HST/COS Observations of the Ly alpha Forest toward the BL Lac Object 1ES1553+113

Source: arXiv

ABSTRACT We present new far-ultraviolet spectra from the Cosmic Origins Spectrograph (HST/COS) of the BL Lac object 1ES1553+113 covering the wavelength range 1135-1795 A. The data show a smooth continuum with a wealth of narrow absorption features arising in the ISM and IGM. These features include 41 Lya absorbers at 0<z<0.43, fourteen of which are detected in multiple Lyman lines and six in one or more metal lines. We analyze a metal-rich triplet of Lya absorbers at z=0.188 in which OVI, NV, and CIII absorption is detected. Silicon ions (SiIII/IV) are not detected to fairly strong upper limits, and we use the measured SiIII/CIII upper limit to derive an abundance limit [C/Si]>0.6 for the strongest component of the absorber complex. Galaxy redshift surveys show a number of massive galaxies at approximately the same redshift as this absorption complex, suggesting that it arises in a large-scale galaxy filament. As one of the brightest extragalactic X-ray and gamma-ray sources, 1ES1553+113 is of great interest to the high-energy astrophysics community. With no intrinsic emission or absorption features, 1ES1553+113 has no direct redshift determination. We use intervening Lya absorbers to place a direct limit on the redshift: z_em>0.395 based on a confirmed Lya+OVI absorber and z_em>0.433 based on a single-line detection of Lya. COS/FUV data are only sensitive to Lya absorbers at z<0.47, but we present statistical arguments that z_em<0.58 based on the non-detection of any Lyb absorbers at z>0.4. Comment: ApJ submitted: 11 pages, 7 figures

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    ABSTRACT: The distribution of TeV spectral slopes versus redshift for currently known TeV blazars (16 sources with z<0.21, and one with z>0.25) is essentially a scatter plot with hardly any hint of a global trend. We suggest that this is the outcome of two combined effects of intergalactic gamma-gamma absorption, plus an inherent feature of the SSC (synchro-self-Compton) process of blazar emission. First, flux dimming introduces a bias that favors detection of progressively more flaring sources at higher redshifts. According to mainstream SSC models, more flaring source states imply sources with flatter TeV slopes. This results in a structured relation between intrinsic TeV slope and redshift. The second effect, spectral steepening by intergalactic absorption, affects sources progressively with distance and effectively wipes out the intrinsic slope-redshift correlation. Comment: A&A, in press (accepted Jan 29, 2008): 6 pages, 2 figures. One source, published subsequent to acceptance, added
    Astronomy and Astrophysics 11/2007; · 5.08 Impact Factor
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    Astronomy and Astrophysics 06/2003; · 5.08 Impact Factor
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    ABSTRACT: The low Galactic halo is enveloped by a sheath of ionized, low-metallicity gas, which can provide a substantial (1 M_sun/yr) cooling inflow to replenish star formation in the disk. Using absorption spectra from the HST and FUSE toward 37 active galactic nuclei at high latitude, we detect widespread interstellar SiIII 1206.5 absorption: 61 high-velocity clouds (HVCs) along 30 sight lines and 22 intermediate-velocity clouds (IVCs) along 20 sight lines. We find a segregation of redshifted and blueshifted absorbers across the Galactic rotation axis at l=180, consistent with a lag in the rotation velocity above the Galactic plane. The HVC sky coverage is large (81+-5% for 30/37 directions) with SiIII optical depth typically 4-5 times that of OVI 1032. The mean HVC column density per sight line, =13.42+-0.21, corresponds to total column density N_HII~6x10^18)/(Z_Si/0.2Z_sun) of ionized low-metallicity gas, similar to that inferred in OVI. This reservoir could total 10^8 M_sun and produce a mass infall rate ~1 M_sun/yr. By modeling SiII, SiIII, SiIV, and HI in a subset of absorbers, we constrain the mean photoionization parameter in the low halo, = -3.0 (+0.3,-0.4), approximately ten times lower than observed in the low-redshift intergalactic medium. The metallicities in some HVCs, derived from [SiII/HI], are 10-30% solar, whereas values found from all three silicon ions are lower in the pure-photoionization models. Thse formally lower metallicities are highly uncertain, since some of the higher ions may be collisionally ionized. The mean Si metallicities, <logZ_Si/Z_sun)>= -2.1(+1.1,-0.3) in 17 HVCs and -1.0(+0.6,-1.0) in 19 IVCs, are somewhat uncertain owing to ionization modeling, but consistent with the median photometric metallicity, [Fe/H]=-1.46+-0.30, for ~2e5 halo F/G stars in SDSS.
    The Astrophysical Journal 04/2009; 699(1). · 6.73 Impact Factor


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