arXiv subjects

Karlen Shahinyan

Publications and source records attributed to Karlen Shahinyan.

5 recordsLinked to original sources

arXiv2016

VERITAS and VLBA Observations of HESS J1943+213

HESS J1943+213 is a very high energy (VHE; 100 GeV) γ-ray source in the direction of the Galactic plane. 38 hours of deep VERITAS observations taken over two seasons detect the source with ~20 σ significance. Monitoring observations of HESS J1943+213 show a remarkably stable flux and spectrum in VHE γ-rays. Studies exploring the classification of HESS J1943+213 are converging towards accepting the source as an extreme synchrotron BL Lac object. Specifically, overall SED characteristics of the source, the detection of a potential host galaxy in near-IR imaging, and VLBI observations of the HESS J1943+213 radio counterpart showing extended jet-like emission at milliarcsecond scale and core flux density variability establish the source as a blazar. Recent Very Long Baseline Array (VLBA) observations of the source (shown here for the first time) confirm the extended structure found in the 1.6 GHz band and detect the jet-like component in the 4.6 GHz and the 7.3 GHz bands. The spectral indices of the core and the jet-like components derived from the VLBA observations are in a range typical for blazars. HESS J1943+213 fits the extreme HBL description; however, with variability detected only at milliarcseconds scales in radio, it appears to be an abnormally stable VHE γ-ray blazar.

astro-ph.HE↗

arXiv2015

VERITAS Observations of HESS J1943+213

HESS J1943+213 is a very-high-energy (VHE; >100 GeV) gamma-ray point source detected during the H.E.S.S. Galactic Plane Survey. Radio, infrared, X-ray, and GeV gamma-ray counterparts have been identified for HESS J1943+213; however, the classification of the source is still uncertain. Recent publications have argued primarily in favor of an extreme BL Lac object behind the Galactic plane, though the scenario that HESS J1943+213 is a young pulsar wind nebula is viable as well. We present deep VERITAS observations of HESS J1943+213, which provide the most significant VHE detection of the source so far, with ~18 sigma excess. The source is detected at ~2% Crab Nebula flux above 200 GeV with VERITAS, with the source spectrum well fit by a power-law function and showing agreement with the H.E.S.S. detection. We also include results from analysis of Swift XRT observations contemporaneous with VERITAS. No significant flux or spectral variability is detected with VERITAS or Swift XRT observations. We place the VERITAS results in a multi-wavelength context to comment on the HESS J1943+213 classification.

astro-ph.HE↗

arXiv2015

VERITAS Observations of the Unidentified Point Source HESS J1943+213

The H.E.S.S. Galactic plane scan has revealed a large population of Galactic very high energy (VHE; E > 100 GeV) emitters. The majority of the galactic sources are extended and can typically be associated with pulsar wind nebulae (35%) and supernova remnants (21%), while some of the sources remain unidentified (31%). A much smaller fraction of point-like sources (5 in total, corresponding to 4%) are identified as gamma-ray binaries. Active galactic nuclei located behind the Galactic plane are also a potential source class. An active galaxy could be identified in the VHE regime by a point-like appearance, a high variability amplitude (up to a factor of 100) and a typically soft spectrum (due to absorption by the extra-galactic background light). Here we report on VERITAS observations of HESS J1943+213, an unidentified point source discovered to emit above 470 GeV during the extended H.E.S.S. Galactic plane scan. This source is thought to be a distant BL Lac object behind the Galactic plane and, though it exhibits a steep spectrum it is a weak GeV source, only recently detected using 5 years of Fermi-LAT data. Deep VERITAS observations at high elevations result in the most significant VHE detection of this object so far, with an excess above 200 GeV of more than 18 standard deviations. We use variability and spectral analyses of VERITAS data on HESS J1943+213 in a multi-wavelength context to address the source classification.

astro-ph.HE↗

arXiv2014

Black Holes at the Centers of Nearby Dwarf Galaxies

Using a distance-limited portion of the Sloan Digital Sky Survey (SDSS) Data Release 7, we have identified 28 active galactic nuclei (AGNs) in nearby (d < 80 Mpc) low-mass, low-luminosity dwarf galaxies. The accreting objects at the galaxy centers are expected to be intermediate-mass black holes (IMBHs) with M_BH < 1e6 M_sun. The AGNs were selected using several optical emission-line diagnostics after careful modeling of the continuum present in the spectra. We have limited our survey to objects with spectral characteristics similar to those of Seyfert nuclei, excluding emission-line galaxies with ambiguous spectra that could be powered by stellar processes. The host galaxies in our sample are thus the least massive objects in the very local universe certain to contain central black holes. Given our focus on the nearest objects included in the SDSS, our survey is more sensitive to low-luminosity emission than previous optical searches for AGNs in low-mass galaxies. The [O III] lambda5007 luminosities of the Seyfert nuclei in our sample have a median value of L_5007 = 2e5 L_sun and extend down to 1e4 L_sun. Using published data for broad-line IMBH candidates, we have derived an [O III] bolometric correction of log (L_bol/L_5007) = 3.0 +/- 0.3, which is significantly lower than values obtained for high-luminosity AGNs. Applying this correction to our sample, we obtain minimum black-hole mass estimates that fall mainly in the 10^3 M_sun -- 10^4 M_sun range, which is roughly where the predicted mass functions for different black-hole seed formation scenarios overlap the most. In the stellar mass range that includes the bulk of the AGN host galaxies in our sample, we derive a lower limit on the AGN fraction of a few percent, indicating that active nuclei in dwarf galaxies are not as rare as previously thought.

astro-ph.GA↗

arXiv2009

Clumpy Galaxies in GOODS and GEMS: Massive Analogs of Local Dwarf Irregulars

Clumpy galaxies in the GEMS and GOODS fields are examined for clues to their evolution into modern spirals. The magnitudes of the clumps and the surface brightnesses of the interclump regions are measured and fitted to models of stellar age and mass. There is an evolutionary trend from clump clusters with no evident interclump emission to clump clusters with faint red disks, to spiral galaxies of the flocculent or grand design types. Along this sequence, the interclump surface density increases and the mass surface density contrast between the clumps and the interclump regions decreases, suggesting a gradual dispersal of clumps to form disks. Also along this sequence, the bulge-to-clump mass ratios and age ratios increase, suggesting a gradual formation of bulges. All of these morphological types occur in the same redshift range, indicating that the clump cluster morphology is not the result of bandshifting. Comparisons to local galaxies with the same rest wavelength and spatial resolution show that clump clusters resemble local dwarf Irregulars. This resemblance is consistent with a model in which the clumpy morphology comes from gravitational instabilities in gas with a high turbulent speed compared to the rotation speed and a high mass fraction compared to the stars.

astro-ph.CO↗