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Electromagnetic ion cyclotron (EMIC) waves play an important role in the dynamics of ultrarelativistic electron population in the radiation belts. However, as EMIC waves are very sporadic, developing a parameterization of such wave properties is a challenging task. Currently, there are no dynamic, activity-dependent models of EMIC waves that can be used in the long-term (several months) simulations, which makes the quantitative modeling of the radiation belt dynamics incomplete. In this study, we investigate Kp, Dst, and AE indices, solar wind speed, and dynamic pressure as possible parameters of EMIC wave presence. The EMIC waves are included in the long-term simulations (1year, including different geomagnetic activity) performed with the Versatile Electron Radiation Belt code, and we compare results of the simulation with the Van Allen Probes observations. The comparison shows that modeling with EMIC waves, parameterized by solar wind dynamic pressure, provides a better agreement with the observations among considered parameterizations. The simulation with EMIC waves improves the dynamics of ultrarelativistic fluxes and reproduces the formation of the local minimum in the phase space density profiles.
Up until recently, signatures of the ultrarelativistic electron loss driven by electromagnetic ion cyclotron (EMIC) waves in the Earth's outer radiation belt have been limited to direct or indirect measurements of electron precipitation or the narrowing of normalized pitch angle distributions in the heart of the belt. In this study, we demonstrate additional observational evidence of ultrarelativistic electron loss that can be driven by resonant interaction with EMIC waves. We analyzed the profiles derived from Van Allen Probe particle data as a function of time and three adiabatic invariants between 9 October and 29 November 2012. New local minimums in the profiles are accompanied by the narrowing of normalized pitch angle distributions and ground‐based detection of EMIC waves. Such a correlation may be indicative of ultrarelativistic electron precipitation into the Earth's atmosphere caused by resonance with EMIC waves.
gamma Cassiopeia (gamma Cas), is known to be a binary system consisting of a Be-type star and a low-mass (M similar to 1M(circle dot)) companion of unknown nature orbiting in the Be-disc plane. Here, we apply the quasi-spherical accretion theory on to a compact magnetized star and show that if the low-mass companion of gamma Cas is a fast spinning neutron star, the key observational signatures of. Cas are remarkably well reproduced. Direct accretion on to this fast rotating neutron star is impeded by the propeller mechanism. In this case, around the neutron star magnetosphere a hot shell is formed which emits thermal X-rays in qualitative and quantitative agreement with observed properties of the X-ray emission from gamma Cas. We suggest that gamma Cas and its analogues constitute a new subclass of Be-type X-ray binaries hosting rapidly rotating neutron stars formed in supernova explosions with small kicks. The subsequent evolutionary stage of gamma Cas and its analogues should be the X Per-type binaries comprising low-luminosity slowly rotating X-ray pulsars. The model explains the enigmatic X-ray emission from gamma Cas, and also establishes evolutionary connections between various types of rotating magnetized neutron stars in Be-binaries.
Context. Massive stars severely influence their environment by their strong ionizing radiation and by the momentum and kinetic energy input provided by their stellar winds and supernovae. Quantitative analyses of massive stars are required to understand how their feedback creates and shapes large scale structures of the interstellar medium. The giant H II region N206 in the Large Magellanic Cloud contains an OB association that powers a superbubble filled with hot X-ray emitting gas, serving as an ideal laboratory in this context. Aims. We aim to estimate stellar and wind parameters of all OB stars in N206 by means of quantitative spectroscopic analyses. In this first paper, we focus on the nine Of-type stars located in this region. We determine their ionizing flux and wind mechanical energy. The analysis of nitrogen abundances in our sample probes rotational mixing. Methods. We obtained optical spectra with the multi-object spectrograph FLAMES at the ESO-VLT. When possible, the optical spectroscopy was complemented by UV spectra from the HST, IUE, and FUSE archives. Detailed spectral classifications are presented for our sample Of-type stars. For the quantitative spectroscopic analysis we used the Potsdam Wolf-Rayet model atmosphere code. We determined the physical parameters and nitrogen abundances of our sample stars by fitting synthetic spectra to the observations. Results. The stellar and wind parameters of nine Of-type stars, which are largely derived from spectral analysis are used to construct wind momentum luminosity relationship. We find that our sample follows a relation close to the theoretical prediction, assuming clumped winds. The most massive star in the N206 association is an Of supergiant that has a very high mass-loss rate. Two objects in our sample reveal composite spectra, showing that the Of primaries have companions of late O subtype. All stars in our sample have an evolutionary age of less than 4 million yr, with the O2-type star being the youngest. All these stars show a systematic discrepancy between evolutionary and spectroscopic masses. All stars in our sample are nitrogen enriched. Nitrogen enrichment shows a clear correlation with increasing projected rotational velocities. Conclusions. The mechanical energy input from the Of stars alone is comparable to the energy stored in the N206 superbubble as measured from the observed X-ray and H alpha emission.
We present the first SB2 orbital solution and disentanglement of the massive Wolf-Rayet binary R145 (P = 159 d) located in the Large Magellanic Cloud. The primary was claimed to have a stellar mass greater than 300 M-circle dot, making it a candidate for being the most massive star known to date. While the primary is a known late-type, H-rich Wolf-Rayet star (WN6h), the secondary has so far not been unambiguously detected. Using moderate-resolution spectra, we are able to derive accurate radial velocities for both components. By performing simultaneous orbital and polarimetric analyses, we derive the complete set of orbital parameters, including the inclination. The spectra are disentangled and spectroscopically analyzed, and an analysis of the wind-wind collision zone is conducted. The disentangled spectra and our models are consistent with a WN6h type for the primary and suggest that the secondary is an O3.5 If*/WN7 type star. We derive a high eccentricity of e = 0 : 78 and minimum masses of M-1 sin(3) i approximate to M-2 sin(3) i = 13 +/- 2 M-circle dot, with q = M-2/M-1 = 1.01 +/- 0.07. An analysis of emission excess stemming from a wind-wind collision yields an inclination similar to that obtained from polarimetry (i = 39 +/- 6 degrees). Our analysis thus implies M-1 = 53(-20)(+40) and M2 = 54(-20)(+40) M-circle dot, excluding M-1 > 300 M-circle dot. A detailed comparison with evolution tracks calculated for single and binary stars together with the high eccentricity suggests that the components of the system underwent quasi-homogeneous evolution and avoided mass-transfer. This scenario would suggest current masses of approximate to 80 M-circle dot and initial masses of M-i,M-1 approximate to 10(5) and M-i,M-2 approximate to 90 M-circle dot, consistent with the upper limits of our derived orbital masses, and would imply an age of approximate to 2.2 Myr.
Mass-loss rates of massive, late type main sequence stars are much weaker than currently predicted, but their true values are very difficult to measure. We suggest that confined stellar winds of magnetic stars can be exploited to constrain the true mass-loss rates M of massive main sequence stars. We acquired UV, X-ray, and optical amateur data of HD 54879 (09.7 V), one of a few O-type stars with a detected atmospheric magnetic field (B-d greater than or similar to 2 kG). We analyze these data with the Potsdam Wolf-Rayet (PoWR) and XSPEC codes. We can roughly estimate the mass-loss rate the star would have in the absence of a magnetic field as log M-B=0 approximate to -9.0 M-circle dot yr(-1). Since the wind is partially trapped within the Alfven radius rA greater than or similar to 12 R-*,, the true mass-loss rate of HD 54879 is log M less than or similar to -10.2 M-circle dot yr(-1). Moreover, we find that the microturbulent, macroturbulent, and projected rotational velocities are lower than previously suggested (< 4 km s(-1)). An initial mass of 16 M-circle dot and an age of 5 Myr are inferred. We derive a mean X-ray emitting temperature of log T-x = 6.7 K and an X-ray luminosity of log L-x = 32 erg s(-1). The latter implies a significant X-ray excess (log L-x/L-Bol approximate to - 6.0), most likely stemming from collisions at the magnetic equator. A tentative period of P approximate to 5 yr is derived from variability of the Ha line. Our study confirms that strongly magnetized stars lose little or no mass, and supplies important constraints on the weak-wind problem of massive main sequence stars.
The blue hypergiant Cyg OB2 12 (B3Ia(+)) is a representative member of the class of very massive stars in a poorly understood evolutionary stage. We obtained its high-resolution X-ray spectrum using the Chandra observatory. PoWR model atmospheres were calculated to provide realistic wind opacities and to establish the wind density structure. We find that collisional de-excitation is the dominant mechanism depopulating the metastable upper levels of the forbidden lines of the He-like ions Si XIV and Mg XII. Comparison between the model and observations reveals that X-ray emission is produced in a dense plasma, which could reside only at the photosphere or in a colliding wind zone between binary components. The observed X-ray spectra are well-fitted by thermal plasma models, with average temperatures in excess of 10 MK. The wind speed in Cyg OB2 12 is not high enough to power such high temperatures, but the collision of two winds in a binary system can be sufficient. We used archival data to investigate the X-ray properties of other blue hypergiants. In general, stars of this class are not detected as X-ray sources. We suggest that our new Chandra observations of Cyg OB2 12 can be best explained if Cyg OB2 12 is a colliding wind binary possessing a late O-type companion. This makes Cyg OB2 12 only the second binary system among the 16 known Galactic hypergiants. This low binary fraction indicates that the blue hypergiants are likely products of massive binary evolution during which they either accreted a significant amount of mass or already merged with their companions.
Reduced Interface-Mediated Recombination for High Open-Circuit Voltages in CH3NH3PbI3 Solar Cells
(2017)
Perovskite solar cells with all-organic transport layers exhibit efficiencies rivaling their counterparts that employ inorganic transport layers, while avoiding high-temperature processing. Herein, it is investigated how the choice of the fullerene derivative employed in the electron-transporting layer of inverted perovskite cells affects the open-circuit voltage (V-OC). It is shown that nonradiative recombination mediated by the electron-transporting layer is the limiting factor for the V-OC in the cells. By inserting an ultrathin layer of an insulating polymer between the active CH3NH3PbI3 perovskite and the fullerene, an external radiative efficiency of up to 0.3%, a V-OC as high as 1.16 V, and a power conversion efficiency of 19.4% are realized. The results show that the reduction of nonradiative recombination due to charge-blocking at the perovskite/organic interface is more important than proper level alignment in the search for ideal selective contacts toward high V-OC and efficiency.
Perovskite solar cells now compete with their inorganic counterparts in terms of power conversion efficiency, not least because of their small open-circuit voltage (V-OC) losses. A key to surpass traditional thin-film solar cells is the fill factor (FF). Therefore, more insights into the physical mechanisms that define the bias dependence of the photocurrent are urgently required. In this work, we studied charge extraction and recombination in efficient triple cation perovskite solar cells with undoped organic electron/hole transport layers (ETL/HTL). Using integral time of flight we identify the transit time through the HTL as the key figure of merit for maximizing the fill factor (FF) and efficiency. Complementarily, intensity dependent photocurrent and V-OC measurements elucidate the role of the HTL on the bias dependence of non-radiative and transport-related loss channels. We show that charge transport losses can be completely avoided under certain conditions, yielding devices with FFs of up to 84%. Optimized cells exhibit power conversion efficiencies of above 20% for 6 mm(2) sized pixels and 18.9% for a device area of 1 cm(2). These are record efficiencies for hybrid perovskite devices with dopant-free transport layers, highlighting the potential of this device technology to avoid charge-transport limitations and to approach the Shockley-Queisser limit.
Floating ice shelves, which fringe most of Antarctica’s coastline, regulate ice flow into the Southern Ocean1,2,3. Their thinning4,5,6,7 or disintegration8,9 can cause upstream acceleration of grounded ice and raise global sea levels. So far the effect has not been quantified in a comprehensive and spatially explicit manner. Here, using a finite-element model, we diagnose the immediate, continent-wide flux response to different spatial patterns of ice-shelf mass loss. We show that highly localized ice-shelf thinning can reach across the entire shelf and accelerate ice flow in regions far from the initial perturbation. As an example, this ‘tele-buttressing’ enhances outflow from Bindschadler Ice Stream in response to thinning near Ross Island more than 900 km away. We further find that the integrated flux response across all grounding lines is highly dependent on the location of imposed changes: the strongest response is caused not only near ice streams and ice rises, but also by thinning, for instance, well-within the Filchner–Ronne and Ross Ice Shelves. The most critical regions in all major ice shelves are often located in regions easily accessible to the intrusion of warm ocean waters10,11,12, stressing Antarctica’s vulnerability to changes in its surrounding ocean.
Studying the temporal variability of BL Lac objects at the highest energies provides unique insights into the extreme physical processes occurring in relativistic jets and in the vicinity of super-massive black holes. To this end, the long-term variability of the BL Lac object PKS 2155 304 is analyzed in the high (HE, 100MeV < E < 300 GeV) and very high energy (VHE, E > 200 GeV) gamma-ray domain. Over the course of similar to 9 yr of H. E. S. S. observations the VHE light curve in the quiescent state is consistent with a log-normal behavior. The VHE variability in this state is well described by flicker noise (power-spectral-density index beta(VHE) = 1 .10(+ 0 : 10) (0 : 13)) on timescales larger than one day. An analysis of similar to 5.5 yr of HE Fermi-LAT data gives consistent results (beta(HE) = 1 : 20(+ 0 : 21) (0 : 23), on timescales larger than 10 days) compatible with the VHE findings. The HE and VHE power spectral densities show a scale invariance across the probed time ranges. A direct linear correlation between the VHE and HE fluxes could neither be excluded nor firmly established. These long-term-variability properties are discussed and compared to the red noise behavior (beta similar to 2) seen on shorter timescales during VHE-flaring states. The difference in power spectral noise behavior at VHE energies during quiescent and flaring states provides evidence that these states are influenced by different physical processes, while the compatibility of the HE and VHE long-term results is suggestive of a common physical link as it might be introduced by an underlying jet-disk connection.
We study gadolinium thin films as a model system for ferromagnets with negative thermal expansion. Ultrashort laser pulses heat up the electronic subsystem and we follow the transient strain via ultrafast x-ray diffraction. In terms of a simple Grueneisen approach, the strain is decomposed into two contributions proportional to the thermal energy of spin and phonon subsystems. Our analysis reveals that upon femtosecond laser excitation, phonons and spins can be driven out of thermal equilibrium for several nanoseconds.
Massive stars, at least similar to 10 times more massive than the Sun, have two key properties that make them the main drivers of evolution of star clusters, galaxies, and the Universe as a whole. On the one hand, the outer layers of massive stars are so hot that they produce most of the ionizing ultraviolet radiation of galaxies; in fact, the first massive stars helped to re-ionize the Universe after its Dark Ages. Another important property of massive stars are the strong stellar winds and outflows they produce. This mass loss, and finally the explosion of a massive star as a supernova or a gamma-ray burst, provide a significant input of mechanical and radiative energy into the interstellar space. These two properties together make massive stars one of the most important cosmic engines: they trigger the star formation and enrich the interstellar medium with heavy elements, that ultimately leads to formation of Earth-like rocky planets and the development of complex life. The study of massive star winds is thus a truly multidisciplinary field and has a wide impact on different areas of astronomy. In recent years observational and theoretical evidences have been growing that these winds are not smooth and homogeneous as previously assumed, but rather populated by dense "clumps". The presence of these structures dramatically affects the mass loss rates derived from the study of stellar winds. Clump properties in isolated stars are nowadays inferred mostly through indirect methods (i.e., spectroscopic observations of line profiles in various wavelength regimes, and their analysis based on tailored, inhomogeneous wind models). The limited characterization of the clump physical properties (mass, size) obtained so far have led to large uncertainties in the mass loss rates from massive stars. Such uncertainties limit our understanding of the role of massive star winds in galactic and cosmic evolution. Supergiant high mass X-ray binaries (SgXBs) are among the brightest X-ray sources in the sky. A large number of them consist of a neutron star accreting from the wind of a massive companion and producing a powerful X-ray source. The characteristics of the stellar wind together with the complex interactions between the compact object and the donor star determine the observed X-ray output from all these systems. Consequently, the use of SgXBs for studies of massive stars is only possible when the physics of the stellar winds, the compact objects, and accretion mechanisms are combined together and confronted with observations. This detailed review summarises the current knowledge on the theory and observations of winds from massive stars, as well as on observations and accretion processes in wind-fed high mass X-ray binaries. The aim is to combine in the near future all available theoretical diagnostics and observational measurements to achieve a unified picture of massive star winds in isolated objects and in binary systems.
In this paper, we investigate the multiwavelength properties of the magnetic early B-type star HR 7355. We present its radio light curves at several frequencies, taken with the Jansky Very Large Array, and X-ray spectra, taken with the XMM-Newton X-ray telescope. Modelling of the radio light curves for the Stokes I and V provides a quantitative analysis of the HR 7355 magnetosphere. A comparison between HR 7355 and a similar analysis for the Ap star CU Vir allows us to study how the different physical parameters of the two stars affect the structure of the respective magnetospheres where the non-thermal electrons originate. Our model includes a cold thermal plasma component that accumulates at high magnetic latitudes that influences the radio regime, but does not give rise to X-ray emission. Instead, the thermal X-ray emission arises from shocks generated by wind stream collisions close to the magnetic equatorial plane. The analysis of the X-ray spectrum of HR 7355 also suggests the presence of a non-thermal radiation. Comparison between the spectral index of the power-law X-ray energy distribution with the non-thermal electron energy distribution indicates that the non-thermal X-ray component could be the auroral signature of the non-thermal electrons that impact the stellar surface, the same non-thermal electrons that are responsible for the observed radio emission. On the basis of our analysis, we suggest a novel model that simultaneously explains the X-ray and the radio features of HR 7355 and is likely relevant for magnetospheres of other magnetic early-type stars.
We present the results of a 140 ks XMM-Newton observation of the B2 star rho Oph A. The star has exhibited strong X-ray variability: a cusp-shaped increase of rate, similar to that which we partially observed in 2013, and a bright flare. These events are separated in time by about 104 ks, which likely correspond to the rotational period of the star (1.2 days). Time resolved spectroscopy of the X-ray spectra shows that the first event is caused by an increase of the plasma emission measure, while the second increase of rate is a major flare with temperatures in excess of 60 MK (kT similar to 5 keV). From the analysis of its rise, we infer a magnetic field of >= 300 G and a size of the flaring region of similar to 1.4-1.9 x 10(11) cm, which corresponds to similar to 25%-30% of the stellar radius. We speculate that either an intrinsic magnetism that produces a hot spot on its surface or an unknown low mass companion are the source of such X-rays and variability. A hot spot of magnetic origin should be a stable structure over a time span of >= 2.5 yr, and suggests an overall large scale dipolar magnetic field that produces an extended feature on the stellar surface. In the second scenario, a low mass unknown companion is the emitter of X-rays and it should orbit extremely close to the surface of the primary in a locked spin-orbit configuration, almost on the verge of collapsing onto the primary. As such, the X-ray activity of the secondary star would be enhanced by its young age, and the tight orbit as in RS Cvn systems. In both cases rho Oph would constitute an extreme system that is worthy of further investigation.
AH Cep and CW Cep are both early B-type binaries with short orbital periods of 1.8. days and 2.7. days, respectively. All four components are B0.5V types. The binaries are also double-lined spectroscopic and eclipsing. Consequently, solutions for orbital and stellar parameters make the pair of binaries ideal targets for a study of the colliding winds between two B. stars. Chandra ACIS-I observations were obtained to determine X-ray luminosities. AH. Cep was detected with an unabsorbed X-ray luminosity at a 90% confidence interval of (9-33) x 10(30) erg s(-1), or (0.5-1.7) x 10(-7) L-Bol , relative to the combined Bolometric luminosities of the two components. While formally consistent with expectations for embedded wind shocks, or binary wind collision, the near-twin system of CW Cep was a surprising nondetection. For CW Cep, an upper limit was determined with L-X/L-Bol < 10(-8), again for the combined components. One difference between these two systems is that AH Cep is part of a multiple system. The X-rays from AH. Cep may not arise from standard wind shocks nor wind collision, but perhaps instead from magnetism in any one of the four components of the system. The possibility could be tested by searching for cyclic X-ray variability in AH. Cep on the short orbital period of the inner B. stars.
Theoretical models predict that the compressed interstellar medium around runaway O stars can produce highenergy non-thermal diffuse emission, in particular, non-thermal X-ray and gamma-ray emission. So far, detection of nonthermal X-ray emission was claimed for only one runaway star, AE Aur. We present a search for non-thermal diffuse X-ray emission from bow shocks using archived XMM-Newton observations for a clean sample of six welldetermined runaway O stars. We find that none of these objects present diffuse X-ray emission associated with their bow shocks, similarly to previous X-ray studies toward. zeta ph and BD+ 43 degrees 3654. We carefully investigated multi-wavelength observations of AE Aur and could not confirm previous findings of non-thermal X-rays. We conclude that so far there is no clear evidence of non-thermal extended emission in bow shocks around runaway O stars.
Context. Supergiant fast X-ray (SFXT) transients are a peculiar class of supergiant X-ray binaries characterized by a remarkable variability in the X-ray domain, widely ascribed to accretion from a clumpy stellar wind. Aims. In this paper we performed a systematic and homogeneous analysis of the sufficiently bright X-ray flares observed with XMM-Newton from the supergiant fast X-ray transients to probe spectral variations on timescales as short as a few hundred seconds. Our ultimate goal is to investigate whether SFXT flares and outbursts are triggered by the presence of clumps, and to reveal whether strongly or mildly dense clumps are required. Methods. For all sources, we employ a technique developed by our group already exploited in a number of our previous papers, making use of an adaptive rebinned hardness ratio to optimally select the time intervals for the spectral extraction. A total of twelve observations performed in the direction of five SFXTs are reported, providing the largest sample of events available so far. Results. Using the original results reported here and those obtained with our technique from the analysis of two previously published XMM-Newton observations of IGR J17544-2619 and IGR J18410-0535, we show that both strongly and mildly dense clumps can trigger these events. In the former case, the local absorption column density may increase by a factor of >> 3, while in the latter case, the increase is only a factor of similar to 2-3 (or lower). An increase in the absorption column density is generally recorded during the rise of the flares/outbursts, while a drop follows when the source achieves peak flux. In a few cases, a re-increase of the absorption column density after the flare is also detected, and we discovered one absorption event related to the passage of an unaccreted clump in front of the compact object. Overall, there seems to be no obvious correlation between the dynamic ranges in the X-ray fluxes and absorption column densities in supergiant fast X-ray transients, with an indication that lower densities are recorded at the highest fluxes. Conclusions. The spectral variations measured in all sources are in agreement with the idea that the flares/outbursts are triggered by the presence of dense structures in the wind interacting with the X-rays from the compact object (leading to photoionization). The lack of correlation between the dynamic ranges in the X-ray fluxes and absorption column densities can be explained by the presence of accretion inhibition mechanism(s). Based on the knowledge acquired so far on the SFXTs, we propose a classification of the flares/outbursts from these sources in order to drive future observational investigations. We suggest that the difference between the classes of flares/outbursts is related to the fact that the mechanism(s) inhibiting accretion can be overcome more easily in some sources compared to others. We also investigate the possibility that different stellar wind structures, other than clumps, could provide the means to temporarily overcome the inhibition of accretion in supergiant fast X-ray transients.
The long-term X-ray light curves of classical supergiant X-ray binaries and supergiant fast X-ray transients show relatively similar super-orbital modulations, which are still lacking a sound interpretation. We propose that these modulations are related to the presence of corotating interaction regions (CIRs) known to thread the winds of OB supergiants. To test this hypothesis, we couple the outcomes of three-dimensional (3D) hydrodynamic models for the formation of CIRs in stellar winds with a simplified recipe for the accretion onto a neutron star. The results show that the synthetic X-ray light curves are indeed modulated by the presence of the CIRs. The exact period and amplitude of these modulations depend on a number of parameters governing the hydrodynamic wind models and on the binary orbital configuration. To compare our model predictions with the observations, we apply the 3D wind structure previously shown to well explain the appearance of discrete absorption components in the UV time series of a prototypical B0.5I-type supergiant. Using the orbital parameters of IGRJ 16493-4348, which has the same B0.5I donor spectral type, the period and modulations in the simulated X-ray light curve are similar to the observed ones, thus providing support to our scenario. We propose that the presence of CIRs in donor star winds should be considered in future theoretical and simulation efforts of wind-fed X-ray binaries.
Circumstantial evidence suggests that magnetism and enhanced X-ray emission are likely correlated in early B-type stars: similar fractions of them (similar to 10%) are strong and hard X-ray sources and possess strong magnetic fields. It is also known that some B-type stars have spots on their surface. Yet up to now no X-ray activity associated with spots on early-type stars was detected. In this Letter we report the detection of a magnetic field on the B2V star rho Oph A. Previously, we assessed that the X-ray activity of this star is associated with a surface spot, herewith we establish its magnetic origin. We analyze spectra of rho Oph A obtained with the FORS2 spectrograph at ESO Very Large Telescope (VLT) at two epochs, and detect a longitudinal component of the magnetic field of the order of similar to 500 G in one of the datasets. The detection of the magnetic field only at one epoch can be explained by stellar rotation which is also invoked to explain observed periodic X-ray activity. From archival HARPS ESO VLT high resolution spectra we derived the fundamental stellar parameters of rho Oph A and further constrained its age. We conclude that rho Oph A provides strong evidence for the presence of active X-ray emitting regions on young magnetized early type stars.