Identification of the Early Fermi LAT Gamma-Ray Bright Objects with Extragalactic VLBI sources

A list of 205 gamma-ray strong objects was reported recently as a result of a 3-month integration with the Large Area Telescope on board the Fermi Gamma-Ray Space Telescope. We attempted identification of these objects, cross-correlating the gamma-ray positions with VLBI positions of a large all-sky sample of extragalactic radio sources selected on the basis of their parsec-scale flux density. The original associations reported by the Fermi team are confirmed and six new identifications are suggested. A Monte-Carlo analysis shows that the fraction of chance associations in our analysis is less than 5 per cent, and confirms that the vast majority of gamma-ray bright extragalactic sources are radio loud blazars with strong parsec-scale jets. A correlation between the parsec-scale radio and gamma-ray flux is supported by our analysis of a complete VLBI flux-density-limited sample of extragalactic jets. The effectiveness of using a VLBI catalog to find associations between gamma-ray detections and compact extragalactic radio sources, especially near the Galactic plane, is demonstrated. It is suggested that VLBI catalogs should be used for future identification of Fermi LAT objects.Comment: 5 pages, 4 figures, 1 table, accepted by the Astrophysical Journal Letters; minor corrections to the text are made, one panel is added to Figure 1; full table 1 in electronic form can be extracted from the preprint sourc

Search for a Point-Source Counterpart of the Unidentified Gamma-Ray Source TeV J2032+4130 in Cygnus

We have made a multiwavelength study of the overlapping error boxes of the unidentified gamma-ray sources TeV J2032+4130 and 3EG J2033+4118 in the direction of the Cygnus OB2 association (d = 1.7 kpc) in order to search for a point-source counterpart of the first unidentified TeV source. Optical identifications and spectroscopic classifications for the brighter X-ray sources in ROSAT PSPC and Chandra ACIS images are obtained, without finding a compelling counterpart. The classified X-ray sources are a mix of early and late-type stars, with one exception. The brightest source in the Chandra observation is a new, hard absorbed source that is both transient and rapidly variable. It lies 7' from the centroid of the TeV emission, which places it outside of the claimed 2 sigma location (r = 4.8'). A possible eclipse or "dip" transition is seen in its light curve. With a peak 1-10 keV luminosity of 7 x 10^(32) (d/1.7 kpc})^2 ergs s^(-1), this source could be a quiescent low-mass X-ray binary that lies beyond the Cyg OB2 association. A coincident, reddened optical object of R = 20.4, J = 15.4, H = 14.2, and K = 13.4 is observed, but not yet classified due to the lack of obvious emission or absorption features in its spectrum. Alternatively, this Chandra} and optical source might be a considered a candidate for a proton blazar, a long hypothesized type of radio-weak gamma-ray source. More detailed observations will be needed to determine the nature of this variable X-ray source, and to assess the possibility of its connection with TeV J2032+4130.Comment: 24 pages, 8 figures, accepted by Ap

Surveys with the Cherenkov Telescope Array

International audienceSurveys open up unbiased discovery space and generate legacy datasets of long-lasting value. One of the goals of imaging arrays of Cherenkov telescopes like CTA is to survey areas of the sky for faint very high energy gamma-ray (VHE) sources, especially sources that would not have drawn attention were it not for their VHE emission (e.g . the Galactic “dark accelerators”). More than half the currently known VHE sources are to be found in the Galactic Plane. Using standard techniques, CTA can carry out a survey of the region |l|<60° |b|<2° in 250 h (1/4th the available time per year at one location) down to a uniform sensitivity of 3 mCrab (a “Galactic Plane survey”). CTA could also survey 1/4th of the sky down to a sensitivity of 20 mCrab in 370 h of observing time (an “all-sky survey”), which complements well the surveys by the Fermi/LAT at lower energies and extended air shower arrays at higher energies. Observations in (non-standard) divergent pointing mode may shorten the “all-sky survey” time to about 100 h with no loss in survey sensitivity. We present the scientific rationale for these surveys, their place in the multi-wavelength context, their possible impact and their feasibility. We find that the Galactic Plane survey has the potential to detect hundreds of sources. Implementing such a survey should be a major goal of CTA. Additionally, about a dozen blazars, or counterparts to Fermi/LAT sources, are expected to be detected by the all-sky survey, whose prime motivation is the search for extragalactic “dark accelerators”

UVES/VLT high resolution absorption spectroscopy of the GRB080330 afterglow: a study of the GRB host galaxy and intervening absorbers

We study the Gamma Ray Burst (GRB) environment and intervening absorbers by analyzing the optical absorption features produced by gas surrounding the GRB or along its line of sight. We analyzed high resolution spectroscopic observations (R=40000, S/N=3 - 6) of the optical afterglow of GRB080330, taken with UVES at the VLT ~ 1.5 hours after the GRB trigger. The spectrum illustrates the complexity of the ISM of the GRB host galaxy at z = 1.51 which has at least four components in the main absorption system. We detect strong FeII, SiII, and NiII excited absorption lines associated with the bluemost component only. In addition to the host galaxy, at least two more absorbers lying along the line of sight to the afterglow have been detected in the redshift range 0.8 < z < 1.1, each exhibiting MgII absorption. For the bluemost component in the host galaxy, we derive information about its distance from the site of the GRB explosion. We do so by assuming that the excited absorption lines are produced by indirect UV pumping, and compare the data with a time dependent photo-excitation code. The distance of this component is found to be 280+40-50 pc, which is lower than found for other GRBs (1 - 6 kpc). We identify two additional MgII absorbers, one of them with a rest frame equivalent width larger than 1A. The distance between the GRB and the absorber measured in this paper confirms that the power of the GRB radiation can influence the conditions of the interstellar medium up to a distance of at least several hundred pc. For the intervening absorbers, we confirm the trend that on average one strong intervening system is found per afterglow, as has been noted in studies exhibiting an excess of strong MgII absorbers along GRB sightlines compared to quasars.Comment: 8 Pages, 7 ps figures, A&A in pres

GRB 991216 Joins the Jet Set: Discovery and Monitoring of its Optical Afterglow

The optical light curve of the energetic gamma-ray burst GRB 991216 is consistent with jet-like behavior in which a power-law decay steepens from t**(-1.22 +/- 0.04) at early times to t**(-1.53 +/- 0.05) in a gradual transition at around 2 d. The derivation of the late-time decay slope takes into account the constant contribution of a host or intervening galaxy which was measured 110 d after the event at R = 24.56 +/- 0.14, although the light curve deviates from a single power law whether or not a constant term is included. The early-time spectral energy distribution of the afterglow can be described as F_nu ~ nu**(-0.74 +/- 0.05) or flatter between optical and X-ray, which, together with the slow initial decay, is characteristic of standard adiabatic evolution in a uniformly dense medium. Assuming that a reported absorption-line redshift of 1.02 is correct, the apparent isotropic energy of 6.7 x 10**53 erg is reduced by a factor of ~ 200 in the jet model, and the initial half-opening angle is ~ 6 deg. GRB 991216 is the third good example of a jet-like afterglow (following GRB 990123 and GRB 990510), supporting a trend in which the apparently most energetic gamma-ray events have the narrowest collimation and a uniform ISM environment. This, plus the absence of evidence for supernovae associated with jet-like afterglows, suggests that these events may originate from a progenitor in which angular momentum plays an important role but a massive stellar envelope or wind does not, e.g., the coalescence of a compact binary.Comment: 19 pages, accepted by The Astrophysical Journa

Agreement between a simple dyspnea-guided treatment algorithm for stable COPD and the GOLD guidelines: A pilot study

Introduction: Guidelines recommendations for the treatment of COPD are poorly followed. This could be related to the complexity of classification and treatment algorithms. The purpose of this study was to validate a simpler dyspnea-based treatment algorithm for inhaled pharmacotherapy in stable COPD, comparing its concordance with the current Global Initiative for Obstructive Lung Disease (GOLD) guideline. Methods: We enrolled patients who had been diagnosed with COPD in three primary care facilities and two tertiary hospitals in Spain. We determined anthropometric data, forced expiratory volume in the 1st second (percent), exacerbations, and dyspnea based on the modified Medical Research Council scale. We evaluated the new algorithm based on dyspnea and exacerbations and calculated the concordance with the current GOLD recommendations. Results: We enrolled 100 patients in primary care and 150 attending specialized care in a respiratory clinic. There were differences in the sample distribution between cohorts with 41% vs 26% in grade A, 16% vs 12% in grade B, 16% vs 22% in grade C, and 27% vs 40% in grade D for primary and respiratory care, respectively (P=0.005). The coincidence of the algorithm with the GOLD recommendations in primary care was 93% and 91.8% in the respiratory care cohort. Conclusion: A simple dyspnea-based treatment algorithm for inhaled pharmacotherapy of COPD could be useful in the management of COPD patients and concurs very well with the recommended schema suggested by the GOLD initiative

Active Galactic Nuclei under the scrutiny of CTA

Active Galactic Nuclei (hereafter AGN) produce powerful outflows which offer excellent conditions for efficient particle acceleration in internal and external shocks, turbulence, and magnetic reconnection events. The jets as well as particle accelerating regions close to the supermassive black holes (hereafter SMBH) at the intersection of plasma inflows and outflows, can produce readily detectable very high energy gamma-ray emission. As of now, more than 45 AGN including 41 blazars and 4 radiogalaxies have been detected by the present ground-based gamma-ray telescopes, which represents more than one third of the cosmic sources detected so far in the VHE gamma-ray regime. The future Cherenkov Telescope Array (CTA) should boost the sample of AGN detected in the VHE range by about one order of magnitude, shedding new light on AGN population studies, and AGN classification and unification schemes. CTA will be a unique tool to scrutinize the extreme high-energy tail of accelerated particles in SMBH environments, to revisit the central engines and their associated relativistic jets, and to study the particle acceleration and emission mechanisms, particularly exploring the missing link between accretion physics, SMBH magnetospheres and jet formation. Monitoring of distant AGN will be an extremely rewarding observing program which will inform us about the inner workings and evolution of AGN. Furthermore these AGN are bright beacons of gamma-rays which will allow us to constrain the extragalactic infrared and optical backgrounds as well as the intergalactic magnetic field, and will enable tests of quantum gravity and other "exotic" phenomena.Comment: 28 pages, 23 figure

Prospects for Observations of Pulsars and Pulsar Wind Nebulae with CTA

The last few years have seen a revolution in very-high gamma-ray astronomy (VHE; E>100 GeV) driven largely by a new generation of Cherenkov telescopes (namely the H.E.S.S. telescope array, the MAGIC and MAGIC-II large telescopes and the VERITAS telescope array). The Cherenkov Telescope Array (CTA) project foresees a factor of 5 to 10 improvement in sensitivity above 0.1 TeV, extending the accessible energy range to higher energies up to 100 TeV, in the Galactic cut-off regime, and down to a few tens GeV, covering the VHE photon spectrum with good energy and angular resolution. As a result of the fast development of the VHE field, the number of pulsar wind nebulae (PWNe) detected has increased from one PWN in the early '90s to more than two dozen firm candidates today. Also, the low energy threshold achieved and good sensitivity at TeV energies has resulted in the detection of pulsed emission from the Crab Pulsar (or its close environment) opening new and exiting expectations about the pulsed spectra of the high energy pulsars powering PWNe. Here we discuss the physics goals we aim to achieve with CTA on pulsar and PWNe physics evaluating the response of the instrument for different configurations.Comment: accepted for publication in Astroparticle Physic

The detailed optical light curve of GRB 030329

(Abridged) We present densely sampled BVRI light curves of the optical transient associated with the gamma-ray burst GRB 030329, the result of a coordinated observing campaign conducted at five observatories. Augmented with published observations of this GRB, the compiled optical dataset contains 2687 photometric measurements, obtained between 78 minutes and 79 days after the burst. We show that the underlying supernova 2003dh evolved faster than, and was probably somewhat fainter than the type Ic SN 1998bw, associated with GRB 980425. We find that our data can be described by a broken power-law decay perturbed by a complex variable component. The early- and late-time decay slopes are determined to be ~1.1 and ~2, respectively. Assuming this single power-law model, we constrain the break to lie between ~3 and ~8 days after the burst. This simple, singly-broken power-law model, derived only from the analysis of our optical observations, may also account for available multi-band data, provided that the break happened ~8 days after the burst. The more complex double-jet model of Berger et al. provides a comparable fit to the optical, X-ray, mm and radio observations of this event. We detect a significant change in optical colors during the first day. Our color analysis is consistent with a cooling break frequency sweeping through the optical band during the first day. The light curves of GRB 030329 reveal a rich array of variations, superposed over the mean power-law decay. We find that the early variations are asymmetric, with a steep rise followed by a relatively slower (by a factor of about two) decline. The variations maintain a similar time scale during the first four days, and then get significantly longer.Comment: 14 pages, 12 figures, accepted for publication in ApJ with minor changes. See the GRB030329 Data Treasury at http://wise-obs.tau.ac.il/GRB030329

Multifrequency Strategies for the Identification of Gamma-Ray Sources

More than half the sources in the Third EGRET (3EG) catalog have no firmly established counterparts at other wavelengths and are unidentified. Some of these unidentified sources have remained a mystery since the first surveys of the gamma-ray sky with the COS-B satellite. The unidentified sources generally have large error circles, and finding counterparts has often been a challenging job. A multiwavelength approach, using X-ray, optical, and radio data, is often needed to understand the nature of these sources. This chapter reviews the technique of identification of EGRET sources using multiwavelength studies of the gamma-ray fields.Comment: 35 pages, 22 figures. Chapter prepared for the book "Cosmic Gamma-ray Sources", edited by K.S. Cheng and G.E. Romero, to be published by Kluwer Academic Press, 2004. For complete article and higher resolution figures, go to: http://www.astro.columbia.edu/~muk/mukherjee_multiwave.pd