arXiv subjects

Avishay Gal-Yam

Publications and source records attributed to Avishay Gal-Yam.

At least 235 records · Page 13Linked to original sources

arXiv2006

The Local Rate and the Progenitor Lifetimes of Short-Hard Gamma-Ray Bursts: Synthesis and Predictions for LIGO

The recent discovery of the first four afterglows of short-hard gamma-ray bursts (SHBs), suggests that they typically result from long-lived progenitor systems. The most popular progenitor model invokes the merger of either double neutron star (DNS) binaries or neutron star-black hole (NS-BH) systems. Such events are strong sources of gravitational-waves (GWs) and might be detected by ground-based GW observatories. In this work we combine the census of SHB observations with refined theoretical analysis to perform a critical evaluation of the compact binary model. We then explore the implications for GW detection of these events. Beginning from the measured star-formation rate through cosmic time, we consider what intrinsic luminosity and lifetime distributions can reproduce the known SHB redshifts and luminosities as well as the peak flux distribution of the large BATSE SHB sample. We find that: (1) The typical progenitor lifetime is long. Assuming lognormal lifetime distribution the typical lifetime is >4[1] Gyr (2[3]σc.l.). If the lifetime distribution is a power-law with index ηthen η>-0.5 [-1] (2[3] σc.l.). This result is difficult to reconcile with the properties of the observed galactic DNS population, suggesting that if SHBs do result from DNS mergers then the observed galactic binaries do not represent the cosmic one. (2) We find that the local rate of SHBs is larger than 10Gpc^-3/yr and may be higher by several orders of magnitude, significantly above previous estimates. (3) Assuming that SHBs do result from compact binaries, our predictions for the LIGO and VIRGO event rates are encouraging: The chance for detection by current facilities is not negligible, while a coincident detection of GW and electromagnetic radiation from an SHB is guaranteed for next-generation observatories.

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arXiv2006

Towards a Cosmological Hubble Diagram for Type II-P Supernovae

We present the first high-redshift Hubble diagram for Type II-P supernovae (SNe II-P) based upon five events at redshift up to z~0.3. This diagram was constructed using photometry from the Canada-France-Hawaii Telescope Supernova Legacy Survey and absorption line spectroscopy from the Keck observatory. The method used to measure distances to these supernovae is based on recent work by Hamuy & Pinto (2002) and exploits a correlation between the absolute brightness of SNe II-P and the expansion velocities derived from the minimum of the Fe II 516.9 nm P-Cygni feature observed during the plateau phases. We present three refinements to this method which significantly improve the practicality of measuring the distances of SNe II-P at cosmologically interesting redshifts. These are an extinction correction measurement based on the V-I colors at day 50, a cross-correlation measurement for the expansion velocity and the ability to extrapolate such velocities accurately over almost the entire plateau phase. We apply this revised method to our dataset of high-redshift SNe II-P and find that the resulting Hubble diagram has a scatter of only 0.26 magnitudes, thus demonstrating the feasibility of measuring the expansion history, with present facilities, using a method independent of that based upon supernovae of Type Ia.

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arXiv2006

Models for the Type Ic Hypernova SN 2003lw associated with GRB 031203

The Gamma-Ray Burst 031203 at a redshift z=0.1055 revealed a highly reddened Type Ic Supernova, SN 2003lw, in its afterglow light. This is the third well established case of a link between a long-duration GRB and a type Ic SN. The SN light curve is obtained subtracting the galaxy contribution and is modelled together with two spectra at near-maximum epochs. A red VLT grism 150I spectrum of the SN near peak is used to extend the spectral coverage, and in particular to constrain the uncertain reddening, the most likely value for which is E_{G+H}(B-V) about 1.07 +/- 0.05. Accounting for reddening, SN 2003lw is about 0.3 mag brighter than the prototypical GRB-SN 1998bw. Light curve models yield a 56Ni mass of about 0.55 solar mass. The optimal explosion model is somewhat more massive (ejecta mass about 13 solar mass) and energetic (kinetic energy about 6 times 10^52 erg) than the model for SN 1998bw, implying a massive progenitor (40 - 50 solar mass). The mass at high velocity is not very large (1.4 solar mass above 30000 km/s, but only 0.1 solar mass above 60000 km/s), but is sufficient to cause the observed broad lines. The similarity of SNe 2003lw and 1998bw and the weakness of their related GRBs, GRB031203 and GRB980425, suggest that both GRBs may be normal events viewed slightly off-axis or a weaker but possibly more frequent type of GRB.

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arXiv2006

A non-spherical core in the explosion of supernova SN 2004dj

An important and perhaps critical clue to the mechanism driving the explosion of massive stars as supernovae is provided by the accumulating evidence for asymmetry in the explosion. Indirect evidence comes from high pulsar velocities, associations of supernovae with long-soft gamma-ray bursts, and asymmetries in late-time emission-line profiles. Spectropolarimetry provides a direct probe of young supernova geometry, with higher polarization generally indicating a greater departure from spherical symmetry. Large polarizations have been measured for 'stripped-envelope' (that is, type Ic) supernovae, which confirms their non-spherical morphology; but the explosions of massive stars with intact hydrogen envelopes (type II-P supernovae) have shown only weak polarizations at the early times observed. Here we report multi-epoch spectropolarimetry of a classic type II-P supernova that reveals the abrupt appearance of significant polarization when the inner core is first exposed in the thinning ejecta (~90 days after explosion). We infer a departure from spherical symmetry of at least 30 per cent for the inner ejecta. Combined with earlier results, this suggests that a strongly non-spherical explosion may be a generic feature of core-collapse supernovae of all types, where the asphericity in type II-P supernovae is cloaked at early times by the massive, opaque, hydrogen envelope.

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arXiv2005

The Progenitors of Short-Hard Gamma-Ray Bursts from an Extended Sample of Events

The detection of the afterglow emission and host galaxies of short-hard gamma-ray bursts (SHBs) is one of the most exciting recent astronomical discoveries. Indications that SHB progenitors belong to old stellar populations, in contrast to those of the long-soft GRBs, provide a strong clue about their physical nature. Definitive conclusions however are limited by the small number of SHBs with known hosts. Here, we present our investigation of SHBs previously localized by the interplanetary network (IPN) using new and archival optical and X-ray observations. We show that we can likely identify the host galaxies/clusters for additional two bursts, significantly increasing the sample of SHBs with known hosts and/or distances. In particular, we determine that the bright SHB 790613 occurred within the rich galaxy cluster Abell 1892, making it probably the nearest SHB currently known. We show that the brightest galaxy within the error box of SHB 000607, at z=0.14, is most likely the host galaxy of this event. Additionally, we rule out the existence of galaxy overdensities (down to ~21mag) near the locations of two other SHBs, and set a lower limit on their probable redshift. We combine our SHB sample with events discovered recently by the Swift and HETE-2 missions, and investigate the properties of the extended sample. Comparison to SNe Ia shows that the progenitors of SHBs are typically older, implying a typical life time of several Gy. We also show that it is unlikely that there is a significant population of progenitors with life time shorter than 1Gy. This result disfavors the popular model of NS-NS mergers as the progenitors of SHBs.

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arXiv2005

The distances of short-hard GRBs and the SGR connection

We present a search for nearby (D<100 Mpc) galaxies in the error boxes of six well-localized short-hard gamma-ray bursts (GRBs). None of the six error boxes reveals the presence of a plausible nearby host galaxy. This allows us to set lower limits on the distances and, hence, the isotropic-equivalent energy of these GRBs. Our lower limits are around $1 \times 10^{49}$ erg (at $2σ$ confidence level); as a consequence, some of the short-hard GRBs we examine would have been detected by BATSE out to distances greater than 1 Gpc and therefore constitute a bona fide cosmological population. Our search is partially motivated by the December 27, 2004 hypergiant flare from SGR 1806-20, and the intriguing possibility that short-hard GRBs are extragalactic events of a similar nature. Such events would be detectable with BATSE to a distance of \~50 Mpc, and their detection rate should be comparable to the actual BATSE detection rate of short-hard GRBs. The failure of our search, by contrast, suggests that such flares constitute less than 15% of the short-hard GRBs (<40% at 95% confidence). We discuss possible resolutions of this discrepancy.

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arXiv2005

A Complete Survey of the Transient Radio Sky and Implications for Gamma-Ray Bursts, Supernovae, and other Relativistic Explosions

We had previously reported on a survey for radio transients, used to set an upper limit on the number of orphan gamma-ray burst (GRB) radio afterglows, and thus a lower limit on the typical GRB beaming factor. Here we report radio and optical follow-up observations of these possible transients, achieving the first full characterization of the transient radio sky. We find that only two source are likely to be real radio transients, an optically obscured radio supernova (SN) in the nearby galaxy NGC 4216, and a source not associated with a bright host galaxy, which is too radio luminous to be a GRB afterglow. We speculate that this may be a flare from a peculiar active galactic nucleus, or a burst from an unusual Galactic compact object. We place an upper limit of 65 radio transients above 6 mJy over the entire sky at the 95% confidence level. The implications are as follows. First, we derive a limit on the typical beaming of GRBs; we find f_b^{-1} >~ 60, ~5 times higher than our earlier results. Second, we impose an upper limit on the rate of events that eject >~ 10^{51} erg in unconfined relativistic ejecta, whether or not accompanied by detectable emission in wavebands other than the radio. Our estimated rate, <=1000/y/Gpc, is about two orders of magnitude smaller than the rate of core-collapse SNe (and type Ib/c events in particular), indicating that only a minority of such events eject significant amounts of relativistic material, which are required by fireball models of long-soft GRBs. Finally, we show that future wider and/or deeper radio variability surveys are expected to detect numerous orphan radio GRB afterglows. Our survey also illustrates the great potential of sensitive surveys with new instruments to revolutionize the study of nearby SNe (abridged).

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arXiv2004

Photometric Typing Analyses of Three Young Supernovae with the Robotic Palomar 60-Inch Telescope

We present photometric typing analyses of three young supernovae observed with the Robotic 60-inch Telescope at Palomar Observatory (P60). This represents the first time that such photo-typing has been attempted, in a blind fashion, on newly-discovered supernovae. For one of the target supernovae, SN2004cs, our photometry provided the first constraint on the SN type, which we predicted would be type Ia. To the contrary, however, our subsequent Keck spectroscopy shows it to be an unusual type II supernova. For each of the other two supernovae, SN2004dh (type II) and SN2004dk (type Ib), our photo-typing results are consistent with the known type of the event as determined from ground-based spectroscopy; however, the colors of SN2004dk are also consistent with a type Ic or type II classification. We discuss our approach to the challenges of photo-typing - contamination by host galaxy light and the unknown photometric quality of the data - in the case where it is desirable to complete the analysis with just one night of observations. The growing interest in the properties and behavior of very young supernovae, and the increased discovery rate for such events, mean that prompt photo-typing analyses can provide useful input to observational campaigns. Our results demonstrate the value and feasibility of such a project for P60, at the same time illustrating its chief inherent shortcoming: an inability to identify new and unusual events as such without later spectroscopic observations.

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arXiv2004

Photometric Identification of Young Stripped-Core Supernovae

We present a method designed to identify the spectral type of young (less than ~30 days after explosion) and nearby (z < ~0.05) supernovae (SNe) using their broad-band colors. In particular, we show that stripped-core SNe (i.e., hydrogen deficient core-collapse events, spectroscopically defined as SNe Ib and SNe Ic, including broad-lined SN 1998bw-like events) can be clearly distinguished from other types of SNe. Using the full census of nearby SNe discovered during the year 2002, we estimate the impact that prompt multi-band photometry, obtained by 1m class telescopes, would have on the early identification of stripped-core events. Combining this new approach with ongoing spectroscopic follow-up programs, one can expect ~20 nearby, stripped-core events to be identified, each year, around, or before, maximum light. Follow-up studies, including prompt, multi-epoch optical spectroscopy and spectropolarimetry, as well as radio and X-ray observations, could greatly increase our understanding of these events, and shed new light on their association with cosmological gamma-ray bursts.

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arXiv2003

Constraints on SN Ia Progenitors and ICM enrichment from field and cluster SN rates

The iron mass in galaxy clusters is about 6 times larger than could have been produced by core-collapse SNe, assuming the stars in cluster galaxies formed with a standard IMF. Type-Ia SNe have been proposed as the alternative dominant iron source. We use our HST measurements of the cluster SN-Ia rate at high redshift to study the cluster iron enrichment scenario. The measurements can constrain the star-formation epoch and the SN-Ia progenitor models via the mean delay time between the formation of a stellar population and the explosion of some of its members as SNe-Ia. The low observed rate of cluster SNe-Ia at z~1 pushes back the star-formation epoch in clusters to z>2, and implies a short delay time. We also show a related analysis for high-z field SNe which implies, under some conditions, a long SN-Ia delay time. Thus, cluster enrichment by core-collapse SNe from a top-heavy IMF may remain the only viable option.

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arXiv2003

The Type-Ia Supernova Rate in z < 1 Galaxy Clusters: Implications for Progenitors and the Source of Cluster Iron

The iron mass in galaxy clusters is about 6 times larger than could have been produced by core-collapse supernovae (SNe), assuming the stars in the cluster formed with a standard initial mass function (IMF). SNe Ia have been proposed as the alternative dominant iron source. Different SN Ia progenitor models predict different ``delay functions'' between the formation of a stellar population and the explosion of some of its members as SNe Ia. We use our previous measurements of the cluster SN Ia rate at high redshift to constrain SN Ia progenitor models and the star-formation epoch in clusters. The low observed rate of cluster SNe Ia at z ~ 0 - 1 means that, if SNe Ia produced the observed amount of iron, they must have exploded at even higher z. This puts a >95% upper limit on the mean SN Ia delay time of tau <2 Gyr (<5 Gyr) if the stars in clusters formed at redshift z<2 (z<3), assuming Ho=70 km/s/Mpc. In a companion paper (astro-ph/0309796), we show that, for some current versions of cosmic (field) star formation history (SFH), observations of field SNe Ia place a lower bound on the delay time, tau>3 Gyr. If these SFHs are confirmed, the entire range of tau will be ruled out. Cluster enrichment by core-collapse SNe from a top-heavy IMF will then remain the only viable option.

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arXiv2003

The Redshift Distribution of Type-Ia Supernovae: Constraints on Progenitors and Cosmic Star Formation History

We use the redshift distribution of type-Ia supernovae (SNe) discovered by the Supernova Cosmology Project to constrain the star formation history (SFH) of the Universe and SN Ia progenitor models. Given some of the recent determinations of the SFH, the observed SN Ia redshift distribution indicates a long (>~1 h^-1 Gyr) mean delay time between the formation of a stellar population and the explosion of some of its members as SNe Ia. For example, if the Madau et al. (1998) SFH is assumed, the delay time tau is constrained to be tau > 1.7 (tau > 0.7) h^-1 Gyr at the 95%(99%) confidence level (CL). SFHs that rise at high redshift, similar to those advocated by Lanzetta et al. (2002), are inconsistent with the data at the 95% CL unless tau > 2.5 h^-1 Gyr. Long time delays disfavor progenitor models such as edge-lit detonation of a white dwarf accreting from a giant donor, and the carbon core ignition of a white dwarf passing the Chandrasekhar mass due to accretion from a subgiant. The SN Ia delay may be shorter, thereby relaxing some of these constraints, if the field star formation rate falls, between z=1 and the present, less sharply than implied, e.g., by the original Madau plot. We show that the discovery of larger samples of high-z SNe Ia by forthcoming observational projects should yield strong constraints on the progenitor models and the SFH. In a companion paper (astro-ph/0309797), we demonstrate that if SNe Ia produce most of the iron in galaxy clusters, and the stars in clusters formed at z~2, the SN Ia delay time must be lower than 2 Gyr. If so, then the Lanzetta et al. (2002) SFH will be ruled out by the data presented here.

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arXiv2003

Using Multi-Band Photometry to Classify Supernovae

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arXiv2002

A Population of Intergalactic Supernovae in Galaxy Clusters

We have discovered seven type Ia cluster supernovae (SNe) in the course of the Wise Observatory Optical Transients Search in the fields of galaxy clusters with redshifts between z=0.06 and z=0.2. Two of these events, SN 1998fc in Abell 403 (z=0.10) and SN 2001al in Abell 2122/4 (z = 0.066), have no obvious hosts. Both events appear projected on the halos of the central cD galaxies, but have velocity offsets of 750-2000 km/s relative to those galaxies, suggesting they are not bound to them. We use deep Keck imaging of the locations of the two SNe to put upper limits on the luminosities of possible dwarf hosts, M_R > -14 mag for SN 1998fc and M_R > -11.8 mag for SN 2001al. The fractions of the cluster luminosities in dwarf galaxies fainter than our limits are less than 3 x 10^-3 and 3 x 10^-4, respectively. Thus, 2/7 of the SNe would be associated with less than 3 x 10^-3 of the luminosity attributed to galaxies. We argue, instead, that the progenitors of both events were probably members of a diffuse population of intergalactic stars, recently detected in local clusters via planetary nebulae and red giants. Considering the higher detectability of hostless SNe compared to normal SNe, we estimate that 20^{+12}_{-15} percent of the SN Ia parent stellar population in clusters is intergalactic. This fraction is consistent with other measurements of the intergalactic stellar population, and implies that the process that produces intergalactic stars (e.g., tidal disruption of cluster dwarfs) does not disrupt or enhance significantly the SN Ia formation mechanism. Hostless SNe are potentially powerful tracers of the formation of the intergalactic stellar population out to high redshift.

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arXiv2002

Not Color Blind: Using Multiband Photometry to Classify Supernovae

Large numbers of supernovae (SNe) have been discovered in recent years, and many more will be found in the near future. Once discovered, further study of a SN and its possible use as an astronomical tool (e.g., as a distance estimator) require knowledge of the SN type. Current classification methods rely almost solely on the analysis of SN spectra to determine their type. However, spectroscopy may not be possible or practical when SNe are faint, numerous, or discovered in archival studies. We present a classification method for SNe based on the comparison of their observed colors with synthetic ones, calculated from a large database of multi-epoch optical spectra of nearby events. We discuss the capabilities and limitations of this method. For example, type Ia SNe at redshifts z < 0.1 can be distinguished from most other SN types during the first few weeks of their evolution, based on V-R vs. R-I colors. Type II-P SNe have distinct (very red) colors at late (t > 100 days) stages. Broadband photometry through standard Johnson-Cousins $UBVRI$ filters can be useful to classify SNe out to z ~ 0.6. The use of Sloan Digital Sky Survey (SDSS) ugriz filters allows the extension of our classification method to even higher redshifts (z = 0.75), and the use of infrared bands, to z = 2.5. We demonstrate the application of this method to a recently discovered SN from the SDSS. Finally, we outline the observational data required to further improve the sensitivity of the method, and discuss prospects for its use on future SN samples. Community access to the tools developed is provided by a dedicated website (http://wise-obs.tau.ac.il/~dovip/typing) .

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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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arXiv2002

Orphan GRB radio afterglows: Candidates and constraints on beaming

The number of orphan radio afterglows associated with gamma-ray bursts (GRBs) that should be detected by a flux limited radio survey, is calculated. It is shown that for jetted GRBs this number is smaller for smaller jet opening angle (theta), contrary to naive expectation. For a beaming factor f_b^{-1}=(theta^2/2)^{-1} = 500, roughly the value inferred by Frail et al. (2001) from analysis of afterglow light curves, we predict that between several hundreds to several thousands orphan radio afterglows should be detectable (over all sky) above 1 mJy at GHz frequencies at any given time. This orphan population is dominated by sources lying at distances of a few hundred Mpc, and having an age of ~1 yr. A search for point-like radio transients with flux densities greater than 6 mJy was conducted using the FIRST and NVSS surveys, yielding a list of 25 orphan candidates. We argue that most of the candidates are unlikely to be radio supernovae. However, the possibility that they are radio loud AGNs cannot be ruled out without further observations. Our analysis sets an upper limit for the all sky number of radio orphans, which corresponds to a lower limit f_b^{-1}>10 on the beaming factor. Rejection of all candidates found in our search would imply f_b^{-1}>100. This, and the fact that some candidates may indeed be radio afterglows, strongly motivate further observations of these transients.

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arXiv2002

SDSS J124602.54+011318.8: A Highly Variable AGN, Not an Orphan GRB Afterglow

The optically variable source SDSS J124602.54+011318.8 first appears in Sloan Digital Sky Survey (SDSS) data as a bright point source with nonstellar colors. Subsequent SDSS imaging and spectroscopy showed that the point source declined or disappeared, revealing an underlying host galaxy at redshift 0.385. Based on these properties, the source was suggested to be a candidate ``orphan afterglow'': a moderately beamed optical transient, associated with a gamma-ray burst (GRB) whose highly beamed radiation cone does not include our line of sight. We present new imaging and spectroscopic observations of this source. When combined with a careful re-analysis of archival optical and radio data, the observations prove that SDSS J124602.54+011318.8 is in fact an unusual radio-loud AGN, probably in the BL Lac class. The object displays strong photometric variability on time scales of weeks to years, including several bright flares, similar to the one initially reported. The SDSS observations are therefore almost certainly not related to a GRB. The optical spectrum of this object dramatically changes in correlation with its optical brightness. At the bright phase, weak, narrow oxygen emission lines and probably a broader H$α$ line are superposed on a blue continuum. As the flux decreases, the spectrum becomes dominated by the host galaxy light, with emerging stellar absorption lines, while both the narrow and broad emission lines have larger equivalent widths. We briefly discuss the implications of this discovery on the study of AGNs and other optically variable or transient phenomena.

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