arXiv subjects

Jeremy Drake

Publications and source records attributed to Jeremy Drake.

36 records · Page 2Linked to original sources

arXiv2015

Stellar Activity and Coronal Heating: an overview of recent results

Observations of the coronae of the Sun and of solar-like stars provide complementary information to advance our understanding of stellar magnetic activity, and of the processes leading to the heating of their outer atmospheres. While solar observations allow us to study the corona at high spatial and temporal resolution, the study of stellar coronae allows us to probe stellar activity over a wide range of ages and stellar parameters. Stellar studies therefore provide us with additional tools for understanding coronal heating processes, as well as the long-term evolution of solar X-ray activity. We discuss how recent studies of stellar magnetic fields and coronae contribute to our understanding of the phenomenon of activity and coronal heating in late-type stars.

astro-ph.SR↗

arXiv2014

Exploring the Small Magellanic Cloud to the Faintest X-ray Fluxes: Source Catalog, Timing and Spectral Analysis

We present the results of a pair of 100 ksec Chandra observations in the Small Magellanic Cloud to survey high mass X-ray binaries (HMXBs), stars and LMXBs/CVs down to Lx = 4.3 x 10^32 erg/s The two SMC Deep Fields are located in the most active star forming region of the bar, with Deep Field-1 positioned at the most pulsar-rich location identified from previous surveys. Two new pulsars were discovered in outburst: CXOU J004929.7-731058 (P=892s), CXOU J005252.2-721715 (P=326s), and 3 new HMXB candidates were identified. Of 15 Be-pulsars now known in the field, 13 were detected, with pulsations seen in 9 of them. Ephemerides demonstrate that 6 of the 10 pulsars known to exhibit regular outbursts were seen outside their periastron phase, and quiescent X-ray emission at Lx=10^33 - 10^34 is shown to be common. Comparison with ROSAT, ASCA, XMM-Newton catalogs resulted in positive identification of several previously ambiguous sources. Bright optical counterparts exist for 40 of the X-ray sources, of which 33 are consistent with early-type stars Mv<-2, B-V<0.2), and are the subject of a companion paper. The results point to an underlying HMXB population-density up to double that of active systems. The full catalog of 394 point-sources is presented along with detailed analyses of timing and spectral properties.

astro-ph↗

arXiv2013

The Hot and Energetic Universe: A White Paper presenting the science theme motivating the Athena+ mission

This White Paper, submitted to the recent ESA call for science themes to define its future large missions, advocates the need for a transformational leap in our understanding of two key questions in astrophysics: 1) How does ordinary matter assemble into the large scale structures that we see today? 2) How do black holes grow and shape the Universe? Hot gas in clusters, groups and the intergalactic medium dominates the baryonic content of the local Universe. To understand the astrophysical processes responsible for the formation and assembly of these large structures, it is necessary to measure their physical properties and evolution. This requires spatially resolved X-ray spectroscopy with a factor 10 increase in both telescope throughput and spatial resolving power compared to currently planned facilities. Feedback from supermassive black holes is an essential ingredient in this process and in most galaxy evolution models, but it is not well understood. X-ray observations can uniquely reveal the mechanisms launching winds close to black holes and determine the coupling of the energy and matter flows on larger scales. Due to the effects of feedback, a complete understanding of galaxy evolution requires knowledge of the obscured growth of supermassive black holes through cosmic time, out to the redshifts where the first galaxies form. X-ray emission is the most reliable way to reveal accreting black holes, but deep survey speed must improve by a factor ~100 over current facilities to perform a full census into the early Universe. The Advanced Telescope for High Energy Astrophysics (Athena+) mission provides the necessary performance (e.g. angular resolution, spectral resolution, survey grasp) to address these questions and revolutionize our understanding of the Hot and Energetic Universe. These capabilities will also provide a powerful observatory to be used in all areas of astrophysics.

astro-ph.HE↗

arXiv2012

Addressing Decadal Survey Science through Community Access to Highly Multiplexed Spectroscopy with BigBOSS on the KPNO Mayall Telescope

This document summarizes the results of a community-based discussion of the potential science impact of the Mayall+BigBOSS highly multiplexed multi-object spectroscopic capability. The KPNO Mayall 4m telescope equipped with the DOE- and internationally-funded BigBOSS spectrograph offers one of the most cost-efficient ways of accomplishing many of the pressing scientific goals identified for this decade by the "New Worlds, New Horizons" report. The BigBOSS Key Project will place unprecedented constraints on cosmological parameters related to the expansion history of the universe. With the addition of an open (publicly funded) community access component, the scientific impact of BigBOSS can be extended to many important astrophysical questions related to the origin and evolution of galaxies, stars, and the IGM. Massive spectroscopy is the critical missing ingredient in numerous ongoing and planned ground- and space-based surveys, and BigBOSS is unique in its ability to provide this to the US community. BigBOSS data from community-led projects will play a vital role in the education and training of students and in maintaining US leadership in these fields of astrophysics. We urge the NSF-AST division to support community science with the BigBOSS multi-object spectrograph through the period of the BigBOSS survey in order to ensure public access to the extraordinary spectroscopic capability.

astro-ph.CO↗

arXiv2012

Spectroscopic follow-up of UV-excess objects selected from the UVEX survey

We present the results of the first spectroscopic follow-up of 132 optically blue UV-excess sources selected from the UV-excess survey of the Northern Galactic Plane (UVEX). The UV-excess spectra are classified into different populations and grids of model spectra are fit to determine spectral types, temperatures, surface gravities and reddening. From this initial spectroscopic follow-up 95% of the UV-excess candidates turn out to be genuine UV-excess sources such as white dwarfs, white dwarf binaries, subdwarfs type O and B, emission line stars and QSOs. The remaining sources are classified as slightly reddened main-sequence stars with spectral types later than A0V. The fraction of DA white dwarfs is 47% with reddening smaller than E(B-V)<0.7 mag. Relations between the different populations and their UVEX photometry, Galac- tic latitude and reddening are shown. A larger fraction of UVEX white dwarfs is found at magnitudes fainter than g>17 and Galactic latitude smaller than |b|<4 compared to main-sequence stars, blue horizontal branch stars and subdwarfs.

astro-ph.SR↗

arXiv2009

Structure and Evolution of Pre-Main Sequence Stars

Low-mass pre-main sequence (PMS) stars are strong and variable X-ray emitters, as has been well established by EINSTEIN and ROSAT observatories. It was originally believed that this emission was of thermal nature and primarily originated from coronal activity (magnetically confined loops, in analogy with Solar activity) on contracting young stars. Broadband spectral analysis showed that the emission was not isothermal and that elemental abundances were non-Solar. The resolving power of the Chandra and XMM X-ray gratings spectrometers have provided the first, tantalizing details concerning the physical conditions such as temperatures, densities, and abundances that characterize the X-ray emitting regions of young star. These existing high resolution spectrometers, however, simply do not have the effective area to measure diagnostic lines for a large number of PMS stars over required to answer global questions such as: how does magnetic activity in PMS stars differ from that of main sequence stars, how do they evolve, what determines the population structure and activity in stellar clusters, and how does the activity influence the evolution of protostellar disks. Highly resolved (R>3000) X-ray spectroscopy at orders of magnitude greater efficiency than currently available will provide major advances in answering these questions. This requires the ability to resolve the key diagnostic emission lines with a precision of better than 100 km/s.

astro-ph.HE↗

arXiv2009

X-ray Studies of Planetary Systems: An Astro2010 Decadal Survey White Paper

While it may seem counterintuitive that X-ray astronomy should give any insights into low-temperature planetary systems, planets orbit stars whose magnetized surfaces divert a small fraction of the stellar energy into high energy products: coronal UV and X-rays, flare X-rays and energetic particles, and a high-velocity stellar wind. In our Solar System, X-ray emission gives unique insights into the solar activity, planetary atmospheres, cometary comae, charge exchange physics, and space weather across the Solar System. The stellar activity of young stars is greatly elevated and can substantially affect protoplanetary disks and planet formation processes. We highlight six studies achievable with the planned International X-ray Observatory which address in unique ways issues in planetary sciences: probing X-ray irradiation of protoplanetary disks with the iron fluorescent line and its effects on disk turbulence; study the complex charge-exchange X-ray emission from Jupiter and the Martian exosphere; elucidate charge-exchange processes in cometary comae; understanding heliospheric charge-exchange emission and the interpretation of the soft X-ray background; and examining the magnetic activity of planet-hosting stars and its evaporation of planetary atmospheres.

astro-ph.HE↗

arXiv2009

The Dynamic X-ray Sky of the Local Universe

Over the next decade, we can expect time domain astronomy to flourish at optical and radio wavelengths. In parallel with these efforts, a dedicated transient "machine" operating at higher energies (X-ray band through soft gamma-rays) is required to reveal the unique subset of events with variable emission predominantly visible above 100 eV. Here we focus on the transient phase space never yet sampled due to the lack of a sensitive, wide-field and triggering facility dedicated exclusively to catching high energy transients and enabling rapid coordinated multi-wavelength follow-up. We first describe the advancements in our understanding of known X-ray transients that can only be enabled through such a facility and then focus on the classes of transients theoretically predicted to be out of reach of current detection capabilities. Finally there is the exciting opportunity of revealing new classes of X-ray transients and unveiling their nature through coordinated follow-up observations at longer wavelengths.

astro-ph.HE↗

arXiv2008

IPHAS discoveries of young stars towards Cyg OB2 and its southern periphery

We report on the discovery of over 50 strong Halpha emitting objects towards the large OB association Cyg OB2 and the HII region DR 15 on its southern periphery. This was achieved using the INT Photometric Halpha Survey of the Northern Galactic Plane (IPHAS), combined with follow-up spectroscopy using the MMT multi-object spectrometer HectoSpec. We present optical spectra, supplemented with optical r', i' and Halpha photometry from IPHAS, and near-infrared J, H, and K photometry from 2MASS. The position of the objects in the (J - H) versus (H - K) diagram strongly suggests most of them are young. Many show CaII IR triplet emission indicating that they are in a pre-main sequence phase of evolution of T Tauri and Herbig Ae nature. Among these, we have uncovered pronounced clustering of T Tauri stars roughly a degree south of the centre of Cyg OB2, in an arc close to the HII region DR 15, and the radio ring nebula G79.29+0.46, for which we discuss its candidacy as a luminous blue variable (LBV). The emission line objects toward Cyg OB2 itself could be the brightest most prominent component of a population of lower mass pre-main sequence stars that has yet to be uncovered. Finally, we discuss the nature of the ongoing star formation in Cyg OB2 and the possibility that the central OB stars have triggered star formation in the periphery.

astro-ph↗

arXiv2008

TeV J2032+4130: a not-so-dark Accelerator?

The HEGRA gamma-ray source TeV J2032+4130 is considered the prototypical 'dark accelerator', since it was the first TeV source detected with no firm counterparts at lower frequencies. The Whipple collaboration observed this source in 2003-5 and the emission hotspot appears displaced about 9 arcminutes to the northeast of the HEGRA position, though given the large positional uncertainties the HEGRA and Whipple positions are consistent. Here we report on Westerbork Synthesis Radio Telescope (WSRT), Very Large Array (VLA), Chandra and INTEGRAL data covering the locations of the Whipple and HEGRA hotspots. We confirm a dual-lobed radio source (also see Marti et al., 2007) coincident with the Whipple hotspot, as well as a weak, partially non-thermal shell-like object, with a location and morphology very similar to the HEGRA source, in our WSRT and mosaicked VLA datasets, respectively. Due to its extended nature, it is likely that the latter structure is a more plausible counterpart of the reported very high energy (VHE) gamma-ray emissions in this region. If so, TeV J2032+4130 may not be a 'dark accelerator' after all. Further observations with the new generation of imaging Cherenkov telescopes are needed to pin down the precise location and morphology of the TeV emission region and thus clear up the confusion over its possible lower frequency counterparts.

astro-ph↗

arXiv2005

Deeper Chandra Follow-up of Cygnus TeV Source Perpetuates Mystery

A 50 ksec Chandra observation of the unidentified TeV source in Cygnus reported by the HEGRA collaboration reveals no obvious diffuse X-ray counterpart. However, 240 Pointlike X-ray sources are detected within or nearby the extended TeV J2032+4130 source region, of which at least 36 are massive stars and 2 may be radio emitters. That the HEGRA source is a composite, having as counterpart the multiple point-like X-ray sources we observe, cannot be ruled out. Indeed, the distribution of point-like X-ray sources appears non-uniform and concentrated broadly within the extent of the TeV source region. We offer a hypothesis for the origin of the very high energy gamma-ray emission in Cyg OB2 based on the local acceleration of TeV range cosmic rays and the differential distribution of OB vs. less massive stars in this association.

astro-ph↗

arXiv2005

Chandra Detection of the First X-ray Forest along the Line of Sight To Mkn 421

We present the first >=3.5 sigma (conservative) or >=5.8 sigma (sum of lines significance) detection of two Warm-Hot Intergalactic Medium (WHIM) filaments at z>0, which we find along the line of sight to the blazar Mkn 421. These systems are detected through highly ionized resonant metal absorption in high quality Chandra-ACIS and -HRC Low Energy Transmission Grating (LETG) spectra of Mkn 421, obtained following our two Target of Opportunity requests during two outburst phases. The two intervening WHIM systems that we detect, have OVII and NVII columns of N(OVII)=(1.0 +/- 0.3) x 1e15 cm-2} N(NVII)=(0.8 +/- 0.4) x 1e15 cm-2, and N(OVII)=(0.7 +/- 0.3) x 1e15 cm-2, N(NVII)=(1.4 +/- 0.5) x 1e15 cm-2 respectively. From the detected number of WHIM filaments along this line of sight we can estimate the number of OVII filaments per unit redshift with columns larger than 7e14 cm-2, dP(OVII)/dz(N(OVII)>=7e14) = 67^{+88}_{-43}, consistent, within the large 1-sigma errors, with the hydrodynamical simulation predictions of dP(OVII)/dz(N(OVII)>=7e14) = 30. Finally, we measure a cosmological mass density of X-ray WHIM filaments Omega_b = 0.027^{+0.038}_{-0.019} x 10^{[O/H]_{-1}}, consistent with both model predictions and the estimated number of 'missing' baryons at low redshift.

astro-ph↗

arXiv2003

CHANDRA/VLA Follow-up of TeV J2032+4131, the Only Unidentified TeV Gamma-ray Source

The HEGRA Cherenkov telescope array group recently reported a steady and extended unidentified TeV gamma-ray source lying at the outskirts of Cygnus OB2. This is the most massive stellar association known in the Galaxy, estimated to contain ~2600 OB type members alone. It has been previously argued that the large scale shocks and turbulence induced by the multiple interacting supersonic winds from the many young stars in such associations may play a role in accelerating Galactic cosmic rays. Indeed, Cyg OB2 also coincides with the non-variable MeV-GeV range unidentified EGRET source, 3EG 2033+4118. We report on the near-simultaneous follow-up observations of the extended TeV source region with the CHANDRA X-ray Observatory and the Very Large Array (VLA) radio telescope obtained in order to explore this possibility. Analysis of the CO, HI, and IRAS 100 micron emissions shows that the TeV source region coincides with an outlying sub-group of powerful OB stars which have evacuated or destroyed much of the ambient atomic, molecular and dust material, and which may be related to the very high-energy emissions. An interesting SNR-like structure is also revealed near the TeV source region in the CO, HI and radio emission maps. Applying a numerical simulation which accurately tracks the radio to gamma-ray emission from primary hadrons as well as primary and secondary e+/-, we find that the broadband spectrum of the TeV source region favors a predominantly nucleonic - rather than electronic - origin of the high-energy flux, though deeper X-ray and radio observations are needed to confirm this. A very reasonable, ~0.1%, conversion efficiency of Cyg OB2's extreme stellar wind mechanical luminosity to nucleonic acceleration to ~PeV (10^15 eV) energies is sufficient to explain the multifrequency emissions.

astro-ph↗

arXiv2003

Cosmic Ray Acceleration by Stellar Associations? The Case of Cygnus OB2

The origins of all extra-solar cosmic rays -- both the ultra high-energy and lower energy Galactic variety -- remain unclear. It has been argued that the large scale shocks and turbulence induced by the multiple interacting supersonic winds from the many hot stars in young stellar associations may play a role in accelerating Galactic cosmic rays -- with or without the associated multiple supernova remnants (eg. Cesarsky & Montmerle, 1983; Bykov 2001). In this context, the report by the HEGRA Cherenkov telescope group of a steady and extended unidentified TeV gamma-ray source lying at the outskirts of Cygnus OB2 is particularly significant. This is the most massive stellar association known in the Galaxy, estimated to contain ~2600 OB type members alone indeed, Cyg OB2 also coincides with the non-variable MeV-GeV range unidentified EGRET source, 3EG 2033+4118. We summarise here the near-simultaneous follow-up observations of the extended TeV source region with the CHANDRA X-ray Observatory and the Very Large Array (VLA) radio telescope. Applying a numerical simulation which accurately tracks the radio to gamma-ray emission from primary hadrons as well as primary and secondary e+/-, we find that the broadband spectrum of the TeV source region favors a predominantly nucleonic rather than electronic origin of the high-energy flux, though deeper X-ray and radio observations will help confirm this. A very reasonable, ~0.1%, conversion efficiency of Cyg OB2's extreme stellar wind mechanical luminosity to nucleonic acceleration to ~PeV (10^15 eV) energies is sufficient to explain the multifrequency emissions.

astro-ph↗

arXiv2002

Chandra Multi-wavelength Plane (ChaMPlane) Survey: Design and Initial Results

The Chandra Multiwavength Plane (ChaMPlane) Survey of the galactic plane incorporates serendipitous sources from selected Chandra pointings in or near the galactic plane (b < 12deg; >20 ksec; lack of bright diffuse or point sources) to measure or constrain the luminosity function of low-luminosity accretion sources in the Galaxy. The primary goal is to detect and identify accreting white dwarfs (cataclysmic variables, with space density still uncertain by a factor of >10-100), neutron stars and black holes (quiescent low mass X-ray binaries) to constrain their space densities and thus origin and evolution. Secondary objectives are to identify Be stars in high mass X-ray binaries and constrain their space densities, and to survey the H-R diagram for stellar coronal sources. A parallel optical imaging under the NOAO Long Term Survey program provides deep optical images using the Mosaic imager on the CTIO and KPNO 4-m telescopes. The 36arcmin X 36arcmin optical images (Halpha, R, V and I) cover ~5X the area of each enclosed Chandra ACIS FOV, providing an extended survey of emission line objects for comparison with Chandra. Spectroscopic followup of optical counterparts is then conducted, thus far with WIYN and Magellan. The X-ray preliminary results from both the Chandra and optical surveys will be presented, including logN-logS vs. galactic position (l,b) and optical idenifications.

astro-ph↗

arXiv2002

Flare Heating in Stellar Coronae

We investigate the contribution of very weak flares to the coronal luminosity of low-mass active stars. We analyze EUVE/DS events data from FK Aqr, V1054 Oph, and AD Leo and conclude that in all these cases the coronal emission is dominated by flares to such an extent that in some cases the entire emission may be ascribed to flare heating. We have developed a new method to directly model for the first time stochastically produced flare emission, including undetectable flares, and their effects on the observed photon arrival times. We find that the index of the power-law distribution of flare energies (dN/dE ~ E^{-alpha}) is 2.6+-0.34, 2.74+-0.35, and 2.03-2.32 for FK Aqr, V1054 Oph, and AD Leo respectively. We also find that the flare component accounts for a large fraction (generally >50 percent) of the total flux.

astro-ph↗

arXiv2002

Bayesian Spectral Analysis of Metal Abandance Deficient Stars

Metallicity can be measured by analyzing the spectra in the X-ray region and comparing the flux in spectral lines to the flux in the underlying Bremsstrahlung continuum. In this paper we propose new Bayesian methods which directly model the Poisson nature of the data and thus are expected to exhibit improved sampling properties. Our model also accounts for the Poisson nature of background contamination of the observations, image blurring due to instrument response, and the absorption of photons in space. The resulting highly structured hierarchical model is fit using the Gibbs sampler, data augmentation and Metropolis-Hasting. We demonstrate our methods with the X-ray spectral analysis of several "Metal Abundance Deficient" stars. The model is designed to summarize the relative frequency of the energy of photons (X-ray or gamma-ray) arriving at a detector. Independent Poisson distributions are more appropriate to model the counts than the commonly used normal approximation. We model the high energy tail of the ASCA spectrum of each of the stars as a combination of a Bremsstrahlung continuum and ten narrow emission lines, included at positions of known strong lines. Statistical analysis is based on two source observations and one background observation. We use sequential Bayesian analysis for the two source observations; the posterior distribution from the first analysis is used to construct a prior for the second. Sensitivity of the final results to the choise of prior is investigated by altering the prior.

astro-ph↗

arXiv1999

On X-ray Variability in Active Binary Stars

We compare the count-rates of active binaries observed by Einstein and ROSAT. We find significant evidence for short-term variability (approx 0.32) and marginal evidence for a larger variability at longer timescales (approx 0.38). Modeling this excess as cyclic variability, we place an upper limit on the relative amplitude of the cyclic component (I_cyc/I_min < 4) for active binaries, significantly less than the variations seen in the Solar X-ray output over the Solar cycle.

astro-ph↗