEX, and at radio frequencies with ATCA. Utilizing also Swift X-ray observations, we attempt an interpretation of the observational data within the fireball scenario. The afterglow of GRB 100621A shows a very complex temporal as well as spectral evolution. We identify three different emission components, the most spectacular one causing a sudden intensity jump about one hour after the prompt emission. The spectrum of this component is much steeper than the canonical afterglow. We interpret this component using the prescription of Vlasis et al. (2011) for a two-shell collision after the first shell has been decelerated by the circumburst medium. We use the fireball scenario to derive constraints on the microphysical parameters of the first shell. Long-term energy injection into a narrow jet seems to provide an adequate description. Another noteworthy result is the large ($A_V$ = 3.6 mag) line-of-sight host extinction of the afterglow in an otherwise extremely blue host galaxy.

astro-ph.HE↗

arXiv2013

Discovery of a 168.8 s X-ray pulsar transiting in front of its Be companion star in the Large Magellanic Cloud

We report the discovery of LXP169, a new high-mass X-ray binary (XRB) in the LMC. The optical counterpart has been identified and appears to exhibit an eclipsing light curve. We performed follow-up observations to clarify the eclipsing nature of the system. Energy spectra and time series were extracted from two XMM-Newton observations to search for pulsations, characterise the spectrum, and measure spectral and timing changes. Long-term X-ray variability was studied using archival ROSAT data. The XMM positions were used to identify the optical counterpart. We obtained UV to NIR photometry to characterise the companion, along with its 4000 d long I-band light curve. We observed LXP169 with Swift at two predicted eclipse times. We found a spin period of 168.8 s that did not change between two XMM observations. The X-ray spectrum, well characterised by a power law, was harder when the source was brighter. The X-ray flux of LXP169 is found to be variable by a factor of at least 10. The counterpart is highly variable on short and long timescales, and its photometry is that of an early-type star with a NIR excess. This classifies the source as a BeXRB pulsar. We observed a transit in the UV, thereby confirming that the companion star itself is eclipsed. We give an ephemeris for the transit of MJD 56203.877 + N*24.329. We propose and discuss the scenario where the matter captured from the companion's equatorial disc creates an extended region of high density around the neutron star (NS), which partially eclipses the companion as the NS transits in front of it. This is most likely the first time the compact object in an XRB is observed to eclipse its companion star. LXP169 would be the first eclipsing BeXRB, and a wealth of important information might be gained from additional observations, such as a measure of the possible Be disc/orbital plane misalignment, or the mass of the NS.

astro-ph.HE↗