arXiv subjects

Ohad Shemmer

Publications and source records attributed to Ohad Shemmer.

68 records · Page 4Linked to original sources

arXiv2009

High-Redshift SDSS Quasars with Weak Emission Lines

We identify a sample of 74 high-redshift quasars (z>3) with weak emission lines from the Fifth Data Release of the Sloan Digital Sky Survey and present infrared, optical, and radio observations of a subsample of four objects at z>4. These weak emission-line quasars (WLQs) constitute a prominent tail of the Lya+NV equivalent width distribution, and we compare them to quasars with more typical emission-line properties and to low-redshift active galactic nuclei with weak/absent emission lines, namely BL Lac objects. We find that WLQs exhibit hot (T~1000 K) thermal dust emission and have rest-frame 0.1-5 micron spectral energy distributions that are quite similar to those of normal quasars. The variability, polarization, and radio properties of WLQs are also different from those of BL Lacs, making continuum boosting by a relativistic jet an unlikely physical interpretation. The most probable scenario for WLQs involves broad-line region properties that are physically distinct from those of normal quasars.

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arXiv2009

X-ray Insights into the Nature of Weak Emission-Line Quasars at High Redshift

We present Chandra observations of nine high-redshift quasars (z=2.7-5.9) discovered by the Sloan Digital Sky Survey with weak or undetectable high-ionization emission lines in their UV spectra (WLQs). Adding archival X-ray observations of six additional sources of this class has enabled us to place the strongest constraints yet on the X-ray properties of this remarkable class of AGNs. Although our data cannot rule out the possibility that the emission lines are overwhelmed by a relativistically boosted continuum, as manifested by BL Lac objects, we find that WLQs are considerably weaker in the X-ray and radio bands than the majority of BL Lacs found at much lower redshifts. If WLQs are high-redshift BL Lacs, then it is difficult to explain the lack of a large parent population of X-ray and radio bright weak-lined sources at high redshift. We also consider the possibility that WLQs are quasars with extreme properties, and in particular that the emission lines are suppressed by high accretion rates. Using joint spectral fitting of the X-ray spectra of 11 WLQs we find that the mean photon index in the hard X-ray band is consistent with those observed in typical radio-quiet AGNs with no hint of an unusually steep hard-X-ray spectrum. This result poses a challenge to the hypothesis that WLQs have extremely high accretion rates, and we discuss additional observations required to test this idea.

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arXiv2008

The Hard X-ray Spectrum as a Probe for Black-Hole Growth in Radio-Quiet Active Galactic Nuclei

We study the hard-X-ray spectral properties of ten highly luminous radio-quiet (RQ) active galactic nuclei (AGNs) at z=1.3-3.2, including new XMM-Newton observations of four of these sources. We find a significant correlation between the normalized accretion rate (L/L_Edd) and the hard-X-ray photon index (Gamma) for 35 moderate-high luminosity RQ AGNs including our ten highly luminous sources. Within the limits of our sample, we show that a measurement of Gamma and L_X can provide an estimate of L/L_Edd and black-hole (BH) mass (M_BH) with a mean uncertainty of a factor of <~3 on the predicted values of these properties. This may provide a useful probe for tracing the history of BH growth in the Universe, utilizing samples of X-ray-selected AGNs for which L/L_Edd and M_BH have not yet been determined systematically. It may prove to be a useful way to probe BH growth in distant Compton-thin type 2 AGNs. We also find that the optical-X-ray spectral slope (a_ox) depends primarily on optical-UV luminosity rather than on L/L_Edd in a sample of RQ AGNs spanning five orders of magnitude in luminosity and over two orders of magnitude in L/L_Edd. We detect a significant Compton-reflection continuum in two of our highly luminous sources, and in the stacked X-ray spectrum of seven other sources with similar luminosities, we obtain a mean relative Compton reflection of R=0.9^{+0.6}_{-0.5} and an upper limit on the rest-frame equivalent width of a neutral Fe Ka line of 105 eV. We do not detect a significant steepening of the X-ray power-law spectrum below rest-frame 2 keV in any of our highly luminous sources, suggesting that a soft-excess feature, commonly observed in local AGNs, either does not depend strongly on L/L_Edd, or is not accessible at high redshifts using current X-ray detectors. [Abridged]

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arXiv2007

Black-Hole Mass and Growth Rate at High Redshift

We present new H and K bands spectroscopy of 15 high luminosity active galactic nuclei (AGNs) at redshifts 2.3-3.4 obtained on Gemini South. We combined the data with spectra of additional 29 high-luminosity sources to obtain a sample with 10^{45.2}<λL_λ(5100A)<10^{47.3} ergs/sec and black hole (BH) mass range, using reverberation mapping relationships based on the H_beta method, of 10^{8.8}-10^{10.7} M_sun. We do not find a correlation of L/L_Edd with M_BH but find a correlation with λL_λ(5100A) which might be due to selection effects. The L/L_Edd distribution is broad and covers the range ~0.07-1.6, similar to what is observed in lower redshift, lower luminosity AGNs. We suggest that this consistently measured and calibrated sample gives the best representation of L/L_Edd at those redshifts and note potential discrepancies with recent theoretical and observational studies. The lower accretion rates are not in accord with growth scenarios for BHs at such redshifts and the growth times of many of the sources are longer than the age of the universe at the corresponding epochs. This suggests earlier episodes of faster growth at z>~3 for those sources. The use of the C IV method gives considerably different results and a larger scatter; this method seems to be a poor M_BH and L/L_Edd estimator at very high luminosity.

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arXiv2006

Reverberation Mapping of High-Luminosity Quasars: First Results

Reverberation mapping of nearby active galactic nuclei has led to estimates of broad-line-region (BLR) sizes and central-object masses for some 37 objects to date. However, successful reverberation mapping has yet to be performed for quasars of either high luminosity (above L_opt~10^{46} erg/s) or high redshift (z>0.3). Over the past six years, we have carried out, at the Hobby-Eberly Telescope, rest-frame-ultraviolet spectrophotometric monitoring of a sample of six quasars at redshifts z=2.2--3.2, with luminosities of L_opt~10^{46.4}--10^{47.6} erg/s, an order of magnitude greater than those of previously mapped quasars. The six quasars, together with an additional five having similar redshift and luminosity properties, were monitored photometrically at the Wise Observatory during the past decade. All 11 quasars monitored show significant continuum variations of order 10%--70%. This is about a factor of two smaller variability than for lower luminosity quasars monitored over the same rest-frame period. In the six objects which have been spectrophotometrically monitored, significant variability is detected in the CIV1550 broad emission line. In several cases the variations track the continuum variations in the same quasar, with amplitudes comparable to, or even greater than, those of the corresponding continua. In contrast, no significant Lyαvariability is detected in any of the four objects in which it was observed. Thus, UV lines may have different variability trends in high-luminosity and low-luminosity AGNs. For one quasar, S5~0836+71 at z=2.172, we measure a tentative delay of 595 days between CIV and UV-continuum variations, corresponding to a rest-frame delay of 188 days and a central black-hole mass of 2.6\times10^9 M_\odot.

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arXiv2006

The Hard X-ray Spectral Slope as an Accretion-Rate Indicator in Radio-Quiet Active Galactic Nuclei

We present new XMM-Newton observations of two luminous and high accretion-rate radio-quiet active galactic nuclei (AGNs) at z~2. Together with archival X-ray and rest-frame optical spectra of three sources with similar properties as well as 25 moderate-luminosity radio-quiet AGNs at z<0.5, we investigate, for the first time, the dependence of the hard (>~2 keV) X-ray power-law photon index on the broad H_beta emission-line width and on the accretion rate across ~3 orders of magnitude in AGN luminosity. Provided the accretion rates of the five luminous sources can be estimated by extrapolating the well-known broad-line region size-luminosity relation to high luminosities, we find that the photon indices of these sources, while consistent with those expected from their accretion rates, are significantly higher than expected from the widths of their H_beta lines. We argue that, within the limits of our sample, the hard-X-ray photon index depends primarily on the accretion rate.

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arXiv2006

Chandra Observations of the Highest Redshift Quasars from the Sloan Digital Sky Survey

We present new Chandra observations of 21 z>4 quasars, including 11 sources at z>5. These observations double the number of X-ray detected quasars at z>5, allowing investigation of the X-ray spectral properties of a substantial sample of quasars at the dawn of the modern Universe. By jointly fitting the spectra of 15 z>5 radio-quiet quasars (RQQs), including sources from the Chandra archive, with a total of 185 photons, we find a mean X-ray power-law photon index of Gamma=1.95^{+0.30}_{-0.26}, and a mean neutral intrinsic absorption column density of N_H<~6x10^{22} cm^{-2}. These results show that quasar X-ray spectral properties have not evolved up to the highest observable redshifts. We also find that the mean optical-X-ray spectral slope (alpha_ox) of optically-selected z>5 RQQs, excluding broad absorption line quasars, is alpha_ox=-1.69+/-0.03, which is consistent with the value predicted from the observed relationship between alpha_ox and ultraviolet luminosity. Four of the sources in our sample are members of the rare class of weak emission-line quasars, and we detect two of them in X-rays. We discuss the implications our X-ray observations have for the nature of these mysterious sources and, in particular, whether their weak-line spectra are a consequence of continuum boosting or a deficit of high-ionization line emitting gas.

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arXiv2005

New XMM-Newton Spectroscopy of the Most Luminous and Distant Quasars

In the two parts of this contribution we describe two related XMM-Newton programs. The first part summarizes our study of the X-ray spectral properties and variability of z>4 quasars (Shemmer et al. 2005). The second part presents preliminary results from our ongoing XMM-Newton program to investigate the X-ray spectral properties and variability of luminous, high accretion-rate quasars at z~2--3. We find that the X-ray photon index does not depend on luminosity or redshift, and there is suggestive evidence that it may depend on the accretion rate. None of our quasars is significantly absorbed, and none shows signatures of reflection. By jointly fitting high-quality spectra of eight radio-quiet z>4 quasars, including three from our XMM-Newton observations, we place tight constraints on the mean X-ray spectral properties of such sources. Most of our quasars are significantly X-ray variable on timescales of months--years, but none shows rapid (~1 hr timescale) variations.

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arXiv2002

Reverberation Mapping of High-Redshift, High-Luminosity Quasars

We present preliminary results from a reverberation mapping program to measure the Broad Line Region size in high-redshift, high-luminosity quasars. The observations are carried out at the Hobby-Eberly Telescope and at the Wise Observatory. The data cover 8 yr of photometric monitoring of 11 quasars, and 2.5 yr of spectrophotometric monitoring of 7 of these sources. Thus far we detected continuum variations but no line variations. We find that the continua of the high-luminosity quasars have smaller variability amplitudes and longer variability timescales compared with low-luminosity AGNs.

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arXiv2002

Is There a Metallicity--Luminosity Relationship in AGN? The Case of Narrow-Line Seyfert 1 Galaxies

The well known relationship between metallicity and luminosity in AGN is addressed by introducing new metallicity measurements (based on the method of Hamann & Ferland 1993; HF93) for a sample of narrow-line Seyfert 1 (NLS1) galaxies. Our new results, based on a sample of 162 AGN, including 9 NLS1s, indicate that, while broad-line AGN trace a metallicity--luminosity power law with an index of ~0.2, NLS1s deviate significantly from this relationship at low luminosities. Adopting the HF93 method based on the N V/C IV line ratio, we find that NLS1 metallicities are similar to those of some high-redshift high-luminosity quasars. We also examined the N IV]/C IV line ratio and compared it with N V/C IV in a sample of 30 sources including several NLS1s. We find that the two do not give a consistent answer regarding the N/C abundance ratio. This result is marginal because of the quality of the data. We suggest two alternative explanations to these results: 1) The HF93 metallicity--luminosity dependence is not a simple two-parameter dependence and there is an additional hidden variable in this relationship that has not yet been discovered. The additional parameter may be the accretion rate, the age of the central stellar cluster or, perhaps, something else. 2) The strong line ratios involving N V 1240A suggested by HF93 are not adequate metallicity indicators for NLS1s and perhaps also other AGN for reasons that are not yet fully understood.

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arXiv2002

Supernova 2002ap - The First Month

Supernova (SN) 2002ap in M74 was discovered on January 29, 2002. Being one of the nearest (10 Mpc) SN events in the last decades, and spectroscopically similar to the so-called ``hypernovae'' 1997ef and 1998bw, both possibly associated with gamma-ray bursts (GRBs), it is of great interest. Shortly after its discovery, we launched an intensive photometric and spectroscopic monitoring campaign of this event, and here we report the results of the first month of observations. We use our UBVRI photometry to estimate the magnitudes at, and dates of, peak brightness. Our data suggest that this object reached its peak B-band luminosity on Feb. 7.1(-1.3)(+2) UT. Based on its similarity to SN 1998bw, we estimate the range of possible dates for a GRB that may have been associated with SN 2002ap. We find that it may include dates outside the time frame for which all available gamma-ray data have been intensively scanned, according to recent reports. The absolute magnitude at peak brightness of SN 2002ap (M_B = -16.9) shows that it was significantly fainter than SN 1998bw, or normal type-Ia SNe, but similar to SN 1997ef. Our spectroscopic observations confirm that SN 2002ap is strikingly similar to SNe 1998bw and 1997ef. We briefly describe the spectral evolution of this object. To assist other observers and to stimulate theoretical models, we make our entire data set publicly available in digital form.

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arXiv2000

Optical Monitoring of NLS1s at the Wise Observatory

We report the preliminary results of a spectrophotometric monitoring program of Narrow-Line Seyfert 1 galaxies conducted at the Wise Observatory in 1999. The data we collected will be used to evaluate the line-to-continuum time lag in an attempt to test the idea that the accretion rate in NLS1s is larger than in other Seyfert 1 galaxies.

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arXiv1999

Periodic Microvariation of B416, a New LBV in M33

We report optical photometric and spectroscopic observations of the M33 star B416, establishing it as the fifth identified LBV in that galaxy. The B magnitude of the star varies with a period of 8.26 day and an amplitude of 0.03 magnitude. The red end of the optical spectrum does not seem to partake in these oscillations, which we identify as the microvariations, known to persist in LBV stars. B416 is the first LBV with such oscillations that are strictly periodic and coherent over at least 10 years. The brightness of the star is also varying on a longer (years) time scale, again mainly in its blue light. The spectrum has most of the characteristics of an LBV in quiescence, showing Balmer, He I, Fe II and [Fe II] emission lines. The flux and profile of some of these lines are varying but the scarcity of the observations prevents us from establishing their characteristic time scale. B416 is at the centre of an H II region, another indication of the relatively young age of this stellar system.

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arXiv1998

The Discovery of a Nearby M Dwarf

We report the discovery of a nearby M dwarf star, found accidentally while observing the old nova DN Gem at the Wise Observatory. The star is designated 1200-05296925 in the PMM USNO-A1.0 catalogue and its coordinates, calculated for 1997 November 27 are: RA=6 55 05.13 Dec = +32 09 54.1 (Equinox J2000, Epoch J1997.90). Astrometric measurements for the star yielded a yearly proper motion rate of 0.155$\pm$0.002 arcseconds in right ascension and negligible yearly proper motion rate in declination. The apparent V magnitude of the star was measured as m_V=13.87$\pm$0.16 mag and spectral identification yielded a spectral type of M3.5Ve$\pm$0.5 subclasses. Using relations between spectral type and absolute V magnitude in M dwarfs, we arrive at an absolute magnitude of M_V=12.3^{+1.2}_{-1.1} mag, which corresponds to a distance of 21^{+15}_{-10} pc.

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