skip to main content


Search for: All records

Creators/Authors contains: "Kumar, Harsh"

Note: When clicking on a Digital Object Identifier (DOI) number, you will be taken to an external site maintained by the publisher. Some full text articles may not yet be available without a charge during the embargo (administrative interval).
What is a DOI Number?

Some links on this page may take you to non-federal websites. Their policies may differ from this site.

  1. The emergency of machine type and ultra-reliable low latency communication is imposing stringent constraints for service provisioning. Addressing such constraints is challenging for network and cloud service providers. As a trending paradigm, software-defined networking (SDN) plays a significant role in future networks and services. However, the classical implementation of the SDN controller has limitations in-terms-of latency and reliability since the controller is decoupled from the forwarding device. Several research works have tried to tackle these challenges by proposing solutions such as Devoflow, DIFANE, and hierarchical and distributed controller deployment. Nonetheless, these approaches are not fully addressing these challenges. This paper tries to address the problem of latency and reliability by proposing a dynamic controller role delegation architecture for forwarding devices. To align with the microservice or multi-agent-based service-based architecture, the role delegation function as a service is proposed. The dynamic role delegation enables to predict and (pre-)installed flow rules in the forwarding devices based on various considerations such as network state, packet type, and service's stringent requirements. The proposed architecture is implemented and evaluated for latency and resiliency performance in comparison to the centralized and distributed deployment of the SDN controller. We used ComNetsEmu, a softwarized network emulation tool, to emulate SDN and NFV (Network Function Virtualization). The result indicated a significant decrease in latency and improved resilience in case of failure, yielding better network performance. 
    more » « less
    Free, publicly-accessible full text available March 8, 2024
  2. Abstract

    One of the open questions following the discovery of GW170817 is whether neutron star (NS) mergers are the only astrophysical sites capable of producingr-process elements. Simulations have shown that 0.01–0.1Mofr-process material could be generated in the outflows originating from the accretion disk surrounding the rapidly rotating black hole that forms as a remnant to both NS mergers and collapsing massive stars associated with long-duration gamma-ray bursts (collapsars). The hallmark signature ofr-process nucleosynthesis in the binary NS merger GW170817 was its long-lasting near-infrared (NIR) emission, thus motivating a systematic photometric study of the light curves of broad-lined stripped-envelope (Ic-BL) supernovae (SNe) associated with collapsars. We present the first systematic study of 25 SNe Ic-BL—including 18 observed with the Zwicky Transient Facility and 7 from the literature—in the optical/NIR bands to determine what quantity ofr-process material, if any, is synthesized in these explosions. Using semi-analytic models designed to account forr-process production in SNe Ic-BL, we perform light curve fitting to derive constraints on ther-process mass for these SNe. We also perform independent light curve fits to models without ther-process. We find that ther-process-free models are a better fit to the light curves of the objects in our sample. Thus, we find no compelling evidence ofr-process enrichment in any of our objects. Further high-cadence infrared photometric studies and nebular spectroscopic analysis would be sensitive to smaller quantities ofr-process ejecta mass or indicate whether all collapsars are completely devoid ofr-process nucleosynthesis.

     
    more » « less
  3. Abstract

    We report observations of the optical counterpart of the long gamma-ray burst (GRB) GRB 230812B and its associated supernova (SN) SN 2023pel. The proximity (z= 0.36) and high energy (Eγ,iso∼ 1053erg) make it an important event to study as a probe of the connection between massive star core collapse and relativistic jet formation. With a phenomenological power-law model for the optical afterglow, we find a late-time flattening consistent with the presence of an associated SN. SN 2023pel has an absolute peakr-band magnitude ofMr= −19.46 ± 0.18 mag (about as bright as SN 1998bw) and evolves on quicker timescales. Using a radioactive heating model, we derive a nickel mass powering the SN ofMNi= 0.38 ± 0.01Mand a peak bolometric luminosity ofLbol∼ 1.3 × 1043erg s−1. We confirm SN 2023pel’s classification as a broad-line Type Ic SN with a spectrum taken 15.5 days after its peak in therband and derive a photospheric expansion velocity ofvph= 11,300 ± 1600 km s−1at that phase. Extrapolating this velocity to the time of maximum light, we derive the ejecta massMej= 1.0 ± 0.6Mand kinetic energyEKE=1.31.2+3.3×1051erg. We find that GRB 230812B/SN 2023pel has SN properties that are mostly consistent with the overall GRB-SN population. The lack of correlations found in the GRB-SN population between SN brightness andEγ,isofor their associated GRBs across a broad range of 7 orders of magnitude provides further evidence that the central engine powering the relativistic ejecta is not coupled to the SN powering mechanism in GRB-SN systems.

     
    more » « less
  4. Abstract

    We report observations of the optical counterpart of the long gamma-ray burst GRB 221009A. Due to the extreme rarity of being both nearby (z= 0.151) and highly energetic (Eγ,iso≥ 1054erg), GRB 221009A offers a unique opportunity to probe the connection between massive star core collapse and relativistic jet formation across a very broad range ofγ-ray properties. Adopting a phenomenological power-law model for the afterglow and host galaxy estimates from high-resolution Hubble Space Telescope imaging, we use Bayesian model comparison techniques to determine the likelihood of an associated supernova (SN) contributing excess flux to the optical light curve. Though not conclusive, we find moderate evidence (KBayes= 101.2) for the presence of an additional component arising from an associated SN, SN 2022xiw, and find that it must be substantially fainter (<67% as bright at the 99% confidence interval) than SN 1998bw. Given the large and uncertain line-of-sight extinction, we attempt to constrain the SN parameters (MNi,Mej, andEKE) under several different assumptions with respect to the host galaxy’s extinction. We find properties that are broadly consistent with previous GRB-associated SNe:MNi= 0.05–0.25M,Mej= 3.5–11.1M, andEKE= (1.6–5.2) × 1052erg. We note that these properties are weakly constrained due to the faintness of the SN with respect to the afterglow and host emission, but we do find a robust upper limit onMNiofMNi< 0.36M. Given the tremendous range in isotropic gamma-ray energy release exhibited by GRBs (seven orders of magnitude), the SN emission appears to be decoupled from the central engine in these systems.

     
    more » « less
  5. ABSTRACT

    S190426c/GW190426_152155 was the first probable neutron star–black hole merger candidate detected by the LIGO-Virgo Collaboration. We undertook a tiled search for optical counterparts of this event using the 0.7-m GROWTH-India Telescope. Over a period of two weeks, we obtained multiple observations over a 22.1 deg2 area, with a 17.5 per cent probability of containing the source location. Initial efforts included obtaining photometry of sources reported by various groups, and a visual search for sources in all galaxies contained in the region. Subsequently, we have developed an image subtraction and candidate vetting pipeline with $\sim 94{{\ \rm per\ cent}}$ efficiency for transient detection. Processing the data with this pipeline, we find several transients, but none that are compatible with kilonova models. We present the details of our observations, the working of our pipeline, results from the search, and our interpretations of the non-detections that will work as a pathfinder during the O4 run of LVK.

     
    more » « less
  6. Abstract Dirty fireballs are a hypothesized class of relativistic massive-star explosions with an initial Lorentz factor Γ init below the Γ init ∼ 100 required to produce a long-duration gamma-ray burst (LGRB), but which could still produce optical emission resembling LGRB afterglows. Here we present the results of a search for on-axis optical afterglows using the Zwicky Transient Facility (ZTF). Our search yielded seven optical transients that resemble on-axis LGRB afterglows in terms of their red colors ( g − r > 0 mag), faint host galaxies ( r > 23 mag), rapid fading ( dr / dt > 1 mag day −1 ), and in some cases X-ray and radio emission. Spectroscopy of the transient emission within a few days of discovery established cosmological distances (redshift z = 0.876 to 2.9) for six of the seven events, tripling the number of afterglows with redshift measurements discovered by optical surveys without a γ -ray trigger. A likely associated LGRB (GRB 200524A, GRB 210204A, GRB 210212B, and GRB 210610B) was identified for four events (ZTF 20abbiixp/AT 2020kym, ZTF 21aagwbjr/AT 2021buv, ZTF 21aakruew/AT 2021cwd, and ZTF 21abfmpwn/AT 2021qbd) post facto, while three (ZTF 20aajnksq/AT 2020blt, ZTF 21aaeyldq/AT 2021any, and ZTF 21aayokph/AT 2021lfa) had no detected LGRB counterpart. The simplest explanation for the three “orphan” events is that they were regular LGRBs missed by high-energy satellites owing to detector sensitivity and duty cycle, although it is possible that they were intrinsically subluminous in γ -rays or viewed slightly off-axis. We rule out a scenario in which dirty fireballs have a similar energy per solid angle to LGRBs and are an order of magnitude more common. In addition, we set the first direct constraint on the ratio of the opening angles of the material producing γ -rays and the material producing early optical afterglow emission, finding that they must be comparable. 
    more » « less
  7. Abstract The Fermi Gamma-ray Burst Monitor (GBM) triggers on-board in response to ∼40 short gamma-ray bursts (SGRBs) per year; however, their large localization regions have made the search for optical counterparts a challenging endeavour. We have developed and executed an extensive program with the wide field of view of the Zwicky Transient Facility (ZTF) camera, mounted on the Palomar 48 inch Oschin telescope (P48), to perform target-of-opportunity (ToO) observations on 10 Fermi-GBM SGRBs during 2018 and 2020–2021. Bridging the large sky areas with small field-of-view optical telescopes in order to track the evolution of potential candidates, we look for the elusive SGRB afterglows and kilonovae (KNe) associated with these high-energy events. No counterpart has yet been found, even though more than 10 ground-based telescopes, part of the Global Relay of Observatories Watching Transients Happen (GROWTH) network, have taken part in these efforts. The candidate selection procedure and the follow-up strategy have shown that ZTF is an efficient instrument for searching for poorly localized SGRBs, retrieving a reasonable number of candidates to follow up and showing promising capabilities as the community approaches the multi-messenger era. Based on the median limiting magnitude of ZTF, our searches would have been able to retrieve a GW170817-like event up to ∼200 Mpc and SGRB afterglows to z = 0.16 or 0.4, depending on the assumed underlying energy model. Future ToOs will expand the horizon to z = 0.2 and 0.7, respectively. 
    more » « less