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Durham University

Department of Physics

Staff profile

Publication details for Prof David Alexander

Baloković, M., Paneque, D., Madejski, G., Furniss, A., Chiang, J., The NuSTAR team: Ajello, M., Alexander, D.M., Barret, D., Blandford, R., Boggs, S.E., Christensen, F.E., Craig, W.W., Forster, K., Giommi, P., Grefenstette, B.W., Hailey, C.J., Harrison, F.A., Hornstrup, A., Kitaguchi, T., Koglin, J.E., Madsen, K.K., Mao, P.H., Miyasaka, H., Mori, K., Perri, M., Pivovaroff, M.J., Puccetti, S., Rana, V., Stern, D., Tagliaferri, G., Urry, C.M., Westergaard, N.J., Zhang, W.W., Zoglauer, A., The VERITAS collaboration: Archambault, S., Archer, A.A., Barnacka, A., Benbow, W., Bird, R., Buckley, J., Bugaev, V., Cerruti, M., Chen, X., Ciupik, L., Connolly, M.P., Cui, W., Dickinson, H.J., Dumm, J., Eisch, J.D., Falcone, A., Feng, Q., Finley, J.P., Fleischhack, H., Fortson, L., Griffin, S., Griffiths, S.T., Grube, J., Gyuk, G., Huetten, M., Haakansson, N., Holder, J., Humensky, T.B., Johnson, C.A., Kaaret, P., Kertzman, M., Khassen, Y., Kieda, D., Krause, M., Krennrich, F., Lang, M.J., Maier, G., McArthur, S., Meagher, K., Moriarty, P., Nelson, T., Nieto, D., Ong, R.A., Park, N., Pohl, M., Popkow, A., Pueschel, E., Reynolds, P.T., Richards, G.T., Roache, E., Santander, M., Sembroski, G.H., Shahinyan, K., Smith, A.W., Staszak, D., Telezhinsky, I., Todd, N.W., Tucci, J.V., Tyler, J., Vincent, S., Weinstein, A., Wilhelm, A., Williams, D.A., Zitzer, B., The MAGIC collaboration: Ahnen, M.L., Ansoldi, S., Antonelli, L.A., Antoranz, P., Babic, A., Banerjee, B., Bangale, P., Barres de Almeida, U., Barrio, J., Becerra González, J., Bednarek, W., Bernardini, E., Biasuzzi, B., Biland, A., Blanch, O., Bonnefoy, S., Bonnoli, G., Borracci, F., Bretz, T., Carmona, E., Carosi, A., Chatterjee, A., Clavero, R., Colin, P., Colombo, E., Contreras, J.L., Cortina, J., Covino, S., Da Vela, P., Dazzi, F., de Angelis, A., De Lotto, B., de Oña Wilhelmi, E.D., Delgado Mendez, C., Di Pierro, F., Dominis Prester, D. Dorner, D., Doro, M., Einecke, S., Elsaesser, D., Fernández-Barral, A., Fidalgo, D., Fonseca, M.V., Font, L., Frantzen, K., Fruck, C., Galindo, D., García López, R.J., Garczarczyk, M., Garrido Terrats, D., Gaug, M., Giammaria, P., Eisenacher, D., Godinović, N., González Muñoz, A., Guberman, D., Hahn, A., Hanabata, Y., Hayashida, M., Herrera, J., Hose, J., Hrupec, D., Hughes, G., Idec, W., Kodani, K., Konno, Y., Kubo, H., Kushida, J., La Barbera, A., Lelas, D., Lindfors, E., Lombardi, S., Longo, F., López, M., López-Coto, R., López-Oramas, A., Lorenz, E. Majumdar, P., Makariev, M., Mallot, K., Maneva, G., Manganaro, M., Mannheim, K., Maraschi, L., Marcote, B., Mariotti, M., Martínez, M., Mazin, D., Menzel, U., Miranda, J.M., Mirzoyan, R., Moralejo, A., Moretti, E., Nakajima, D., Neustroev, V., Niedzwiecki, A., Nievas-Rosillo, M., Nilsson, K., Nishijima, K., Noda, K., Orito, R., Overkemping, A., Paiano, S., Palacio, S., Palatiello, M., Paoletti, R., Paredes, J.M., Paredes-Fortuny, X., Persic, M., Poutanen, J., Prada Moroni, P.G., Prandini, E., Puljak, I., Rhode, W., Ribó, M., Rico, J., Rodriguez Garcia, J., Saito, T., Satalecka, K., Scapin, V., Schultz, C., Schweizer, T., Shore, S.N., Sillanpää, A., Sitarek, J., Snidaric, I. Sobczynska, D., Stamerra, A., Steinbring, T., Strzys, M., Takalo, L.O., Takami, H., Tavecchio, F., Temnikov, P., Terzić, T., Tescaro, D., Teshima, M., Thaele, J., Torres, D.F., Toyama, T., Treves, A., Verguilov, V., Vovk, I., Ward, J.E., Will, M., Wu, M.H., Zanin, R., External collaborators: Perkins, J., Verrecchia, F., Leto, C., Böttcher, M., Villata, M., Raiteri, C.M., Acosta-Pulido, J.A., Bachev, R., Berdyugin, A., Blinov, D.A., Carnerero, M.I., Chen, W.P., Chinchilla, P., Damljanovic, G., Eswaraiah, C., Grishina, T.S., Ibryamov, S., Jordan, B., Jorstad, S.G., Joshi, M., Kopatskaya, E.N., Kurtanidze, O.M., Kurtanidze, S.O., Larionova, E.G., Larionova, L.V., Larionov, V.M., Latev, G., Lin, H.C., Marscher, A.P., Mokrushina, A.A., Morozova, D.A., Nikolashvili, M.G., Semkov, E., Strigachev, A., Troitskaya, Yu V., Troitsky, I.S., Vince, O., Barnes, J., Güver, T., Moody, J.W., Sadun, A.C., Sun, S., Hovatta, T., Richards, J.L., Max-Moerbeck, W., Readhead, A.C., Lähteenmäki, A., Tornikoski, M. Tammi, J., Ramakrishnan, V., Reinthal, R., Angelakis, E., Fuhrmann, L., Myserlis, I., Karamanavis, V., Sievers, A., Ungerechts, H. & Zensus, J.A. (2016). Multiwavelength Study of Quiescent States of Mrk 421 with Unprecedented Hard X-Ray Coverage Provided by NuSTAR in 2013. The Astrophysical Journal 819(2): 156.

Author(s) from Durham

Abstract

We present coordinated multiwavelength observations of the bright, nearby BL Lacertae object Mrk 421 taken in
2013 January–March, involving GASP-WEBT, Swift, NuSTAR, Fermi-LAT, MAGIC, VERITAS, and other
collaborations and instruments, providing data from radio to very high energy (VHE) γ-ray bands. NuSTAR
yielded previously unattainable sensitivity in the 3–79 keV range, revealing that the spectrum softens when the
source is dimmer until the X-ray spectral shape saturates into a steep G » 3 power law, with no evidence for an
exponential cutoff or additional hard components up to ∼80 keV. For the first time, we observed both the
synchrotron and the inverse-Compton peaks of the spectral energy distribution (SED) simultaneously shifted to
frequencies below the typical quiescent state by an order of magnitude. The fractional variability as a function of
photon energy shows a double-bump structure that relates to the two bumps of the broadband SED. In each bump,
the variability increases with energy, which, in the framework of the synchrotron self-Compton model, implies that
the electrons with higher energies are more variable. The measured multi band variability, the significant X-ray-toVHE
correlation down to some of the lowest fluxes ever observed in both bands, the lack of correlation between
optical/UV and X-ray flux, the low degree of polarization and its significant (random) variations, the short
estimated electron cooling time, and the significantly longer variability timescale observed in the NuSTAR light
curves point toward in situ electron acceleration and suggest that there are multiple compact regions contributing to the broadband emission of Mrk 421 during low-activity states.