192 research outputs found
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.Comment: 5 pages, including 4 figures. MNRAS (pink pages) in press. Data
available electronically from http://wise-obs.tau.ac.il/~avishay/local.htm
Complex Optical-X-ray Correlations in the Narrow-Line Seyfert 1 Galaxy NGC 4051
This paper presents the results of a dense and intensive X-ray and optical
monitoring of the narrow-line Seyfert 1 galaxy NGC 4051 carried out in 2000.
Results of the optical analysis are consistent with previous measurements. The
amplitude of optical emission line variability is a factor of two larger than
that of the underlying optical continuum, but part or all of the difference can
be due to host-galaxy starlight contamination or due to the lines being driven
by the unseen UV continuum, which is more variable than the optical continuum.
We measured the lag between optical lines and continuum and found a lower, more
accurate broad line region size of 3.0+-1.5 light days in this object. The
implied black hole mass is M_BH=5(+6,-3)x10^5 M_sun; this is the lowest mass
found, so far, for an active nucleus. We find significant evidence for an
X-ray-optical (XO) correlation with a peak lag of about <1 day, although the
centroid of the asymmetric correlation function reveals that part of the
optical flux varies in advance of the X-ray flux by 2.4+-1.0 days. This complex
XO correlation is explained as a possible combination of X-ray reprocessing and
perturbations propagating from the outer (optically emitting) parts of the
accretion disc into its inner (X-ray emitting) region.Comment: 8 pages, 4 figures included, LaTeX mn.sty, accepted for publication
in MNRA
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 0.1550.002 arcseconds in right ascension and negligible yearly proper motion rate in declination. The apparent V magnitude of the star was measured as m_V=13.870.16 mag and spectral identification yielded a spectral type of M3.5Ve0.5 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
A Chandra Survey of the X-ray Properties of Broad Absorption Line Radio-Loud Quasars
This work presents the results of a Chandra study of 21 broad absorption line
(BAL) radio-loud quasars (RLQs). We conducted a Chandra snapshot survey of 12
bright BAL RLQs selected from SDSS/FIRST data and possessing a wide range of
radio and CIV absorption properties. Optical spectra were obtained nearly
contemporaneously with the Hobby-Eberly Telescope; no strong flux or BAL
variability was seen between epochs. We also include in our sample 9 additional
BAL RLQs possessing archival Chandra coverage. We compare the properties of
(predominantly high-ionization) BAL RLQs to those of non-BAL RLQs as well as to
BAL radio-quiet quasars (RQQs) and non-BAL RQQs for context.
All 12 snapshot and 8/9 archival BAL RLQs are detected, with observed X-ray
luminosities less than those of non-BAL RLQs having comparable optical/UV
luminosities by typical factors of 4.1-8.5. (BAL RLQs are also X-ray weak by
typical factors of 2.0-4.5 relative to non-BAL RLQs having both comparable
optical/UV and radio luminosities.) However, BAL RLQs are not as X-ray weak
relative to non-BAL RLQs as are BAL RQQs relative to non-BAL RQQs. While some
BAL RLQs have harder X-ray spectra than typical non-BAL RLQs, some have
hardness ratios consistent with those of non-BAL RLQs, and there does not
appear to be a correlation between X-ray weakness and spectral hardness, in
contrast to the situation for BAL RQQs. RLQs are expected to have X-ray
continuum contributions from both disk-corona and small-scale jet emission.
While the entire X-ray continuum in BAL RLQs cannot be obscured to the same
degree as in BAL RQQs, we calculate that the jet is likely partially covered in
many BAL RLQs. We comment briefly on implications for geometries and source
ages in BAL RLQs.Comment: 48 pages, 5 tables, 14 figures, accepted by Ap
Near Infrared Spectroscopy of High Redshift Active Galactic Nuclei. II. Disappearing Narrow Line Regions and the Role of Accretion
We present new near infrared spectroscopic measurements for 29 luminous
high-z quasars and use the data to discuss the size and other properties of the
NLRs in those sources. The high resolution spectra have been used to carefully
model the Fe II blends and to provide reliable [O III], Fe II and Hb
measurements. We find that about 2/3 of all high luminosity sources show strong
[O III] lines while the remaining objects show no or very weak such line. While
weak [O III] emitters are also found among lower luminosity AGN, we argue that
the implications for very high luminosity objects are different. In particular,
we suggest that the averaging of these two populations in other works gave rise
to claims of a Baldwin relationship in [O III] which is not confirmed by our
data. We also argue that earlier proposed relations of the type R_NLR \propto
L_[O III]^{1/2}, where R_NLR is the NLR radius, are theoretically sound yet
they must break down for R_NLR exceeding a few kpc. This suggests that the NLR
properties in luminous sources are different from those observed in nearby AGN.
In particular, we suggest that some sources lost their very large, dynamically
unbound NLR while others are in a phase of violent star-forming events that
produce a large quantity of high density gas in the central kpc. This gas is
ionized and excited by the central radiation source and its spectroscopic
properties may be different from those observed in nearby, lower luminosity
NLRs. We also discuss the dependence of EW(Hb) and Fe II/Hb on L, M_BH, and
accretion rate for a large sample of AGNs. The strongest dependence of the two
quantities is on the accretion rate and the Fe II/Hb correlation is probably
due to the EW(Hb) dependence on accretion rate. We show the most extreme values
measured so far of Fe II/Hb and address its correlation with EW([O III]).Comment: 10 pages (emulateapj), 9 figures. Accepted by Ap
Locating Star-Forming Regions in Quasar Host Galaxies
We present a study of the morphology and intensity of star formation in the
host galaxies of eight Palomar-Green quasars using observations with the Hubble
Space Telescope. Our observations are motivated by recent evidence for a close
relationship between black hole growth and the stellar mass evolution in its
host galaxy. We use narrow-band [O II] 3727, H, [O III]
5007 and Pa images, taken with the WFPC2 and NICMOS
instruments, to map the morphology of line-emitting regions, and, after
extinction corrections, diagnose the excitation mechanism and infer
star-formation rates. Significant challenges in this type of work are the
separation of the quasar light from the stellar continuum and the
quasar-excited gas from the star-forming regions. To this end, we present a
novel technique for image decomposition and subtraction of quasar light. Our
primary result is the detection of extended line-emitting regions with sizes
ranging from 0.5 to 5 kpc and distributed symmetrically around the nucleus,
powered primarily by star formation. We determine star-formation rates of order
a few tens of M/yr. The host galaxies of our target quasars have
stellar masses of order M and specific star formation rates
on a par with those of M82 and luminous infrared galaxies. As such they fall at
the upper envelope or just above the star-formation mass sequence in the
specific star formation vs stellar mass diagram. We see a clear trend of
increasing star formation rate with quasar luminosity, reinforcing the link
between the growth of the stellar mass of the host and the black hole mass
found by other authors.Comment: Accepted for publication in M.N.R.A.
The X-ray Properties of the Most-Luminous Quasars from the Sloan Digital Sky Survey
Utilizing 21 new Chandra observations as well as archival Chandra, ROSAT, and
XMM-Newton data, we study the X-ray properties of a representative sample of 59
of the most optically luminous quasars in the Universe (M_i~~-29.3 to -30.2)
spanning a redshift range of z~~1.5-4.5. Our full sample consists of 32 quasars
from the Sloan Digital Sky Survey (SDSS) Data Release 3 (DR3) quasar catalog,
two additional objects in the DR3 area that were missed by the SDSS selection
criteria, and 25 comparably luminous quasars at z>~4. This is the largest X-ray
study of such luminous quasars to date. By jointly fitting the X-ray spectra of
our sample quasars, excluding radio-loud and broad absorption line (BAL)
objects, we find a mean X-ray power-law photon index of
Gamma=1.92^{+0.09}_{-0.08} and constrain any neutral intrinsic absorbing
material to have a mean column density of N_H<~2x10^{21} cm^{-2}. We find,
consistent with other studies, that Gamma does not change with redshift, and we
constrain the amount of allowed Gamma evolution for the most-luminous quasars.
Our sample, excluding radio-loud and BAL quasars, has a mean X-ray-to-optical
spectral slope of a_ox=-1.80+/-0.02, as well as no significant evolution of
a_ox with redshift. We also comment upon the X-ray properties of a number of
notable quasars, including an X-ray weak quasar with several strong narrow
absorption-line systems, a mildly radio-loud BAL quasar, and a well-studied
gravitationally lensed quasar.Comment: 18 pages (emulateapj), 11 figures. Accepted for publication in The
Astrophysical Journa
The X-ray Spectral Properties and Variability of Luminous High-Redshift Active Galactic Nuclei
We perform a detailed investigation of moderate-to-high quality X-ray spectra
of ten of the most luminous active galactic nuclei (AGNs) known at z>4 (up to
z~6.28). This study includes five new XMM observations and five archived X-ray
observations (four by XMM and one by Chandra). We find that the X-ray power-law
photon indices of our sample, composed of eight radio-quiet sources and two
that are moderately radio loud, are not significantly different from those of
lower redshift AGNs. The upper limits obtained on intrinsic neutral hydrogen
column densities, N_H<~10^{22}-10^{23} cm^{-2}, indicate that these AGNs are
not significantly absorbed. A joint fit performed on our eight radio-quiet
sources, with a total of ~7000 photons, constrains the mean photon index of z>4
radio-quiet AGNs to Gamma=1.97^{+0.06}_{-0.04}, with no detectable intrinsic
dispersion from source to source. We also obtain a strong constraint on the
mean intrinsic column density, N_H<~3x10^{21} cm^{-2}, showing that optically
selected radio-quiet AGNs at z>4 are, on average, not more absorbed than their
lower-redshift counterparts. All this suggests that the X-ray production
mechanism and the central environment in radio-quiet AGNs have not
significantly evolved over cosmic time. The mean equivalent width of a putative
neutral narrow Fe Ka line is constrained to be <~190 eV, and similarly we place
constraints on the mean Compton reflection component (R<~1.2). None of the AGNs
varied on short (~1 hr) timescales, but on longer timescales (months-to-years)
strong variability is observed in four of the sources. In particular, the X-ray
flux of the z=5.41 radio-quiet AGN SDSS 0231-0728 dropped by a factor of ~4
over a rest-frame period of 73 d. This is the most extreme X-ray variation
observed in a luminous z>4 radio-quiet AGN.Comment: 10 pages (emulateapj), 5 figures. Accepted by Ap
Fast-Growing SMBHs in Fast-Growing Galaxies, at High Redshifts: The Role of Major Mergers As Revealed by ALMA
We present a long-term, multi-wavelength project to understand the epoch of
fastest growth of the most massive black holes by using a sample of 40 luminous
quasars at z~4.8. These quasars have rather uniform properties, with typical
accretion rates and black hole masses of L/L_Edd~0.7 and M_BH~10^9 M_sol. The
sample consists of "FIR-bright" sources with a previous Herschel/SPIRE
detection, suggesting SFR > 1000 M_sol/yr, as well as of "FIR-faint" sources
for which Herschel stacking analysis implies a typical SFR of ~400 M_sol/yr.
Six of the quasars have been observed by ALMA in [CII]{\lambda}157.74 micron
line emission and adjacent rest-frame 150 micron continuum, to study the dusty
cold ISM. ALMA detected companion, spectroscopically confirmed sub-mm galaxies
(SMGs) for three sources - one FIR-bright and two FIR-faint. The companions are
separated by ~14-45 kpc from the quasar hosts, and we interpret them as major
galaxy interactions. Our ALMA data therefore clearly support the idea that
major mergers may be important drivers for rapid, early SMBH growth. However,
the fact that not all high-SFR quasar hosts are accompanied by interacting
SMGs, and their ordered gas kinematics observed by ALMA, suggest that other
processes may be fueling these systems. Our analysis thus demonstrates the
diversity of host galaxy properties and gas accretion mechanisms associated
with early and rapid SMBH growth
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